AN ASTRONAUT GARDENER ON THE MOON - SUMMITS OF SUNLIGHT AND VAST LUNAR CAVES IN LOW GRAVITY

Originally published by me online as: An Astronaut Gardener On The Moon - Summits Of Sunlight And Vast Lunar Caves In Low Gravity

You may have heard that the Moon is hopeless for gardening and for growing crops, and that Mars is the "go to" place for a prospective astronaut gardener. But is it? As it turns out, the Moon has some advantages over Mars, especially if you can plant your garden in a habitat or greenhouse on its summits of sunlight at the poles. Yes, it is rather chilly there, at -30° C (-22 °F), but there is no weather, good or bad, and the vacuum of space is a good insulator (like a thermos flask).

The temperature at the poles is steady, varying by only 10 °C (18 °F) up or down. That's warm enough to keep a well insulated habitat or greenhouse at a comfortable temperature of 20° C (68 °F) year round with sunlight piped in from solar collectors. And it's sunny at the poles, 24/7, year round, except during solar eclipses, and for a few days of the year when the sun dips below the horizon. This is a frame from a charming Russian movie about the Moon made in 1965, before humans landed there.

Looking out on the lunar surface from inside a Moon city, in a frame from the 1965 Russian film Luna

And here is a section of the movie itself with the peaches a few seconds in:

(click to watch on YouTube)

You can also watch The full movie, in restored colour, with machine translation subtitles for part of it.

The ESA now wants to go back to the Moon in collaboration with Russia and many other international partners, and build a base at one of the summits of sunlight - the "peaks of (almost) eternal light".

The modern ESA moon village idea.

Lunar gardening could make a major difference on the Moon, reduce the amount of food they need to import, and generate their oxygen too. You might think that Mars is far better for gardening than the Moon. We are surely going to do our best shot at gardening on the Moon whatever, but it may be encouraging to compare it with Mars. Actually the Moon isn't that bad a place for gardening as deep space locations go. It may be better than Mars in many ways.

The main thing Mars has going for it is its day just short of 24 hours 40 minutes, similar to Earth, while the Moon's night is two weeks long. However, Mars's dust storms can block out 99% of the sunlight for weeks on end, leading to occasional periods of darkness as long as the lunar night, or longer. Here is a photo showing progression of a dust storm as seen by Opportunity.

In the middle of this dust storm, less than 1% of the light that reaches the top of the Mars atmosphere made its way to the ground where Opportunity photographed it.

These storms often continue for weeks on end, and the dust storm season happens every two Earth years. In dust storms like this, artificial light is needed to grow plants, much as it is during the lunar night.

Mars is also further from the sun, with only half the levels of sunlight Earth receives. Its nights get so cold, below -78.5 °C (-109.3 °F), that its atmosphere starts to freeze out as dry ice for two hundred days of the year, even in the Martian "tropics", and it has a vacuum so hard that ice sublimes straight into gas, rather like dry ice does on Earth, without turning into water first.

Frosts on Mars - this photograph from Viking 2 is mildly enhanced to bring out the colour of the frost. These often form at night when the air gets so cold that even in the Martian "tropics", carbon dioxide freezes out as dry ice for 200 days of the 687 day long Mars year, carrying water with it as ice. In the day time on Mars the temperatures can get well above zero at times, so there are large day to night swings in temperature which would make it harder to maintain a steady temperature inside a greenhouse.

You might think that the thin carbon dioxide atmosphere of Mars is an advantage, but plants only need trace amounts in the atmosphere (we have less than 0.04%). A habitat will have only a few kilograms of carbon dioxide total in its atmosphere. Actually, if an astronaut gardener can grow enough crops to eat, the people eating the crops exhale enough carbon dioxide for the next generation of crops. While if they don't grow all their own food, the carbon dioxide builds up and needs to be scrubbed. So carbon dioxide is not likely to be much of an asset for greenhouse construction. It's far more likely to be a problem gas to scrub from the atmosphere of a space habitat, and indeed that's been the situation in all the spacecraft and space stations built to date.

Also the atmospheric pressure from the thin Mars atmosphere makes almost no difference to greenhouse construction. Even if your greenhouse is pressurized to the minimum pressure usually suggested for greenhouses in space, of 10% of Earth's atmosphere, so that you can go into it without the moisture lining your lungs boiling, the Mars atmospheric pressure doesn't help much. It still has to withstand nearly a ton of outwards pressure per square meter on the walls of the greenhouse, not much different from the situation on the Moon.

So, greenhouses on both Mars or the Moon are likely to be dome shaped to hold in the immense outwards pressure of the atmosphere inside. For more on this, with the calculations, see Greenhouse construction - comparison of the Moon and Mars in Case for Moon First

Detail of lunar colony showing a greenhouse inside a base. Detail from image from NASA, 1989. This was for the Lunar Oasis proposal for a ten year program to establish a self sufficient science outpost on the Moon to act as a test bed for space settlements.

If you build a greenhouse on the Moon or on Mars it has to withstand getting on for a ton per square meter of outwards pressure even at a tenth of an atmosphere. It might be easiest to just put it inside your habitat as shown here. It could be illuminated with efficient modern LED lights which require little by way of electricity and don't have problems of excess heat to get rid of. You could also pipe the light from the sun into the habitat using optical fibres connected to solar collectors

The Moon is a major challenge for gardeners too, it's no paradise. I'm not saying it is going to be easy there. It has no atmosphere at all, and apart from the polar regions, it has huge swings of temperature, and those two week long nights. But perhaps it can be made into a place to grow food more readily than Mars, especially if you can set up your garden in a habitat or greenhouse on the sunlit summits of the peaks of almost eternal sunlight at its poles.

The lunar caves also work out well compared to Mars, as we'll see. They have a constant temperature, actually a little warmer than the sunlit summits, at -20° C. Again, at those temperatures, it would be easy to warm up a lunar cave habitat or greenhouse enough for plants, if it is well insulated, as it would be. The night time darkness is the main problem with the caves, but as it turns out, its two weeks long night is not nearly as problematic for plants as you might think. That's the result of some rather surprising experiments with wheat, beet etc done by the Russians, and advances in LED technology.

We now know that the moon has water ice and other volatiles at the poles,. so there may well be plenty of water there for gardening. It is in darkness, and so can't be photographed from orbit, and the two main ways of detecting it from orbit (radar and reduced levels of neutron emissions) come up with different answers. We don't know for sure where it came from either (though comet impacts might be a good guess). But some think it is up to two meters thick layers of ice, and if so it may be easy to extract, with possibly hundreds of millions of tons, or a billion tons of them enough water for everyone in a city of a million to have the equivalent of an Olympic swimming lane filled with water.

24/7 SOLAR POWER AT THE LUNAR POLES

The Moon is very different from Earth with its month long "day". But it has another difference which makes all the difference to the polar regions . Unlike Earth and Mars which have an axial tilt of over twenty degrees, the Moon has a tilt of only a bit over 1 degree ( 1.5424°). It's remarkable that the Moon's axis is so vertical.

Diagram from NASA. With the Moon's orbit tilted by 5.14°, Earth tilted by 23.4°, yet the Moon's axis is pretty much vertical, only tilted by 1.5424° to the ecliptic (the apparent path of the sun through the sky). This remarkable coincidence is the reason the Moon's poles are so habitable.

Earth has seasons because of its axial tilt of around 23.4° at present (varies slightly by around one degree or so between 22° 2′ 33″ and 24° 30′ 16″ with a period of 41.040 years).

As a result, the Moon has no seasons, and you get points at the poles in almost constant sunlight, the peaks of eternal night. Habitats in those sunny spots can be kept warm just using solar collectors. Right next to them are permanently shaded craters that haven't seen sunlight for billions of years. There's indirect evidence that they may trap millions of tons of water, ammonia and carbon dioxide, possibly the results of comet impacts on the Moon.

Hamiwari sun tracking solar collector - the light is collected, focused and sent through fiber optics to the interior of the spacecraft or habitat, where it can be used as a light source for algae or growing crops, or to help keep it warm. For details of how this would work for spacecraft, see page 319 of Peter Eckart's book: Spacecraft Life Support and Biospherics.

Solar collectors like this could be used to pipe sunlight into a polar base, or into cave habitats or greenhouses

See peaks of (almost) eternal light in the online NASA astrobiology magazine).

When you are at the lunar poles, it's like a perpetual Arctic or Antarctic equinox, with the sun skimming the horizon, seeming to circle around you once every 28 days. This makes it easy to mount solar photovoltaic panels and solar collectors, to follow the sun and get maximum solar power. They just need to be mounted vertically, and turned slowly once a month.

DEALING WITH PERIODS OF DARKNESS AT THE LUNAR POLES

Even though the "peaks of (almost) eternal light" experience nearly continuous sunlight, they do go dark during eclipses. Also depending on the local topography, they have periods of darkness of several days at a time.

One way to deal with this is to raise the fibre optics solar collectors and solar photovoltaic panels on towers.

This idea goes back at least to 1990. If the Moon was a perfect sphere, then you would be able to achieve sunlight 24/7 (except during eclipses) with a tower 622 meters high.

Figure 1 from Arnold Reinhold's paper

See the section Description of idea in this paper A Solar Powered Station at a Lunar Pole by Arnold Reinhold. That's a major construction project but it would be easier to build on the Moon.

However the Moon isn't perfectly spherical of course, and the peaks of (almost) eternal light are quite high already. So your tower wouldn't need to be as tall as that. As it turns out, you can make quite a difference with a tower only 10 or 20 meters high.

This figure shows the difference it makes to put the solar collectors or solar panels on a pole 10 meters high and then 20 meters high, to raise them above nearby obstacles, from this paper.

With the best location in this paper, near the South pole (see page 1077), a mast 10 meters high increased the average solar visibility from 92.66 to 95.83, and reduces the number of days of darkness in the year from 26 down to 15. So it can make quite a difference.

Another approach is to find points close to each other such that one is in daylight when the other is in darkness. You can then route power from one to the other via a power line, or you can transmit the power using microwaves or lasers. One example here, page 9 two points 12 km apart, together have sunlight 97% of the year. Another example here, points A and B 10 km apart on page 562, figure 6b - B is illuminated for 82% of the year but when you combine it with A, then the result is illuminated 94% of the year.

This is another example

The white lines here show the local horizon as seen from this site in the South pole area on the Shackleton rim, and the yellow regions show the parts of the horizon that block the sun (less than 50% illuminated) on the worst day of the year. (Figure 11, page 16, explanation on page 6 of this paper). As you can see, many of the features that block the light are close to the site itself. Based on the analysis in the paper, this site needs a fast recharge power supply for a maximum of 62 hours or about two and a half days. With a coarser plot, the result was 156 hours, which shows how much a difference the local terrain makes. The results need to be validated against more independent images.

This is work in progress, as results are sensitive to details of the local topography, and the field is changing a lot with new high resolution data. We could do with even better elevation mapping of the poles.

So, the actual figures here might change as we learn more, but the basic idea is clear, that solar collectors on quite a short mast of 10 meters can often significantly reduce the length of the longest period of darkness and that it can also be reduced by using several collectors a few kilometers apart and transmitting power from one to the other.

LONG SHADOWS ON THE MOON, ESPECIALLY AT THE POLES

I haven't found any papers about this yet, so will just describe the issues. If you know of good citations do say. So, the thing is that shadows on the Moon are so very dark, that it's hard to see any features in them. Apollo 11 had to land during a narrow time window of sixteen hours every 29.5 days, so that they could see the lunar features during the descent flight path to the Moon. They could only land when the sun was at an elevation of between 5° to 14°. Too high and the sun would be directly behind them so that they couldn't make out any shadows at all, the landscape would be "washed out". Too low and most of the landscape would be shadowed.

The sun angle at the poles would be far lower, from 0° to around 1.5°, which would probably make it hard to impossible to do an Apollo style visual landing. Most of the landscape would be hidden by the long shadows of boulders, at ground level. We don't need to worry so much about the landing with modern technology and detailed 3D maps of the Moon, but the long shadows have other problems.

It will be hard to just walk around outside the base without artificial light. Although you would be walking in full sunlight, most of the ground at your feet would be in pitch darkness. There'd be some scattered light from the brighter pats of the landscape, but the bright landscape will also stop your eyes from dark adapting, so you won't see much in the darkness. So I think you will need bright lights to shine into the shadows to walk anywhere around the base. I don't see this as much of an issue, but it is something to think about.

Another problem is that if you park a rover next to the habitat, then at certain times of the lunar month, the rover will block the sunlight. It's the same for anything else you park or any structure you put up next to the village. The habitats will block each other as well.

So - this is just a suggestion, but I wonder if it would work best to have a multi-level village? Some of the habitats could be raised on legs above the landscape rather like Belgium's first ever "zero emissions" Antarctic base, the Princess Elisabeth Antarctic Station,

You could park your rovers beneath it. You could also have ramps or lifts to get down to the ground level from above. Of course it would have to be more rounded than this one.

The habitats could also be at slightly different layers and if you had a greenhouse, it could be at the top, center, a few meters above everyone else. The solar panels and solar collectors would be much higher on five or ten meter posts to avoid local relief, so they would not be affected.

HEAT REJECTION

Often it's as much of a problem to keep a space habitat cool as to keep it warm. The ISS has large heat rejecting panels, six of them, each able to reject at least 11.8 kilowatts of excess heat from the ISS. (See also "Explore the Space Station" with the radiators labeled)

This is another advantage of the lunar poles is that it's really easy to design heat radiators. You just need to set the radiators flat on the ground, and then the sun will only ever catch them edge on. They are already in the optimum position for heat rejection, with no need for the radiators to move. (See page 584 of this paper).

It's harder to do this at lower latitudes. One approach is to use heat pumps to increase the temperature to the point where the heat can radiate easily even in the lunar day. Or the heat rejection radiators can be shaded so that they remain cool. Or you can use a combination of both approaches.

FERTILITY OF LUNAR AND MARS SOIL

The easiest way to grow plants for food in space is to use soilless gardening with hydroponic solutions or with aeroponics where plants are grown with roots suspended in a fine mist (uses much less water).

This leads to huge savings in the precious area you need to grow crops. You don't need an acre of farmland per person as you do for conventional agriculture (4000 square meters approximately). In the BIOS-3 experiments, the Russians showed that you can grow all your food with only 30 square meters of growing area per person. The crops were grown on a culture conveyor with 2 to 10 plantings of different ages simultaneously. They grew wheat, sedge-nut, beet, carrots, and other crops, ten crops in total. With those 30 square meters per person, they produced 95% of the daily requirement for oxygen, water, food and everything else (by weight). The remaining 5% consisted of animal products, salt for the humans, nutrients for plants and personal hygiene supplies. They could produce 45% of the food and nearly all the oxygen and water with only 13 square meters.

For details see Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions). So early stages of agriculture on the Moon are sure to use more intensive methods such as this.

Soil based gardening can also be used with the methods of biointensive mini gardening. By using good gardening practices and by careful choice of crops you can grow all the food for one person in 4,000 square feet, about 372 square meters, or less than a tenth of an acre. That's intermediate between conventional agriculture and the conveyor belt type system of BIOS-3.

Grow biointensive - sustainable mini farming - this method needs only 372 square meters of growing area per person.

We can get an idea of how efficient these methods are by working out the total land area needed to feed the world on a vegetarian diet by all the methods. With a million square meters to a square kilometer, then we just need to multiply the numbers by 7,500 to get the area in square kilometers needed to feed a population of 7.5 billion. We get

• BIOS-3, 30 m², so 225,000 km²
  (smaller than the UK, see list of sovereign states and dependencies by area)
• Biointensive mini gardening. 325 m² per person, so 2.44 million km²
  (smaller than India at 3.288 million km²)
• Conventional agriculture, 4000 m² per person, so 30 million km²
  (a little under the area of the US, China and Russia combined)

By comparison, the Sahara desert is 9.2 million km². With the BIOS-3 system, we would need only 2.5% of the Sahara desert to feed the world and provide 95% of all that they need to survive. The total land area of the Earth is 148 million km². But of course much of that is desert, mountains, ice etc, some is uncultivated and animals require more land area than plants.

The surface area of the Moon is 38 million square kilometers. Indeed, a medium sized lunar crater 535 km across has sufficient land area to feed the whole world using the BIOS-3 system if you had the materials to cover it with greenhouses and enough air and water to produce growing conditions inside all those greenhouses. Mare Imbrium with a diameter of 1146 km has enough surface area to feed the world four times over.

Early experiments involving adding lunar material to hydroponic solutions suggested that the lunar soil was very fertile indeed as the plant growth was enhanced.

However later research suggested that the reason for this was a deficiency of trace elements in the hydroponics solutions used and that the plants were equally stimulated by some terrestrial soils.

A recent experiment using lunar soil simulants instead of the lunar regolith itself, and growing the plants in them directly found that the soil was not particularly good for plants, and that Mars simulants were better. However the Mars simulant contained trace amounts of organics and also held water better than the lunar simulant. Also heavy metals are an issue for Mars soil (they could not eat the results of their experiments because of the possibility of heavy metal concentration) as also is the case for the Moon.

None of this simulates the situation of gardening on the Moon or Mars using the soil itself as that would involve composting the residues of the first generation plants with micro-organisms to make a good top soil. You can't really expect plants to grow well directly in ground up rock without topsoil. Even after a single cycle of composting the residues, the result could be much more fertile.

Experiments with material from Apollo 16 showed that cabbage seedlings accumulated high concentrations of aluminium. Other experiments by Kozyrovska et al show that marigolds grown in terrestrial anorthosite accumulated heavy metals such as zinc, iron, nickel and chromium (lunar anorthosite forms the light areas on the Moon).

However when the marigolds were grown in a community of microorganisms in a model plant microcosm, this protected the marigolds against toxic doses of the heavy metals and also helped deliver essential nutrients to the plants.

Marigold flowers (tagetes patula) photo by Dori - when grown in terrestrial anorthosite (a type of mineral common on the moon) they accumulated heavy metals. However when grown with a community of microorganisms, this protected them against the heavy metals and helped to deliver essential nutrients to the plants.

Kozyrovska et al write:

"Our idea was to use the lunar soil for the pioneer plant growth and to convert the resulting plant biomass into so called protosoil, utilizing an optimized consortium of microorganisms."

Their experiments were promising. See Bioaugmentation in growing plants for lunar bases. The toxic heavy metals seem to be a surmountable issue. Another approach suggested by Haisong Liu et al. is to supply soil, and even seedlings, from Earth to get the process off to a quick start.

I'd like to suggest another idea here. Since hydroponics work well in the BIOS-3 experiments, one could also start by growing the first generation of plants hydroponically. Then you could use the plant residues to inoculate the lunar soil with organic material along with beneficial bacteria.

Also after whatever method is used for the first areas of good top soil on the Moon, the soil itself can then be used to inoculate larger areas of lunar soil with some of the top soil created earlier. The same methods could of course also be used on Mars, and Mars also has heavy metals in its soil. So in this comparison the two come out roughly equal.

Wherever we grow our plants, we just possibly might grow the tastiest chocolate in the universe.

LUNAR CAVES

We can only see a few meters into the lunar caves from the surface, so we don’t know how far they extend, especially since the regions near the pits are probably partly filled in with debris as well. But they could be huge; potentially they can be large enough to fit in a large city, the size of Philadelphia, with space to spare

Lava tube caves on the Moon could be stable up to five kilometers wide in the lower gravity. The black silhouette here shows the city of Philadelphia superimposed in one of these suggested tubes. We know there are rills on Mars this wide and can see cave entrances into them on the surface, in photographs taken from orbit, but we can't see far into them so don't know how large the caves are yet. These huge lava tube caves may have been detected indirectly though, through gravitational anomalies in the Grail spacecraft measurements: Scientists May Have Spotted Buried Lava Tubes on the Moon - see also Grail data points to possible lava tubes on the moon.

Such huge caves are only possible because of the low lunar gravity, as they would collapse on Earth. Similar caves on Earth are far smaller as would be any similar caves on Mars. We don't know for sure if such large caves do exist, but it does have many cave entrances photographed from orbit, which proves that at the least, it has caves with entrances similar in size to Earth cave entrances. Then the extensive systems of rills and the Grail data are suggestive of larger caves to be discovered.

Some of the possible lava tube gravitational signatures are over 100 kilometers long and several kilometers wide. If the Moon does indeed have caves 100 km long and kilometers wide, that's similar in size to the O'Neil cylinder space habitat with a land area of several hundred square miles (the O'Neil cylinder consists of a pair of cylinders, each 20 miles long and 4 miles in diameter, with total land area 500 square miles).

Each such cave could house several million people. This may be a long shot, but isn't it amazing, to think that the Moon could have caves as vast as this, similar in size to an O'Neil cylinder, and we simply wouldn't know yet?

EXAMPLE LUNAR CAVE SKYLIGHTS - LACUS MORTIS, MARIUS PIT AND THE KING-Y NATURAL BRIDGE

The Lacus Mortis area has possible volcanic cinder cones, as well as the more common shield volcano features, rilles, and a partially collapsed cave entrance with a gentle slope leading into it. This is the destination for the Astrobiotics mission in 2014.

Partially collapsed "skylight" in the Lacus Mortis region of the Moon. Photos of the Lacus Mortis pit from various angles were used to build a 3D model of the pit, assuming that it is a cave entrance.

3D model shown from various angles. The cave was assumed to be oval shaped as a result of fill by debris form the collapse. It may be a shallow slope down into a lava tube type cave shown at bottom right. If it's a lava tube cave, it should widen out to a circular cross section further from the entrance.

Another interesting pit is the Marius Hills pit entrance, original destination for the astrobiotics lander now due to land on the Moon some time in 2017:

The "skylight" on Marius hills (see page 7) was the original objective for the astrobiotics Skylight mission as envisioned in 2013 - it may be an entrance to a much larger lunar cave as it is located on a lunar rille. It's about 40 meters deep The crispness of the landform suggests the collapse happened less than a billion years ago, and the lack of any raised rim or eject suggests it formed through collapse, not through a meteorite impact.

This image shows an oblique view. It's viewed from an angle of 45 degrees, and the light from the sun is at an angle of 34 degrees from the vertical. As a result they were able to confirm that the area of the floor illuminated in this image continues at least twelve meters under the overhang. Papers here, and here .

This shows the location of the Marius pit along a lunar rille. Image from page 5 of Exploration of Planetary Skylights and Tunnels

Another "honorable mention" goes to the region of King crater, which is of special interest for its remarkable natural bridge.

Lunar natural bridge feature King Y, probably caused by a double collapse. It's about 7 meters in width and a 20 meters walk to cross it.

The lunar caves may also have unusual minerals that formed as the lava that created the cave slowly cooled and differentiated.

The NASA PERISCOPE project, currently a phase II concept study, could potentially give us a way to see into lunar caves from orbit using femtosecond laser photography which lets you "see around corners" to parts of the cave that were never within the line of sight of the orbiter.

We may may get our first views into the interior of a lunar cave from ground level some time in 2017, with the Japanese Hayuto Lunar X prize contender Moonraker, which will explore the Lacus Mortis pit "skylight" and then lower its two wheeled rover Tetris into the pit . For details of this mission, see Robotic missions to the Moon, already planned, or near future, from 2017 onwards.

LUNAR CAVES AS A SITE FOR A HUMAN BASE

So, another place we can set up a base is in a lunar cave. See for instance, Technologies Enabling Exploration of Skylights, Lava Tubes and Caves (from 2011). Advantages of the caves include:

• Natural protection from cosmic radiation and solar storms.
• Temperature is lower, but constant and the base would be easy to insulate and keep warm.
• Easy to make a low maintenance airtight enclosure, for instance, by covering the interior of the caves with a layer of glass formed from the regolith dust
• Sunlight can be brought in from the surface through the roof using light pipes / collectors
• However, a base in a cave only has access to nearby sunlight and solar power for 14 days of a lunar month. Some power storage would also be needed for the lunar poles, to deal with the short periods of night they get, including lunar eclipses.

Everything else is great, but the long 14 day lunar night is an issue, unless the cave happens to be right at the poles of the Moon close to the peaks of eternal light - or we dig a huge artificial cave there. However it is easier to cope with a 14 day lunar night than you might think.

PLANTS CAPABLE OF GOOD YIELDS WHEN KEPT IN DARKNESS FOR THE LUNAR NIGHT

It's also possible to deal with the lunar night by reducing the temperature of the plants from 24 °C in the lunar day to 2.5-3 °C in the lunar night (which helps maintain plant vitality during darkness). This was tested in an experiment by the Russians for BIOS-3. Of the ten crops tested, most were able to cope with this regime. The ones that couldn't cope were tomatoes, sedge nuts, and cucumbers. Wheat, barley, peas, turnips, dill, carrots, beet and radish were all able to survive a simulated repeated 14 day lunar night. The edible crops were reduced 30-50%. The most promising ones were carrots (73.5% yield), Beet (yield actually increased to 122%), turnip (57%), dill (72%), and radish (61.5% yield). See table 3 here (I've converted the figures to percentages).

So if you use that approach you'd need up to double the growing area, or around 60 square meters (probably a fair bit less depending on the mix of crops), but would be able to use natural sunlight during the lunar day, so would not need to supply light to the crops during the lunar night at all.

"Wheat plants of various ages showing the "conveyor" approach that was used in the Bios experiments, Young wheat plants are in the foreground, with more mature plants toward the back." Photo from here

In the BIOS-3 experiments they found that Wheat, barley, peas, turnips, dill, carrots, beet and radish were all able to survive a simulated repeated 14 day lunar night and produce reasonable crops of 50 - 70% of the crops achieved with full sunlight. The trick was to reduce the temperature from 24 °C in the lunar day to 2.5-3 °C in the lunar night (which helps maintain plant vitality during darkness)

You could also have a mixed regime, supplying light during the lunar night only to the plants that require it, such as tomatoes, cucumbers, etc, perhaps also extending it to the plants that most benefit from light during the lunar night (for instance, on those figures, the radishes and turnips would benefit most).

CAN WE FILL LUNAR CAVES WITH AIR?

We will surely just bring air from Earth to start with, since it's only a small amount of the launch mass, only 0.26% in the case of the atmosphere in the ISS (not including stored nitrogen). For details, see the Nitrogen section below. But later on, if we have thousands or even millions living on the Moon, perhaps we might want to fill the entirety of a large cave with air. A cubic kilometer of air weighs 1.225 million tons, which is a lot of nitrogen to find somehow. Could we source it on the Moon? This is for a fair way into the future of course, but let's try to get a rough estimate of whether it is possible.

Let's try a cave 250 meters in diameter, and 100 km long, a medium sized lunar cave (if the Moon does have caves as large as the Grail data and modeling suggests), but huge for Earth. That's 4.9 cubic kilometers of air, so 6 million tons of atmosphere using our 1.225 million tons per cubic kilometer figure (1.225*PI*(0.25/2)^2*100).

That much nitrogen could be sourced on the Moon, using lunar railways to truck it from the poles, if they do have and 600 million tons of volatiles, and 6% nitrogen for a total of 25 million tons of nitrogen. Typical heavy trains on Earth can carry 20,000 tons upwards. So if we had their like on the Moon that would be 300 train trips to fill the cave using volatiles from the poles.

You could also fill the cave pressurized to a tenth of Earth normal, which is enough to grow crops and for humans to walk around using only an oxygen mask, and then have pressurized habitats and greenhouses and parks within it at Earth normal pressure. If you do it that way, you need 600,000 tons of atmosphere for our cave 250 meters in diameter and 100 kilometers long, requiring about 30 trips of a heavy goods vehicle to truck the volatiles from the lunar poles to the cave.

Also, we can do another calculation based on the 225,000 km² needed to feed the population of the Earth on a basically vegetarian diet with the BIOS-3 system. If that area was covered with greenhouses, typical height say 2 meters, that's 550 cubic kilometers which at a mass of 1.225 million tons per cubic kilometer is around 674 million tons. If we pressurized the greenhouses at a tenth of Earth normal, that would be 67.4 million tons. There'd be enough water too, if we use aeroponics. So, if there are 25 million tons of nitrogen at the poles, the Moon may have enough by way of volatiles to fill greenhouses sufficient to feed 2.78 billion people or 37% of the Earth's population using the BIOS-3 system with greenhouses pressurized to a tenth of Earth normal - that's 9 kg of nitrogen per person.

That doesn't include the carbon dioxide for growing food. As we will see in CO₂ on the Moon, if you supply enough carbon dioxide to bring the first crop to maturity without astronauts there (so you don't have to feed them), you need about 40 kg of CO₂ per astronaut for the first crop, assuming it takes 40 days for the crop to reach maturity. After that, carbon dioxide becomes a nuisance gas to be removed if you need to import food.

If there are twelve million tons of carbon dioxide at the poles, that's enough for 300 million people on the Moon.

However this might not even be needed at all. If you are sending astronauts to the Moon anyway, and have to send habitats for them, you can surely manage to find the extra 40 kilograms per person for the carbon dioxide and what's more the extra 9 kg of nitrogen per person, or 90 kg if you pressurize the greenhouses to Earth normal. Each person would need at most twice their own weight in volatiles even with all the greenhouses pressurized to Earth normal.

So I don't think that need to provide an atmosphere is a deal breaker for colonization.

Of course I'm not suggesting we colonize the Moon with nearly three billion people, but it does show that the Moon has a fair bit of potential for gardening just using indigenous resources.

So, yes, we could fill some of the medium sized caves in their entirety in this way (depending on what we find by way of volatiles there).

However, eventually if we had millions living on the Moon and wished to fill all the lunar caves with nitrogen and oxygen, and if some are as large as 5 km diameter and 100 km long, then you have the same problem as you have with an O'Neil cylinder, that in such a large space, the mass of the atmosphere dominates, and nitrogen is in short supply in the inner solar system, apart from on Earth and Venus.

For a cave as large as that, 5 km across and 100 km long, you would need 1.225*PI*2^2*100 million tons = 1.54 billion tons of volatiles, most of that nitrogen. Perhaps you could get it from comets, but they have only around 0.5% NH3 or so (hard to estimate). Perhaps it can be exported from Earth if transport costs go down hugely? Or perhaps you would build domed cities and air filled tunnels on the floors of the caves much as you would do it on the surface, and keep them in vacuum conditions. The reason for building in caves then would be for the protection from cosmic radiation, and micrometeorites and more stable thermal environment.

Anyway - that's for the distant future. It's clear we are not going to run out of volatiles on the Moon any time soon, so long as the volatiles are indeed abundant and easy to mine and transport.

LOW POWER LEDS FOR PLANT GROWTH THROUGH THE LUNAR NIGHT

Lights would be needed in the caves or anywhere on the Moon except the poles, during the long 14 day lunar nights. They would be needed on the poles too, for a few days of the year. But they would also be needed for plants on Mars, during the dust storms, which block out 99% of the sunlight, often for weeks on end.

This used to be a major issue, because the lights for growing plants would consume a lot of the power of a space habitat with a greenhouse. But it's not a big issue any more. This is something that has changed a lot recently with the invention of LED lights optimized to produce only the wavelengths of light needed for crops. You need about 100 watts of supplied power for lighting for one square meter. That's 3 kW per person for 30 square meters. However you only need it for 12 hours a day for 14 days. That makes it a total of 504 kWh of storage needed per person to provide lighting for the lunar night.

POWER DURING THE NIGHT

One way or another you need a way to deal with the periods of darkness. For a detailed working out of power available and requirements for early stages of a lunar polar base (with Shackleton crater at the south pole as an example site for the base), including methods of transmission of the power, and storage on the Moon, see Power System Concepts for the Lunar Outpost.

The 500 kWh per colonist for growing plants during the lunar night corresponds to 750 kg of regenerative fuel cells per colonist. If you can use hydrogen electrolyzed from water during the day, it's more like 17 kg of hydrogen storage per colonist to last them through the lunar night, though that doesn't include the mass needed to store the hydrogen, separate it and recombine it. The ISS and Hubble both use nickel hydride batteries. A new type of battery of this type under development stores 140 kWh per kilogram, so if those were available, you would need about 3.5 kg of batteries per colonist.

This level of power is also just within the range you can supply continuously using standard RTGs - the radioisotope thermoelectric generators - as used for Curiosity, New Horizons, Cassini and many other deep space missions, though it does require a fair number of them. The 3 kWh peak power for plants during the day could be be supplied using 100 kg of plutonium 238 (at 3 watts per kg) or about 150 kg of Americium 241 per colonist. Each colonist has lunar night power requirements for growing crops equivalent to five Cassini spacecraft.

If we start with a lunar colony, as for the ESA village, then in the early days, for the first temporary visits to a polar colony on the Moon, astronauts could avoid the local lunar "winter" and could still be there most of an Earth year. Later you could put panels on several of the lunar peaks of almost eternal sunlight. You could use cables to transmit the power over a short range, and then microwaves or lasers for longer distances. Then the only periods without any solar power would be during the rare total lunar eclipses, requiring only hours of storage rather than days. In that way you would sort out the details of long term storage through the lunar night later on after you already have a fair bit of experience of living in the Moon.

For more on all this, see Power during the night in Case for Moon First,

The Moon is an ideal place to make solar panels, with a hard vacuum, abundant silicon, and the surface regolith can be turned to glass using microwaves as easily as you boil a kettle full of water (because of the pure iron imbedded throughout the regolith in the form of nanophase iron). You could have a solar panel paving robots that travel over the surface making solar panels on the surface using little more than the indigenous resources.

As metal refining takes off on the Moon, you could use HVDC for long distance transmission as is often done on Earth, and eventually have strips of solar panels all the way around the Moon - first at high latitudes and then at the equator too. Since solar panels can be made with indigenous resources and don't have to be shipped from Earth, and the solar panel construction can be automated, with plenty of real estate to cover with the panels, that leaves open the possibility that the Moon could export solar power to Earth in the future.

For more on this, see:

• Powering the whole Moon in the far future
• Possibility of using lunar solar power for Earth
• Lunar railways

WHAT ABOUT EARTH DESERTS?

I'm not urging that we colonize either Mars or the Moon, especially not right away. Rather I would say that we are still at an early exploration stage like the first Antarctic explorers. The explorers need crops and gardens too, especially if they need to stay there for any length of time. But they aren't there to grow crops, and set up home, there's a difference.

Going into space is not an optimal way to create large habitable areas for humans in our solar system at present, as much of Earth is uninhabited. Of course some desert areas are of great ecological interest but there are plenty of places where the desert is not of especial interest, and where colonization would be beneficial.

These deserts on Earth are far easier to colonize than anywhere in space. And what's more, we can use similar techniques, set up enclosures in the desert that are far more habitable than the surrounding desert. We are actually doing this already in a small way, with the salt water greenhouses, so I think you can say that not only should we do it, but we already are. This is an Australian desert project. The sea water is used to make water through the sunlight in the desert, and cool down the greenhouses.

(click to watch on YouTube)

These ideas could be used to reverse desertification in the Sahara desert and other deserts. This is how it works:

Diagrams by Raffa be from wikipedia

It not only lets you grow crops in the greenhouses - it can also help make the surrounding areas more habitable, so you’d get trees and crops growing in an area around the greenhouses as well. Doesn’t extract anything from desert aquifers, rather, it adds to them.

Sundrop farms have a large area set out for greenhouses like this now, in the middle of a desert, so this is taking off in a big way in Australia. Early days yet though.

This video just shows the greenhouses, and when they go inside in the video there is nothing growing there yet, not sure why, maybe it is a new installation, but it shows how it’s quite big in Australia.

(click to watch on YouTube)

There are many countries working on reversing desertification Israel does a lot of reversing of desertification.

One of the worst areas of encroaching desertification is the southern edge of the Sahara desert. The first priority there is to stop the spreading desertification - then to reverse it. Many African countries are collaborating in the Great Green Wall project to plant a forest along the southern edge of the desert.

Then, there is a similar project underway there now to the Australian Sundrop farms, using seawater greenhouses.

Technologies - Sahara Forest Project

This gives far more food and living space for the same amount of cost, compared with the billions of dollars to set up a few people in a space habitat. It's also far far easier to build a greenhouse in a desert on a planet with abundant sea water, and breathable air, than to do it on Mars. And as we just saw, with the BIOS-3 system, we would need only 2.5% of the Sahara desert to feed the world. So if we grew plants on Earth in as small an area as we could do for space habitats, we could feed the entire world easily with minimal impact, at least on the mainly vegetarian diet we would need for space colonies.

SEASTEADING

If we fill all the deserts, or you just don't have a handy desert in your country that's suitable for building on, you can build on the sea, This is still far easier than building in space. That's exactly what this couple did in Canada.

(click to watch on Vimeo)

Off the Grid on a Homemade Island moored in the sea off the coast of Vancouver island in Canada. They estimate that the floating structure weighs 500 tons and everything was constructed using a handsaw and hammer without use of power tools.

Imagine how hard it would have been for them to do this on the Moon or on Mars, with a vacuum or near vacuum, having to generate oxygen, scrub the air of noxious gases, somehow get hold of $2 million spacesuits, just to get outside of their door, protect themselves from the cosmic radiation and solar storms, and micrometeorites, somehow hold in the pressure of ten tons of atmosphere per square meter, never mind the difficulty of somehow getting everything there. They wouldn't be able to do it with a handsaw and hammer, for sure.

And actually, there is an idea of floating sea cities, a similar idea to the desert projects, but now just floating in the sea. This is at a much earlier stage, more of a paper project at present. Still it is very similar to the Mars or Moon colony ideas, but much easier to do and floating on the sea.

The Seasteading Institute | Opening humanity's next frontier

We could have sea cities covering much of the seas if we really need more space for people to live. If self sustaining colonies are indeed possible on the Moon and on Mars, as experiments so far seem to suggest, they would certainly be possible and much easier to construct, floating in the sea. If the 3D printing technology works for space habitats, to make them self sustaining with minimal imports, think how much better they will work on Earth!

By a sea colony here, I mean one that only uses the sea water and the air, with a few imports from Earth - as that would be the equivalent of a Mars habitat. There'd be no need for fishing or anything else, just air, and sea water, and the materials to build the original city, and some imports, and if advocates are right about Mars colonies, there would be little by way of those too.

Four fifths of the surface of our planet is ocean, so if we could live on the sea, in more or less self contained habitats, as with the ideas for Mars, that's be like finding four new planets to live on.

The land area of Mars is roughly the same as the land area of Earth. Four fifths of Earth is ocean, so if we colonized the oceans with floating sea cities, that's the equivalent of four new planets, each the size of Mars, and each far easier and far less expensive to colonize than Mars. Image credit JPL.

From the calculations in the section Fertility of lunar and Mars soil then you need 225,000 square kilometers to feed 7.5 billion people. That's by growing all their own crops - on a mainly vegan diet such as they would have to have in space colonies, using hydroponics and aquaponics and a culture conveyor with many stages of the crop growing simultaneously.

So then, i f we populated our oceans with similar population density to the land, those four times the population of Earth would need about 900,000 square kilometers. The surface area of the Pacific is 165.2 million km². So, that's 0.5% of the surface area of the Pacific as the amount of land you need for those four "ocean worlds" to grow all their own food using space colony type technology, if available on Earth rather than in space. Here of course, it would be much simpler, lower cost and lower tech and easier to maintain, to do this on Earth.

So in summary, if you used the efficient methods proposed for Mars colonists on Earth, to grow crops, you could feed four times the population of Earth from 0.5% of the surface area of the Pacific. That's actually the aim of the seasteading project too. See their section 5. Sustainability and ecology

"After the concept design is finished, the next challenge is to find the appropriate adaptation strategy – a strategy that creates a safe and livable urban environment on the sea, while minimizing impact on the ecosystems and making efficient use of the available resources. In this section, we explain the Blue Revolution concept and apply it to the seasteading concept"

They explain it in detail there, With its use of aquaponics and aeroponics, it resembles ideas for space habitats.

In a floating sea city like that, you would be in a far better situation than you could ever be on Mars or the Moon. The air is breathable, with no need to generate oxygen or to scrub the air of harmful gases. The Earth as a global system does that for you, just open your windows or make sure you have a bit of ventilation in your homes. There is no need for several meters thickness of cosmic radiation protection. You don’t need to wear spacesuits to go outside to repair the habitat, and there is no need to hold the breathable air in against tons per square meter of outwards pressure. It is hard to beat that.

A sea city would have minimal impact on sea life if done in the same way as for a space colony, growing all their own food inside the habitats. Perhaps this could be one outcome of space settlement, that by learning how to live in space, with such a high priority on efficient recycling, we can also learn to live on Earth as well, with minimal impact. Perhaps both approaches will influence each other.

CASE FOR SENDING HUMANS INTO SPACE

I think there is a case for sending humans into space, I think the Moon is the obvious first destination. Things we can do include

• Detect and deflect asteroids that could impact on Earth
• Supply satellites and other spacecraft with fuel and other components produced on the Moon or from asteroids - for much less cost - possibly - because you eliminate the need to launch the materials to orbit from Earth
• Move some of our heavy industry into space and find metals and other resources.
• Scientific research and astronomical observations ,building Antarctic style bases, and astronomical telescopes and observatories (the Moon has many advantages for some types of telescope, such as infrared telescopes in the passively cooled polar craters, radio telescopes on the far side shielded from radio transmissions from Earth, liquid mirror infrared telescopes in craters, and telescopes networked together using optical interferometry, taking advantage of the stability of the lunar surface)
• Exploration and adventure
• Tourism, could be a major thing for the Moon.
• Manufacturing electronics on the Moon - it’s a high grade vacuum which makes some processes easier to do there than on Earth.
• Manufacture of spacecraft - as industry develops on the Moon, it becomes a natural place to make spacecraft. The only reason that spacecraft have to be made physically small, and unfold, is to fit inside an aeroshell to protect them from the Earth's atmosphere. Spacecraft made on the Moon could be as physically as large as you like, and could be launched into orbit with the solar panels, telescopes etc already in an extended position. The low delta v also makes a huge difference to the amount of mass you can launch through the rocket equation (compare the lunar ascent module used by the astronauts with the Soyuz rocket), so interplanetary spacecraft could be made with cosmic radiation shielding already in place.
• Export of water and other volatiles and rocket fuel from the Moon to LEO. Advocates for this idea predict that the low delta v from the Moon (or almost nothing if you use Hoyt's cislunar tether system) may more than make up for the extra cost of extracting it from the Moon, especially if it is easy to extract water from the lunar poles.
• Solar power. It can actually make sense to do solar power in orbit, because you can use huge thin film mirrors weighing only grams per square meter, to focus the sunlight, onto a central thermal power generator and then safely beam back to Earth using microwaves. Also can be possible to create solar panels on the lunar surface

Note, I don't include Helium 3 in this list. It's a fuel for a technology that doesn't exist yet, some think will never be developed, and most decisive in my view, covering a given area of the Moon with solar panels would generate as much energy in seven years as you'd get from extracting all the Helium 3 from that region to a depth of three meters. So solar power from the Moon seems more viable than Helium 3 extraction as a business case, though Helium 3 would be a useful byproduct of mining operations on the Moon. For more on this see Helium 3 in Case for Moon First.

Many of these ventures would include gardening, growing their own crops, and some would lead to longer term settlement for many years, and possibly eventually colonization. But colonization for its own sake is not currently a good reason in my view, simply because it is so much easier to live on Earth.

Our Earth makes everything so easy for us, compared with space colonies. Perhaps one thing they might do is help us to appreciate quite how valuable and wonderful our Earth is, and how rare, as we look back at the beautiful Earth from the Moon or further afield.

Earth rise over the Moon as photographed from Apollo 8, first mission to orbit the Moon, on Christmas Eve, Dec. 24, 1968

As Carl Sagan in Pale Blue Dot

"The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment, the Earth is where we make our stand."

NO RUSH TO SEND HUMANS TO MARS

As for the idea we have to go to Mars to go multiplanetary, no, I don’t think we need to do that at all. I think that a rush to send humans to Mars would

• Spend trillions of dollars, eventually hundreds of trillions if they do it in earnest, on trying to make Mars habitable to humans, that would be much more beneficially used to protect and help Earth. It’s not going to work at all in the near term and probably never - the most optimistic estimates by the Mars society suggest a thousand years involving mega technology such as mining cubic kilometers of fluorite ore on Mars and using 500 half gigawatt nuclear power stations to convert it into greenhouse gases - and even then there is a lot of question about whether that would work.
  
  How likely is it that we would keep up such a project for a thousand years when it is much easier to live on Earth? Then it needs to go on for 100,000 years to get all the carbon sequestered for an oxygen atmosphere, the nitrogen needs to be sourced somehow, many cycles somehow got working on Mars in a different way from Earth, and lots to go wrong and needs mega technology into the indefinite future to keep Mars habitable if it works, always on the edge of reverting to a desert planet again if the greenhouse gas factories fail or the large mirrors in space stop functioning.
  
  If you read Kim Stanley Robinson’s “Mars Trilogy” then that is fantasy / science fiction with a lot of fudging of the numbers for the sake of a good story that plays out in generations instead of millennia. In the Earth’s evolutionary past of course it took many millions of years. For more on this see the section on Terraforming and paraterraforming below
  
• Also, by introducing Earth microbes to the planet, interfere with the process of scientific study of Mars which has potential to lead to the next major discovery in biology as momentous as the discovery of evolution or the helical structure of DNA, through discovery of life. based on different principles form Earth life or better understanding of early stages of how life can evolve from non living chemicals
  
  It's also very unlikely that at this stage we know enough about which microbes to introduce when, never mind introducing a random grab bag of whatever happens to be in human occupied spacecraft, if we do decide to do ecoengineering of Mars. I think it will be a fair while before we are mature enough to understand the consequences and know how to do it, to embark on a career of planetary ecoengineering as a species. We can do it small scale first with Stanford Torus type habitats or lunar caves / city domes, projects that we can complete in a more realistic decade, rather than a hundred thousand years.
• Does not help with ensuring the future of our civilization, and indeed by distracting from the problems on Earth, can make things more difficult in the future. There is some merit I think in an offworld backup of knowledge, and seeds. But we don't need to colonize to do that, and can do that on the Moon, the most accessible place to put it outside of Earth, where it will remain unchanged for millions of years into the future. See Backup on the Moon - seed banks, libraries, and a small colony
• No natural cause is going to make humans extinct until half a billion years in the future, such a long timescale that humans could evolve a second time from the smallest multicellular creatures. Our solar system has settled down since the huge craters that hit the Moon during the "late heavy bombardment", with no impactors much larger than around 10 km for the last three billion years on Mars, Mercury, the Moon, Earth and what we have of the history of Venus. Jupiter protects us from large comets and the larger asteroids are all in stable orbits over a millions of years timescale. Jupiter breaks the larger comets up, or diverts them to hit the Sun or ejects them from the solar system or they impact into Jupiter itself. Its defense of Earth is very effective as we can tell by the cratering record the only really big craters are from well over 3 billion years ago the ones for 100 km scale impactors such as Hellas basin on Mars, Aitken crater on the Moon and the Caloris Basin on Mercury all date from well over 3 billion years ago, getting on for 4 billion years ago.
  
  Jupiter is not so effective in preventing smaller impacts, of 10 km or so downwards. But asteroid impacts such as the one that ended the dinosaur era would not make us extinct. 70%, or even 99% of all species extinct equates to zero chance of extinction of such an adaptable creature as us, able to survive on fruit, nuts, seeds, insects, shellfish, animals, birds, and to travel in boats, with minimal technology, and live anywhere from the Arctic to the Kalahari.
  
  As for harm we may do to ourselves, that's as likely to be accelerated as prevented by a rush to send humans into space as quickly as possible. A war or a disease is that it is as likely to engulf colonies or even be started by them as be prevented by them especially as even fatal diseases normally have a latency period before symptoms develop (otherwise they wouldn't be able to spread) and transport to colonies in space would eventually only take weeks or even days. You can do something about it with quarantine, but it's hard to know what time period to set, and there is no need to go into space to set up a habitat with quarantine.
  
  It's possible to make up science fiction scenarios in which space colonies "Save the day", but it's equally possible to make up a scenarios where they cause disaster for Earth. A rush to colonize could lead to either type of outcome.
  
  With a more careful approach we can protect Earth from asteroids, and support it in many ways from space. Then if or when we do have colonies in space, this focus on saving our beautiful Earth as our main priority, I think will lead to a better outcome.

FEEDING THE EARTH

Also there is plenty of space to feed our growing population, if we make more efficient use of the land we have already. There's no need to go into space. Our population growth is leveling off, towards a population of perhaps ten billion by the middle to the end of the century.

We have already reached peak child, and most areas of the world now have fertility levels at or below replacement, and feature a slowly growing, steady or declining population. Our world population continues to grow only because with better health world wide, people on average live longer each year.

The middle of the range projections have the Earth's population trending towards 11 billion by 2100 while lower projections have it level off at ten billion or even start to decline towards the end of the century. 

So whatever happens, we aren't headed for Malthusian type exponential growth because we have reached peak child already. In the graph above, the red dotted lines show the upper and lower limits for the 95% prediction interval. The blue lines are for +- 0.5 children per couple average. You can look up the data here, the graphs page for the UN population division.

Though we may not reach peak population this century, most parts of the world have a good chance of stabilizing before then, especially the more developed countries. The least developed countries are the ones that would get most population growth. The most rapid growth is in Africa in the projections. You can see a break down for each region of the world here,

 Older figures from 2014. Most of the population growth is in Africa by the end of the century by these figures, with everywhere else leveling off by then, the least developed countries are the ones that grow most rapidly, so that's a reflection of the situation in Africa

So, things are actually looking brighter than one might think. Which isn't to say it will be easy, but there is no reason why we have to ruin ecosystems on Earth.

I think we can benefit Earth by moving some heavy industry into space, but there's no real reason why we can't have a sustainable human population here, and we don't have to export our population into space to keep going. It is just as well, since an exponential growth could never keep going indefinitely.

Within 18,000 years of doubling the population every century, so multiplying the population by a thousand every thousand years, you'd need all the matter in the entire observable universe to make colonists. Taking it to an absurd limit for an upper bound, even if they reduce the mass of each colonist to a single atom, that just extends it to 28,000 years, and if they double every thousand years instead of every century, with single atom colonists, it's 280,000 years. So obviously exponential growth has to stop well before then - even with warp drive if such was possible. 

Without warp drive then we run out of mass to make humans much more quickly, well within 11,000 years of doubling every century and within 110,000 years of doubling every thousand years. Calculation indented and coloured purple.

Without a warp drive, then the mass density of space in the vicinity of our solar system is 0.18 to 0.21 solar masses per cubic parsec (calculation by John Bahcall), which works out to roughly one solar mass every 165 cubic light years (a parsec is 34.7 cubic light years). We would need to convert a sun's mass into humans after less than 7,000 years of doubling every century (the mass of the sun is equivalent to around 5*10^19 humans at 5 kg per human).

After 11,000 years of traveling at the speed of light we would have access to = 33.8 billion solar masses, and multiplying the mass of humans by 1000 every 1000 years we would require a trillion solar masses at that point. So without warp drive we run out of matter just to make humans well within 11,000 years. Here I've also ignored the limits on thickness of the galactic disk.

So it is very unlikely that if we meet an extra terrestrial, that they will be in a phase of exponential growth and one way or another we also have to find a way to be able to survive without exponential growth, so we have to find a way to stabilize our population, bring the birth rate to below replacement. Luckily, it seems this happens naturally as countries develop economically and with better health and education.

For more on this: Why ET Populations Can't Continue To Expand For More Than A Few Millennia

This section has material from my Quora answer to Shouldn't we focus on colonizing Antarctica and the Sahara before colonizing Mars? as well as several sections from my Case For Moon First

THE MOON IS RESOURCE RICH

Mars colonization advocates often contrast Mars with the Moon. The Moon may be described as being as uninteresting for human colonization as a lump of concrete. But actually it turns out that the Moon is very rich in resources. We need a "Case for the Moon" here like Robert Zubrin's "Case for Mars".

Moon advocates perhaps don't hit the news as much as the Mars advocates but there are many of them, and they are just as enthusiastic about their vision as the Mars advocates. Paul Spudis is one, with his most recent book, The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources. Another is Dennis Wingo, CEO of Skycorp, and author of Moonrush, see his recent paper, and appearance on the Space Show. Others include Madhu Thangavelu, David Schrunk, and other authors and contributors to The Moon: Resources, Future Development and Settlement. See also David Schrunk's paper Planet Moon Philosophy , and their appearance on The Space Show.

It was also the policy of the US too during the Bush administration, with his Vision for Space Exploration program. And the ESA and Russia are strongly behind the idea of sending humans to the Moon first.

So, let's look at some of the suggested lunar resources for this Moon first approach.

CO₂ ON THE MOON

The Moon doesn't have a CO₂ atmosphere, but it has dry ice at the poles. It has an estimated at least 600 million metric tons of ice (based on the mini SAR and lunar prospector data, indirectly detected through radar), and possibly much more. If the proportions are the same as for the LCROSS impact measurements, 2.12% of that may be CO₂, so if those figures are correct (which needs more confirmation), that could be more than twelve million metric tons of CO₂ in the form of dry ice.

That's plenty, but we might not even need it. It is only a trace gas in our atmosphere, less than 0.04%. And, this may surprise you, but actually, feces is nearly all water. About 1 kg of carbon dioxide is exhaled every day, compared to only 30 grams (0.03 kg) of dry weight in feces.

Plants don't need a constant supply of carbon dioxide from outside of the habitat. Indeed, if you grow enough plants for food, then they get all they need to make the plant matter and the oxygen from the carbon dioxide you breathe out, and a small amount from the feces which can be burnt or composted.

With each crop, to close the cycle, the plants also produce almost exactly the amount of oxygen that humans need to breathe, indeed that's the main advantage of growing your food, that it saves on the 0.84 kg of oxygen you need to supply every day. Oxygen supply is the main issue here, and after that, food.

If you have to import some of your food, as has always been the case so far, then an excess of carbon dioxide builds up, and you have to remove because it is dangerous to humans long term at concentrations of above one or two percent in the atmosphere.

The first crop cycle, for, say, 40 days (it took 39 days for dwarf wheat to reach maturity in an experiment on the ISS in zero g) does need a net input of CO₂ to the plants for them to grow. But this will be provided by the astronauts just breathing it out, so if you supply them with food for the first month, then they will then provide enough carbon dioxide for the next crop cycle onwards.

Or if you use robots to set up the greenhouse before the astronauts arrive and bring the first crop to maturity ready for them to eat, then you need to supply that 1 kg a day of CO₂ per astronaut for one month. So that's 40 kg per astronaut.

You could take carbon out of the atmosphere in the habitat if you store and accumulate the plant wastes. Typically half the crop is plant wastes and if you stored all the plant wastes, yes, you'd need a constant input of CO₂. However that would lock up valuable oxygen into the plant wastes and remove carbon from the system as well, so it's not too likely that you would do that.

If you burn plant wastes too as is the most likely thing to do in a space colony, or compost them, then you have a closed system, and any imported food will mean an excess of carbon in the system which will build up in the atmosphere rather rapidly. This was the main thing that threatened the lives of the Apollo 13 crew, they had plenty of oxygen but had to rig up a way to scrub the carbon dioxide to survive.

On the ISS it used to be vented into space. Nowadays they react it with the hydrogen got from splitting water to generate oxygen, in the Sabatier reaction This converts the carbon of the carbon dioxide to methane which is then vented into space. That's still not a closed system as it depends on constant input of water and food to provide the carbon and hydrogen that's lost to space in the methane.

The ISS's half kilogram for 0.04% (see next section) would then correspond to just half a day's worth of wastes for one person. If you suppose similar amount of atmosphere to the ISS, and six people, importing all their own food, the carbon dioxide would build up at a rate of about half a percent a day, so would over 1% within two days, and reach 10% within 20 days, a level which leads to convulsions, coma and death.

If they import half their food, and don't scrub the carbon dioxide they die within 40 days, but probably much sooner. If they import a quarter of their food, with no carbon dioxide scrubbing, they die within 80 days.

That's based on this paper which says concentrations >10% may cause convulsions, coma and death. That's for short term exposure, so with continuous exposure with gradually increasing concentrations, they would probably die well before then. Levels of up to 3% however can be tolerated for more than a month without any adverse effects (see table 2 page 66b of this paper).

No space habitat to date has had to import CO₂, and until you have near perfect recycling it won't be needed. Once you are able to grow all your own food, then you may need a tiny amount, but the more perfect the recycling, the less you need. If you had perfect recycling, you'd only need as much as is necessary to get the plants started. Usually half of the plants grown for crops consist of plant wastes, but that also doesn't really change anything. The CO₂ you get from burning the wastes or composting it, added to the CO₂ breathed out by humans, is almost exactly the amount the plants need to grow the next generation of crops.

So, it's not too likely that you will have a shortage of CO₂ in space.

One way or another, if you import food, you will have an excess of carbon in the system which has to be scrubbed and got rid of somehow, usually as carbon dioxide or methane. While if you manage a nearly biologically closed system, you need hardly any materials supplied from outside to keep it going, just need to deal with leaks. The water in urine, sweat and grey water can be recycled, something that is already done in the ISS. For techy details: upgrades to the ISS water recovery systems. The feces can be dealt with also, without need to build a sewage plant in space, for instance oxidized at high temperatures 400 C and high pressures using supercritical water.

It's true that in the carbon cycle on Earth, volcanoes recycle carbon dioxide, which was originally taken out of the atmosphere millions of years ago, however not by us eating plants, or plants just growing and decaying as that returns roughly the same amount to the atmosphere as was taken from it. It is taken out through steady build up of plant residues, for instance peat, coal and oil, and through build up of limestone and chalk in the oceans and through organics falling into the ocean from algae growing on the surface. Most of our CO₂ is stored in carbonates such as limestone, or as organics in the oily shales, and that's what gets subducted and then returned to the atmosphere in volcanoes. It's a much slower process.

If we ever attempted to terraform Mars or the Moon or anywhere else long term, we would need to have some other way to return the limestone and chalk and other carbonates to the atmosphere, as it doesn't have continental drift to subduct them and cycle them around through volcanoes. However this is not a concern for space habitats in the near future - they aren't going to be troubled by the effects of a build up of oil rich shales, limestone and chalk.

For more about all this see my Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?

NITROGEN

Moon has nitrogen too, in the form of ammonia at the poles. If it is correct that there are 600 million tons or more in the form of ice up to 2 meters thick, with 6% ammonia, then there may be 25 million tons of nitrogen there .

We need nitrogen as a buffer gas in the atmosphere to protect us from oxygen toxicity.

There is another way to avoid oxygen toxicity, and that is, to use oxygen at low pressures, as they do for spacesuits. Spacesuit gloves are stiff and difficult to use and full Earth pressure would make them much harder to use. Apollo also used a pure oxygen atmosphere even after the fire of Apollo 1, as a simpler system, but you have to take care to use materials that won't burn easily. Since Apollo all space flights have used mixed oxygen / nitrogen for habitats and pure oxygen only for spacesuits. So, lunar habitats will surely do the same.

We don't have to keep resupplying nitrogen to a space habitat because it is a one off amount of mass (plants use nitrogen but it's part of a nitrogen cycle so could be returned to the atmosphere using denitrifying bacteria, and anyway compared to carbon, it is a small amount of the total mass of the plant). So how much mass do we need to provide?

Well, ignoring reserves of nitrogen, we can work out how much is needed for the habitat air itself. The pressurized volume of the ISS is 32,333 cubic feet or around 915.5686 cubic meters. At 1.225 kg / m3, at Earth sea level pressure which is what they use, it's 1.122 tons of air. So that's less than a ton of nitrogen, and of the total mass of the ISS, 419.725 tons, only 0.27% is atmosphere. As for CO₂, at 400 ppm, that much air would contain a negligible half a kilogram (0.04% of 1.122 tons)

So, it's useful to be able to get your nitrogen and oxygen in situ, but it's not a deal breaker if you have to get it from Earth if you have decent closed system recycling. 

It's the same for any size of habitat, for city domes too, the atmosphere is small fraction of the total mass, not including regolith shielding, just the unshielded habitat mass.

For a large habitat such as the Stanford Torus then the atmosphere is a higher percentage of the total mass, but that's because it has less structural mass needed, because the surface area (heaviest part) goes up only as the square when the volume goes up as the cube.

For the Stanford Torus, the structural mass is 150,000 tons, atmosphere 44,000 tons, for 10,000 people. Per person that's 15 tons of structural mass and 4.4 tons of atmosphere, not counting the regolith shielding (which they planned to send from the Moon using bulldozers and a mass driver)..

For the ISS, with 6 people, 419.725 tons, that's about 70 tons per person of which 0.187 tons is atmosphere That's a fair bit of atmosphere. There are more efficient ideas for the Stanford Torus, the Vademecium design which has a flatter torus so reducing the amount of mass for atmosphere. However, the reduction in structural mass compared to the ISS more than compensates.

The situation is the same for most large space habitats. The larger it is, the less structural mass per person but the more atmosphere per person if it is similar in shape to a smaller habitat. Venus cloud colonies are different, they have much lower mass requirements per person similar to an airship, and in principle, they can get all of their atmosphere from the Venus atmosphere too.

Nitrogen doesn't seem likely to be a major issue in the early stages at least. It may be more of an issue if we wish to fill an entire lunar cave with nitrogen, more on that below: Can we fill lunar caves with air.

VOLATILE RESOURCES

We have pretty good evidence now of ice at the poles, in permanently shadowed craters, thought to be relatively pure and at least a couple of meters thick according to radar data from a NASA instrument flying on India's Chandrayaan-1 lunar orbiter.

It's not a direct detection however, so there is still room for skepticism about it, as rough material would have the same radar signature as radar transparent ice. But craters that are rough when new, are rough both inside and outside the crater rim. While these signatures are found only inside the craters and not outside the rims, which they interpret as meaning that they are caused by ice. The temperatures are also right for ice.

If it is ice, it could be "fluffy ice".

"We do not know the physical characteristics of this ice—solid, dense ice, or “fairy castle”—snow-like ice would have similar radar properties. In possible support of the latter, the low radar albedo and lower than typical CPR values for nonanomalous terrain near the polar craters are 0.2–0.3, somewhat lower than normal for the nonpolar highlands terrain of the Moon and are suggesting the presence of a low density, “fluffy” surface."
(page 13 of Evidence for water ice on the moon: Results for anomalous polar)

In either case, it is not just a little ice; if this is what they detected, there's estimated to be at least 600 million metric tons of this, and possibly much more.

It also contains other volatiles. We know for sure that there is some ice on the Moon, by the LCROSS impact experiment. Relative to H₂O at 100% they found H₂S at 16.75%, NH₃ at 6.03% SO₂ at 3.19%, C₂H₄ at 3.12%, CO₂ at 2.17%.

So, if the rest of the ice at the poles has a similar constitution to the impact site that's a lot of nitrogen (in the ammonia) and CO₂ on the Moon at the poles.

The green circles here surround craters at the lunar north pole thought to have layers of ice, with an estimated total of at least 600 million metric tons of water.
(0 degrees longitude at bottom)

On the other hand, caution is needed as this is not direct detection. The LEND results (searching for hydrogen through reduced emissions of neutrons of a particular type) are particularly puzzling, as there is almost no resemblance between their map and the miniSAR map.

LEND map - in this picture blue is reduced neutron emission and shows likely locations of hydrogen. 0 degrees longitude is at the top.

They did detect hydrogen, but puzzlingly, it was not correlated with the permanently shadowed regions - there was some hydrogen in permanently shadowed regions, and some also in illuminated regions. A recent paper suggests that ice mixed in the regolith in illuminated regions may be ancient ice that survived a minor shift of the lunar axis. According to one hypothesis, this may be ancient deposits from over three billion years ago before volcanic activity, which changed the polar axis slightly by shifting material.

A new LEND mission has been proposed involving low passes over the poles at altitudes as low as a few kilometers, for higher resolution results.

The Moon may also have ice at lower latitudes too, as there are permanently shaded regions up to 58 degrees from the poles (only 32 degrees from the equator). Though these regions are too warm to have ice on the surface, there may be ice there underground. See Ice may lurk in shadows beyond Moon's poles (Nature, 2012).

At any rate, the Moon does seem to have resources of ice at the poles (though memorably, Patrick Moore in one of the last Sky at Night programs that he did said that he'd believe there is ice at the poles when someone brought him a glass of water from the Moon). More research is needed to find out how much there is and where it is.

Some scientists - particularly Arlin Crotts, think it may have ice several meters below the surface over the entire planet, and that it may have volatile resources deep down. There are signs that suggest it is still geologically active, and one possibility is that the activity may be due to volatiles deep down escaping to the surface. For more on this, see Geologically active moon

METALS

Critics often say that the Moon is undifferentiated and doesn't have any processes to concentrate ores. Although the Moon doesn't have any liquid water so all the processes involving concentration of resources through water erosion won't work, it still has many processes that can concentrate ores. Including:

• Fractional crystallization - as a melt cools down, some minerals crystallize out at a higher temperature than others so form first. They then settle or float, so remove the chemical components that make them up from the mix, so changing its formula, leading to new crystals to form in a sequence.
• Gravitational settling, lower mass material floats to the top.
• Volcanic outgassing can concentrate materials such as iron, sulfur, chlorine, zinc, cadmium, gold, silver and lead.
• The processes that lead to volatiles condensing at the poles - which it seems can also concentrate silver too
• Processes unique to the Moon (perhaps electrostatic dust levitation may concentrate materials)?
• Volatiles brought in as part of the solar wind
• Asteroid and micrometeorite impacts bring materials from asteroids to the lunar surface such as iron and possibly platinum group metals etc.

The Moon has many valuable ores for metals. For instance, the highland regions (probably the original crust of the Moon) consists mainly of Anorthite (a form of feldspar, formula CaAl₂Si₂O₈) which is 20% Aluminium, compared with 25% Aluminium for Bauxite on Earth. So aluminium ores are abundant on the Moon, indeed orders of magnitude more abundant than they are in typical asteroids, but it does require a lot of energy to extract the aluminium from the ore. Either a nuclear power plant or large areas of solar panels. Crawford, in his "Lunar Resources: a Review", says this about aluminium on the Moon:

"Aluminium (Al) is another potentially useful metal, with a concentration in lunar highland regoliths (typically10-18 wt%) that is orders of magnitude higher than occurs in likely asteroidal sources (i.e. ~1 wt% in carbonaceous and ordinary chondtites, and <0.01 wt% in iron meteorites; . It follows that, as for Ti, the Moon may become the preferred source for Al in cis-lunar space. Extraction of Al will require breaking down anorthitic plagioclase (CaAl2Si2O8), which is ubiquitous in the lunar highlands, but this will be energy intensive (e.g. via magma electrolysis or carbothermal reduction; Alternative, possibly less energy intensive, processes include the fluoridation process proposed by Landis , acid digestion of regolith to produce pure oxides followed by reduction of Al2O3 (Duke et al.), or a variant of the molten salt electrochemical process described by Schwandt et al."

Mining this for the aluminium would create calcium as a byproduct, which is useful as a conductor in vacuum conditions, a better conductor than copper weight for weight - you need half the mass for the same amount of electricity. (Copper does better than calcium on a per volume basis because it is 5.8 times denser, it is also of course much more practical in an atmosphere because calcium reacts vigorously with air, but that's not a problem for conductors that operate in a lunar vacuum, and in space applications the reduced mass may be an advantage).

"Calcium metal is not used as a conductor on Earth simply because calcium burns spontaneously when it comes in contact with oxygen (much like the pure magnesium metal in camera flashbulbs). But in vacuum environments in space, calcium becomes attractive.

"Calcium is a better electrical conductor than both aluminum and copper. Calcium's conductivity also holds up better against heating. A couple of figures mining engineer David Kuck pulled out of the scientific literature: "At [20C, 68F], calcium will conduct 16.7% more electricity than aluminum, and at [100C, 212F] it will conduct 21.6% more electricity through one centimeter length and one gram mass of the respective metal." Compared to copper, calcium will conduct two and a half times as much electricity at 20C, 68F, and 297% as much at 100C, 212F.

"Like copper, calcium metal is easy to work with. It is easily shaped and molded, machined, extruded into wire, pressed, and hammered.

"As would be expected of a highland element, calcium is lightweight, roughly half the density of aluminum. However, calcium is not a good construction material because it is not strong. Calcium also sublimes (evaporates) slowly in vacuum, so it may be necessary to coat calcium parts to prevent the calcium from slowly coating other important surfaces like mirrors. In fact, calcium is sometimes used to deoxidize some metal surfaces. Calcium doesn't melt until 845C (1553F).

"Utilization of lunar materials will see the introduction of industrial applications of calcium metal in space."

From the section on Mining the Moon in Permanent - by Mark Evan Prado, a physicist in the Washington, D.C., region working for the Pentagon in advanced planning in the space program.

The Moon is deficient in copper, at least on the basis of what is known so far, but as well as calcium, aluminium is a good conductor.

The LCROSS experiment found silver (a superb conductor) and mercury at the impact site, but the concentration is not known, except that it is far higher than the levels in the Apollo samples, and is probably in a layer below the surface, as the signal was delayed. See LCROSS mission may have struck silver on the moon.

It has abundant iron - in addition to ores (which would need a lot of power to extract), it actually has free iron metal

• From meteorite impacts
• Nanometer sized "blebs" released from the rock by the hydrogen in the solar wind reacting with iron oxides
• Particles of iron concentrated from the source materials for the regolith.

It's in powder form already, and naturally alloyed with nickel and cobalt. The blebs, or "nanophase iron" are found inside impact glass particles, so would be hard to extract. The rest though is made up of tiny particles of pure iron, so the obvious thing to try to do is to separate them out using powerful magnets. They are rather small though, most are less than a micron in diameter which could be a challenge. If we can separate them out, we can get five kilograms of iron, 300 grams of nickel and about half a gram of platinum, gold etc. (platinum group metals) in every cubic meter of regolith - as pure metal what's more. (This summarizes part of section 5, Metals from Crawford)

He bases that on a paper from 1980 by Morris and particularly its conclusion, which uses a model to interpret the data. Taylor and Meeks in the section Agglutinitic Glass versus Grain Size and Maturity (page 133) in their paper suggest that perhaps most of the iron is in nanophase form, mixed up with the glass and hard to extract.

However we don't need to speculate any more as Jayashree Sridhar et al of the NASA Johnson Space Center have done the experiment using actual samples of lunar regolith. See Extraction of meteoritic metals from lunar regolith, and they succeeded! The nanophase iron was a problem but they were able to work around it by varying the experimental setup. By varying on the size of particle they ground it down to, the strength of the magnets and details of the technique they could extract over 80% of the meteoritic iron in some of the tests. They conclude:

"Experimental results indicate promise for the extraction of meteoritic metals from lunar regolith. However, more work is needed to refine the technique and understand more about the variables that affected our results."

The iron is valuable for steel, and is also a conductor, though not nearly as good as Aluminium or Calcium. It would be useful for some applications such as electric railroads on Mars, and is a conductor easy to access in the early stages.

Also nickel and iron are useful for making nickel / iron batteries. These could be useful for making batteries on the Moon with in situ resources, for instance to help last through the lunar night.

"Iron-nickel batteries are very rugged. Their lifetimes which can exceed 20 years are not affected by heat, cold or deep cycling. They are not easily damaged by rapid discharging or over-charging. On the downside, they have poor performance at low temperatures but they can be kept warm with insulation (e.g. simple regolith) and thermal wadis. Also, they only have a charge to discharge efficiency of 65% and will self discharge at the rate of 20% to 40% per month. Despite these shortcomings, they might be the Moon-made power storage systems of choice due to their simplicity and the availability of their component materials on the Moon. Moreover, these materials are among the easiest of materials to produce on the Moon."
See Electrical Energy Storage Using Only Lunar Materials.

Then, you also have titanium. This is especially interesting as it is rare in asteroids. Apollo 17 samples are 20% high purity Ilmenite, a Titanium ore which is found in the lunar mare. And better than that, the Lunar Reconnaissance Orbiter, with its spectral mapping of the Moon, discovered deposits that are up to 10% titanium, more than ten times higher than titanium ores on Earth. (Phys.org report, NASA image). Titanium is an industrially desirable metal, stronger per unit weight than Aluminium (though it is a poor conductor).

Titanium is also widely used in medicine for hip replacements, dental implants, etc., as "one of the few metals human bone can grow around firmly", see also this new titanium / gold alloy four times tougher than titanium

Titanium is especially useful for medical applications because it

• Forms an inert and stable titanium oxide layer spontaneously
• Has a high strength to weight ratio
• Doesn't leach into blood and other aqueous environments because of its low rate of ion formation
• Is one of the few materials that can integrate directly into living bone tissues (osseointegration) without any soft tissue layers in between

Crawford writes (page 17):

"Therefore, in the context of a future space economy, the Moon may have a significant advantage over asteroids as a source of Ti. The fact that oxygen is also produced as a result of Ti production from ilmenite could make combined Ti/O2 production one of the more economically attractive future industries on the Moon.

For more on this, see major lunar minerals. And for an in depth study, read Crawford's review.

So, yes, there are plenty of metals on the Moon, but it might take a lot of power to extract them, apart from the iron, if that can be separated out using magnets.

And that's mainly based on the Apollo results which explored a small region of the lunar surface which has been found to be in some ways unrepresentative. The Moon may have many other surprises in store. Many ores on Earth would not be detected from orbit, and it seems the Moon has a fairly complex geology as well.

As an example of one way the Moon could surprise us - Earth is often hit by iron meteorites, so the Moon should be also. The main question is, how Dennis Wingo has hypothesized in his Moonrush book, that the Moon may also have valuable platinum group metals which could be mined, the result of the impacts of these iron meteorites.

Taking this further, there's a hypothesis by Wieczorek et al that magnetic anomalies on the Moon around the south pole Aitken basin may be from the remains of the metal core of a large 110 km diameter differentiated asteroid that hit the Moon to form the basin. If so, they could be useful sources for platinum, gold, etc.

From Wieczorek et al, the North and South poles are marked N and S. Notice the magnetic anomalies clustered around part of the rim of the South Pole Aitken Basin. This is thought to be the result of an impact by a 110 km diameter asteroid. Wieczorek et al hypothesize that the magnetic anomalies trace out the remains of the metal core of this asteroid. If so these could be rich ores, including iron, nickel, also platinum and other platinum group metals (gold, rhodium etc). See page 16 of Crawford's Lunar Resources: A Review

Platinum is a particularly useful metal. It is heavy, soft, malleable as gold and silver, easy to draw into wires, very unreactive, and has a high melting point. Out of gold, silver, platinum and copper, platinum is the densest and the hardest and the least reactive (the others are somewhat better in terms of electrical and thermal conductivity, and malleability, but it's not too bad at those either). So, it's not just useful for catalytic converters, fuel cells, dental fillings and jewelry. We'd probably use it a fair bit in other ways too if it didn't cost so much.

The platinum group metals might be valuable enough to return to Earth from the Moon, just as suggested for the asteroids, especially if there is water to split and use as fuel available on the Moon or once they set up a mass driver on the Moo

Of course, you can't just take the current market value of platinum, multiply by the amount of platinum available in a large meteorite - or on the Moon if Wingo and Wieczorek et al are right - and conclude that you'd get trillions of dollars by returning all that platinum to Earth and selling it here. You need to fulfill a need or eventually nobody will buy it. If it's just to replace copper, for instance, in wires, it wouldn't be worth returning unless you could reduce the transport cost back to Earth right down. Dennis Wingo suggested in Moonrush that it could be worth exporting it to Earth for use for fuel cells, as an application that could be high value and yet need a lot of platinum.

The gold could be useful too, on the Moon at least. You don't normally think of gold as more decorative than useful but it is used a fair bit in electronics Also combined with the abundant titanium on the Moon you get Ti₃Au, an alloy with 70% less wear, four times the hardness and increased biocompatibility compared with pure titanium (and twice as hard as titanium / silver and titanium copper alloys). It's also 70% less wear than titanium, lower friction and four times harder with a hardness of 800 HV in the Vickers hardness test. Density about the same as steel.

(density of titanium: 4.43 g/cc. using the atomic masses of gold and titanium, multiplying by (196.96657+3*47.867)/(4*47.867)*4.43 = 7.88 approx. By comparison, density of steel is 7.75 g / cc).

The paper focuses on its medical applications, you can alloy titanium with copper or silver, which are twice as hard as pure titanium, but this is four times as hard. It's also 70% more resistant to wear which will make it last longer and lead to less debris. And has excellent biocompatibility properties. But I wonder if it might also have lunar applications, with the hardness especially and resistance to wear.

Probably only the platinum group metals would be worth returning to Earth, since it's going to be easier to mine the Near Earth Asteroids, especially the ones that consist almost entirely of pure metal. However, whether or not they are useful for Earth, they are well worth using on the lunar surface once you have industry there.

The Moon has some advantages over Mars indeed for metals, such as the pure nanophase iron mixed in with the regolith, which can only exist in oxidized form on Mars except for rare metal meteorites. Also, it's unlikely it will be commercially worthwhile to return metals from Mars while there are definite possibilities of returning metals from the Moon. See Exporting materials from the Moon for future suggested low cost methods for export from the Moon. For discussion of whether anything physical could be worth the expense of export from Mars, see Commercial value for Mars

LUNAR GLASS

This is a beneficial side effect of all the micrometeorite impacts on the Moon (which you don't get so much on Mars with its thin atmosphere, just enough to filter out micrometeorites). The Moon's "soil" or regolith contains large quantities of glass, created during the impacts. It also has free iron, as we saw, at half of one percent of the soil, in tiny micro beads of iron (nanophase iron) which concentrate the microwave energy. Again, you don't have this on Mars.

As a result, it is really fast to melt the regolith using microwaves. It took only 30 seconds to melt small lunar sample at 250 watts (typical of a domestic microwave). You can melt the soil to glass as easily as you can boil water using the microwave in your kitchen. See lunar lawnmower. This only works with genuine lunar soil and not the simulants. We have nothing analogous to lunar soil on Earth, as Larry Taylor, principle author of this paper found: Microwave Sintering of Lunar Soil: Properties, Theory, and Practice. He says the microstructure of the genuine lunar regolith, with nanophase iron beads scattered throughout, would be almost impossible to simulate.

His idea (see Products from Microwave Processing of Lunar Soil on page 194 of the paper) is to run a "lunar lawnmower" over the soil with two rows of magnetrons (such as generate microwaves in a microwave cooker). The first row would sinter it to a depth of half a meter using microwaves. Then the second row completely melts the top 3-5 cm of the soil, which then crystallizes to glass. As it does this, it will heat up and release most of the solar wind particles notably hydrogen, helium, carbon and nitrogen. So it could also capture these assets as it goes along, including the Helium 3, if this turns out to be of economic value.

See also The Lunar Dust Problem: From Liability to Asset. This could also be useful, for instance, for a solar panel paving robot to make solar panels, and other applications.

Then, there's Behrokh Khoshnevis' idea for making a landing pad on the Moon using tiles made of lunar glass in situ. The idea is to make the surface into lots of tiles by injecting a material that can't be sintered easily using microwaves into the soil first to outline the edges of the tiles, then use microwaves to melt the soil in between.

(click to watch on Youtube)

This would make a tiled flat surface for supply vessels to land on. It would also help with the problem of lunar dust by removing dust from the landing area. You can read the details here. He used lunar regolith simulant, so presumably by Larry Taylor's results, it would work even better with genuine lunar samples.

SOLAR CELLS FROM LUNAR MATERIALS - SOLAR PANEL PAVING ROBOT

Once you have glass, it might not be such a big step to make photovoltaic cells on the Moon. And here the Moon has one big advantage, the high grade vacuum so you could use vacuum deposition to make the cells in situ. To start with you'd make the cells themselves from materials sent from Earth, later on mine them on the Moon.

This was first tested in the Wake Shield Facility exploiting the increased levels of vacuum in the wake of the Space Shuttle. They were able to grow very pure thin film materials, so demonstrating a path to easy production of solar cells in high vacuum conditions in space, for instance on the Moon.

This is a report from the Center for Advanced Materials at the University of Houston, suggesting the possibility of an autonomous solar powered lunar photovoltaic cell production rover

It would use silicon extracted from lunar materials to make the cells themselves. Of the various methods you could use, magma electrolysis may be best. He uses low efficiency silicon cells which are vacuum deposited on glass, something that is not easy to do on Earth but would be possible in the ultra high vacuum conditions on the Moon. Techy details of this suggestion are here.

It would require transporting a small mass to the Moon in the form of the rover which then over several years of driving could build a 1 MW facility on the Moon.

Idea for a robot to drive over the surface of the Moon leaving solar panels in its wake wherever it goes, using only indigenous lunar materials to make the panels. The panels would be only 1% efficient, but given that there is no shortage of real estate on the Moon, that might not matter. It might be more important to make the panels in situ without any imports from Earth than to make them highly efficient

Structure of the panels

For making glass on the Moon see the section above: Lunar glass

BASALT (LIKE GLASS FIBER)

The basalt itself is a natural resource. If reasonably pure and consistent in composition, it's ideal for making basalt fibre, which is like glass wool, but much better in some ways. The regolith consists mainly of powdered basalt. So might well be ideal for making basalt fibre. See:

• Basalt Fiber Properties

HELIUM 3

I should mention this, since the topic is brought up so often in discussions of lunar settlement. However I don't see this as a major plus point for the Moon at present.

The Moon is a source for helium 3, deposited in the regolith by the solar wind, and some say that helium 3 will be of value for fusion power in the future because it is not radioactive and doesn't produce radioactive waste products. If so, small amounts of helium 3 from the Moon could be worth a lot on Earth and be a useful commodity to export. Apollo 17's Harrison Schmidt is a keen advocate of helium 3 mining on at a reasonable rate at a reasonable rate the Moon.

However, we don't yet have fusion power plants at all, and one able to use helium 3 is a tougher challenge. Frank Close wrote an article in 2007 describing this idea as "moonshine" saying it wouldn't work anyway. Frank Close says that in a deuterium - helium 3 tokamak, at normal temperatures for a tokamak, the deuterium helium 3 reaction proceeds so slowly that the deuterium would instead fuse with itself producing tritium and then fuse with the tritium (the original article is here, but it's behind a paywall). For a critical discussion see also the Space Review article The helium-3 incantation

See also Mining the Moon by Mark Williams Pontin. If you can use much higher temperatures, six times the temperature at the centre of the sun by some calculations, the helium 3 will fuse at a reasonable rate, but these are temperatures way beyond what is practical in a tokamak at present. The reason such high temperatures are needed for a tokamak is because the plasma is in thermal equilibrium and has a maxwellian distribution which means that to achieve a few particles at very high temperatures you have to heat up a lot of particles to lower temperatures to fill up the maxwellian distribution so that just a few will react. This is potentially feasible for the lower temperatures of DT but not feasible for the higher temperatures of ³He³He.

However if you use electrostatic confinement, a bit like a spherical cathode ray tube with the fusion happening at the center where the negatively charged "virtual cathode" is, then the particles are all at the same high energy and the result is much more feasible with lower power requirements. This is the approach of Gerald Kulcinsky who achieves helium 3 fusion in a reactor 10 cm in diameter. However though it does produce power, it produces only one milliwatt of power for each kW of power input so is a long way from break even at present.

Gerald Kulcinski who has developed a small demonstration electrostatic ³He³He reactor 10 cm in diameter. It is far from break-even at present, producing 1 milliwatt of power output for each kilowatt of input. See A fascinating hour with Gerald Kulcinski

Perhaps this line of development will come to something. Perhaps one way or another we will achieve helium 3 fusion as the enthusiasts for helium 3 mining on the Moon hope. However it is early days yet, and we can't yet depend on this based on a future technology that doesn't exist yet.

However even if we do achieve helium 3 fusion, it might not be such a game changer for the lunar economy as you might think. Crawford says (page 25) that to supply all of our energy from Helium 3 would mean mining 5000 square kilometers a year on the Moon, which seems ambitious (and would mean the whole Moon would only last 200 years). So, even if we develop Helium 3 based fusion, and it turns out to be a valuable export, it's probably not going to be a major part of the energy mix.

Even more telling, he also calculates that covering a given area of the Moon with solar panels would generate as much energy in 7 years as you'd get from extracting all the Helium 3 from that region to a depth of three meters.

Also - there are many other ideas being developed for nuclear fusion, such as laser fusion, and the polywell which has the same advantage that no significant radiation is produced when it uses fusion of boron and hydrogen. I think it is far too soon to know whether or not the helium 3 on the Moon will be an asset in the future when we achieve nuclear fusion power. For a summary, see ESA: Helium-3 mining on the lunar surface.

This doesn't mean that there is no point in helium 3 mining however. As Crawford suggests (page 26), Helium 3 is useful for other things, not just for fusion power. It's used for cryogenics, neutron detection, and MRI scanners, amongst other applications, so some Helium 3 from the Moon could be a valuable export right away, even if it doesn't scale up to the huge quantities you'd need for Helium 3 based power generation on Earth. You'd get it automatically as a byproduct while extracting the more abundant volatiles from the solar wind in the regolith, so it might well be a useful side-line to help support lunar manufacturing economically as part of the mix along with everything else.

THORIUM AND KREEP (POTASSIUM, PHOSPHORUS AND RARE EARTH ELEMENTS) ,AND SOME URANIUM

The Moon has some uranium, which is a bit of a surprise for such a heavy element, but when bound with oxygen it is rather lighter and can occur in the lunar crust as on Earth. It is especially rich in Thorium, in the lunar Mare. This is useful as a fuel for nuclear fission reactors, which have to be designed to burn thorium instead of uranium to use it. It's not likely to be worth returning to Earth as thorium is abundant here. But it could be very useful in space, at some point in the future.

Nuclear power stations built on the Moon wouldn't have the same pollution hazards and hazardous waste issues as stations on the Earth. Perhaps this may be a way to power space colonies, and interplanetary ships fueled from the Moon, so avoiding the need to launch nuclear power plants from Earth to orbit.

Thorium is a tracer for KREEP - potassium, phosphorus and rare earth elements. Also associated with chlorine, fluorine, sodium, uranium, thorium, and zirconium, so KREEP ores could be sources for all those elements on the Moon.

When the Moon cooled down from the original molten state, then olivine and pyroxene crystals form first, and sink to the bottom of the magma ocean (both made of iron and/or magnesium plus silicon and oxygen). Meanwhile anorthite also forms (made of calcium, aluminum, silicon, and oxygen), which is less dense and floats to the top (forming the lunar highlands). Some of the other elements like nickel are able to squeeze into the crystal lattice and get removed at the same time. But the larger elements can't, and are left in liquid state. They are last to solidify and form the KREEP deposits. It forms in between the olivine and pyroxene deep down, and the floating anorthite on top and may have been liquid for a long time.

For some reason, not fully understood, then KREEP deposits on the surface of the Moon are concentrated on the near side of the Moon near the Imbrium basin, with a small amount also in a separate concentration on the far side. The Imbrium impactor probably excavated the KREEP deposits on the near side. But it's puzzling that the much larger Aitken basin didn't lead to large deposits on the far side. Perhaps for some reason KREEP is concentrated on the near side of the Moon. For more about this see The Moon is a KREEPy place by the planetary geologist Emily Lakdwalla which I summarized here.

The abundances of rare earth elements on the Moon are much less than rare earth ores on Earth, and despite the name, they aren't very rare here on Earth. So it's not likely that they'll be worth returning. However the most concentrated spots - the ones marked white in this figure - haven't been sampled on the surface and the spatial resolution is low, tens of kilometers. So it's possible we'll find more concentrated ores on the Moon.

It's a similar situation for uranium and thorium. The abundances on the Moon from this map are too low to count even as a low grade ore on Earth. But with such low resolution, there could be richer ore deposits when we look at it closely. (Here I'm summarizing what Crawford says about lunar KREEP ores in his survey, see section 7, Rare earth elements and following)

LUNAR VACUUM AS AN ASSET

The vacuum of the Moon is also actually an asset, so much so that we might need to take special precautions to preserve it.

"It seems absurd to expect that the lunar vacuum could be lost by small-scale operations on the moon. However, high-vacuum and ultra-high vacuum is needed for many industrial processes, some of which may be accomplished on the moon. Some processes which require vacuum and thus would be simpler to manufacture or use on the moon include vacuum tubes, semiconductor manufacture, solar cell manufacture, and particle accelerators."

Degradation of the Lunar Vacuum by a Moon Base - Geoffrey A. Landis

The carbon dioxide atmosphere of Mars is so thin it counts as a laboratory vacuum, you would need a pressurized spacesuit or you'd die quickly because the water lining your lungs would boil. But it's not thin enough to be a useful vacuum. And as we've seen, carbon dioxide is not needed as an input for greenhouses - in a closed system it just circulates around to food and back again. The Moon would seem to have the advantage here, too.

EXPORTING MATERIALS FROM THE MOON

We've just mentioned the idea of using a lunar mass driver. This is a 1970s artist's concept drawing of the idea, the large area of solar cells to the left powers it.

However, unless it can be constructed largely using in situ materials, then there is a lot of material to send up to the Moon to construct it. This is a 2010 study that (making various assumptions) suggested that a lunar mass driver would have a long payback time. The more frequently you can send the payloads, the shorter payback time. You might want to do regenerative breaking to recover as much of the energy as possible from the launch vehicle. Some ideas for mass drivers on lunarpedia.

Then, there's the possibility of generating fuel using the ice at the lunar poles. With 600 million metric tons at least, if those figures are correct, then it seems that we could make fuel without significantly impacting on the amount of ice there.

There are other ideas also for exporting materials from the Moon, such as the lunar space elevator. which was promoted in a successful kickstarter by the Liftport group. Though we don't yet have the technology to build a space elevator on Earth, we could in principle build one on the Moon. It would extend from the surface through the Earth Lunar L1 point,

There are different design ideas, but one proposal in 2011 was for an 11,000 kg tether made of Zylon HM fiber which would be 264,000 km long, transport microrovers to the lunar surface, and return 10 kg at a time from the lunar surface, so that is a mass to payload ratio of 1,100 to 1. For techy details see LADDER.

For more background information about lunar tethers, see. "Lunar Space Elevators for Cislunar Space Development Phase I Final Technical Report"

Another ingenious idea, and one with a much lower structure to payload ratio than a lunar elevator is Robert Hoyt's Cislunar tether transport system. More details: CISLUNAR TETHER TRANSPORT SYSTEM

It uses momentum exchange tethers. This shows the idea for Earth - a rotating tether that could pick a payload from a suborbital spacecraft up from the upper atmosphere, and boost it into LEO. If the tether rotates at exactly the right spin rate, then it can be stationary relative to Earth when in the upper atmosphere, closest to Earth in the tether spin - and spinning at just the right speed to boost the payload to LEO at the upper part of the spin. This is too hard to do at present. But it could capture a payload from a suborbital spacecraft moving at Mach 12, and boost it to orbit.

Well this gets especially interesting if you do it on the Moon. Robert Hoyt designed a cislunar transport system which looks like this:

"The Cislunar Tether Transport System. (1) A payload is launched into a LEO holding orbit; (2) A Tether Boost Facility in elliptical, equatorial Earth orbit picks up the payload (3) and tosses it (4) into a lunar transfer trajectory. When it nears the Moon, (5), a Lunavator Tether (6) captures it and delivers it to the lunar surface."

At the Moon end of the transport system, the tether can be stationary relative to the Moon's surface when closest to the Moon. There are many complications to the idea - that's just the outline of how it works. For details see his Cislunar tether transport system.

His summary reads:

"We have developed a preliminary design for a 80 km long Earth-orbit tether boost facility capable of picking payloads up from LEO and injecting them into a minimal-energy lunar transfer orbit. Using currently available tether materials, this facility would require a mass 10.5 times the mass of the payloads it can handle. After boosting a payload, the facility can use electrodynamic propulsion to reboost its orbit, enabling the system to repeatedly send payloads to the Moon without requiring propellant or return traffic. When the payload reaches the Moon, it will be caught and transferred to the surface by a 200 km long lunar tether. This tether facility will have the capability to reposition a significant portion of its “ballast” mass along the length of the tether, enabling it to catch the payload from a low-energy transfer trajectory and then “spin-up” so that it can deliver the payload to the Moon with zero velocity relative to the surface. This lunar tether facility would require a total mass of less than 17 times the payload mass. Both equatorial and polar lunar orbits are feasible for the Lunavator™"

The interesting thing about this is that it needs almost no fuel!

"By balancing the flow of mass to and from the Moon, the orbital momentum and energy of the system can be conserved, eliminating the need to expend large quantities of propellant to move the payloads back and forth"

That works because the LEO is lower in the gravitational well. So whenever you send material from the Moon to Earth, the system actually gains energy, a bit like a ball rolling down a hill.

One of the strong points in this design is the low mass to payload ratio of 10.5 to 1 for the tether in LEO and 17 to 1 for the tether in lunar orbit.

Agreed, if you want to go to Mars and it's your only destination you are interested in for humans outside of Earth, and for a short visit only, then perhaps it wouldn't make sense to set up all that infrastructure to mine lunar water, or lunavators, or lunar elevators or to build mass drivers, just for a single mission to Mars with humans, or even several missions perhaps.

But if you are visiting the Moon because it is interesting in its own right, then the fuel on the Moon could then easily become a key towards opening the rest of the solar system to easier spaceflight, as well as supplying fuel and water to LEO as well. And the other ideas may be useful also, especially if it turns out that the Moon is a useful source for water for space. Ideas like the lunavator might even make it economical to return materials from the Moon such as nickel, cobalt, etc., quite low value, but in large quantities.

BALLUTES - RETURN OF HIGH VALUE RESOURCES SUCH AS PLATINUM TO EARTH

Hoyt's cislunar tether or the propellants made on the Moon can return materials to LEO. That's great for materials that need to be supplied to LEO such as propellant and water, but what about exports to Earth itself?

To return materials to Earth, you have to target the Earth's atmosphere and to hit it slowly enough so that the materials you mine don't burn up in the atmosphere. You might do aerobraking first to reduce speed, and then over a period of time, skimming the atmosphere, lower the orbit to make the landing gentler.

When you are ready, then actually landing the materials on Earth is relatively easy. To make sure there is no risk of damage to Earth, if the parachute or aeroshell fails to deploy, make sure you send the materials in small amounts, small enough to burn up in the atmosphere in its entirety without the aeroshell - and equip each one with an aeroshell and parachute.

Conventional aeroshells are likely to be too complex to create on-site at least at the early stages of development, and too heavy and so too costly to export from Earth. But there's an alternative here, the ballute, an inflatable balloon that works like an aeroshell.

See the New Scientist article Inflatable cushions to act as spacecraft heat shields, and this article Profitably Exploiting Near-Earth Object Resources

It's lightweight, so you could make them on Earth and then send them up to the facility for returning to Earth.

Space mining would probably start with mining of water for use in LEO and other spacecraft missions - because of the high cost of supply of materials to orbit, making it far easier for the mining to turn a profit and pay for itself. But later, funded by the sale of water to space agencies, it could then move on to mining metals and other resources useful for Earth itself.

COMPARISON OF EXPORTS FROM THE MOON WITH ASTEROID MINING AND SUPPLY FROM EARTH

For those who worry that lunar exports might deplete the Moon of its volatiles, then mining the lunar volatiles for fuel is only needed for perhaps a few decades, or until we find an easy way to access other sources in the asteroid belt or find easy ways to lift water from Earth. If we get easy transport to orbit at dollars per ton (e.g. orbital airships, space elevators, or Earth based mass drivers), then the Earth would take over from the Moon and asteroids at that point for any materials abundant on Earth such as water, or air.

I think there's a good chance that eventually it will be as easy to get into space as to fly to another continent. If it costs a similar amount to transport of materials to space, or across the Atlantic, then it would surely be economical to send Earth water and air into orbit and would compete with sources in space. I think there's a good chance this will happen long term, perhaps on timescales of decades, and very likely on the centuries timescale. And of course there's no risk of Earth running out of water or air as a result of exporting the comparatively tiny quantities that would be needed in space. See my Projects To Get To Space As Easily As We Cross Oceans - A Billion Flights A Year Perhaps - Will We Be Ready?

Meanwhile, if there are hundreds of millions of tons of lunar volatiles, they should last until then, and will be a useful source of income for lunar development. That's enough water so that even if you had a million people on the Moon, you could have a full lane of an Olympic swimming pool's worth of water for each one from the local resources, and only a tiny amount of that would be exported for fuel.

Less delta v is needed to get water and other materials from Near Earth Asteroids to Earth, some have a delta v of less than 1 km / second. Though the ones that have the least delta v are the ones with orbital period closest to Earth which means that the times when they are most easily accessible from Earth come around less often, and close flybys might be a decade or more apart.

The Moon scores here in the early days because it is always easy to get to. Then, if the lunavator idea worked for the Moon, then you could get materials back to Earth with almost no extra fuel needed, so that could make the Moon even more attractive than an asteroid.

There are similar ideas for asteroids also, this time the idea to harness the spin of an asteroid to send materials to Earth using a tether that is attached to it and spins at the same rate as the asteroid itself. For the basic idea, see Asteroid Slingshot Express - Tether-based Sample Return.

In the long term future, perhaps the Moon would be most useful for materials that are rare in most asteroids but common on the Moon, such as Aluminium and Titanium. Both lunar and asteroid mining has the potential to move some of our industry into space, and help reduce the impact of mining on the Earth.

MOON SCIENCE SURPRISES

This contains material from the section Moon Science Surprises from Case for Moon First.

We are at the same stage here as the very first Antarctica explorers, setting foot on a continent sized land mass that we know little about first hand. Indeed, far larger than Antarctica; the Moon is as large as Russia, the USA and China put together.

RECORD KEEPER OF INNER SOLAR SYSTEM (INCLUDING ASTROBIOLOGY AND EARLY LIFE ON EARTH)

The ice at the poles of the Moon could be the "Record keepers of the early solar system" as Greg Delory put it.Then there is more ice offset from the north and south pole. As mentioned already, this may be ancient deposits from over three billion years ago before the volcanic activity, which changed the polar axis slightly by shifting material.

Then there are ancient regolith layers covered with lava which preserve a record of surface lunar conditions and solar activity over billions of years.

There are other surfaces on the Moon recently formed with few craters - changed by some activity, perhaps emissions of gas, in the very recent geological past Some think it may still have ice, deep down, or even trapped water, from a layer of water rich material that might have accumulated on the early Moon at the same time our oceans formed soon after the original impact of the Mars sized protoplanet with early Earth - Water on the Moon, Yvonne Pendleton, page 3.

We don't yet know the age of the ancient South Pole - Aitken basin, a huge 2,500 km diameter crater which extends over the far side, the "oldest, deepest and largest basin recognized" on the Moon. Just a few samples of rocks returned from it would establish this.

The Moon might also be of interest for evolution and exobiology. That's because meteorites from Earth must hit the Moon in large numbers, after the largest impacts on Earth such as the dinosaur extinction Chicxulub event, (perhaps a million fragments around a cm in size, in that case). Though there is no atmosphere, the gravity is so low that meteorites from Earth can hit the surface at quite slow speeds (comparatively). This has lead to many ideas in the literature of ways that organics from life on Earth or other sources of interest to biology may be there for us to discover. We can only find out for sure by exploring the Moon on the surface:

• Possibility of an ancient regolith layer buried beneath later lava preserving actual organics from the first few hundred million years of Earth evolution.
  
• Suggestion that the lunar ice at the poles could preserve ancient biogenic organics.
  
• Suggestion that effects of cosmic radiation on the ices and other chemicals at the poles of the Moon may be creating complex organics - not quite life probably but the ingredients for life.
  
• Possibility of meteorites from Earth on the Moon - which again, if they landed in frozen areas of the Moon, may preserve organics from millions or even billions of years. They could also preserve fossils as fossil diatoms are still recognizable, and the smallest ones intact after a simulated impact on the Moon. There may be as much as 200 kilograms of material from Earth per square kilometer of the lunar surface. See also Paul Spudis's Ancient Life on the Moon — From Earth (In Airspace Magazine)
  
• It may also have early rocks from Mars, Venus and small bodies in the early solar system, including possibly biological evidence from those objects also . See section 3.1.1 of Back to the Moon: The Scientific Rationale for Resuming Lunar Surface Exploration.
  
• It may also record the composition of the solar wind, galactic environment, and volatiles from comets in the early solar system. See section 3.1.1 again
  
• Must have organics from meteorite and comet impacts at least and probably from the solar wind too which brings not just hydrogen but also heavier elements to the Moon.

When you consider the meteorites and the regolith, the Moon must be a treasure trove of proto Earth secrets, and of solar system secrets too. It would start with ejecta from the early Earth as it cooled down after the impact that formed the Moon, Venus too if its atmosphere back then was thin enough for an asteroid impact to eject material with escape velocity, and Mars for sure. Also material from comets, and asteroids, nearly every major thing of note that happened in the inner solar system probably left a record there. The record should also include ejecta from every major impact on Earth, right up to perhaps fragments of ammonites from the Chicxulub impact which must have showered the Moon with large numbers of meteorites.

The Apollo samples were recently re-analysed and the composition of amino acids suggests some extraterrestrial sources, The analysis was a tricky one due to contaminants from Earth in the form of rocket fuel, organics taken to the Moon by the astronauts, and organics introduced while handling them on Earth, which suggests we need to take care to avoid this sort of thing as we explore the Moon.

So, another thing we can do on the Moon is to find out how much organic contamination accompanies human explorations. E.g. before we send humans to Phobos or Deimos or wherever they go next, we'd better know what humans will do to a celestial body when they set up a settlement there. Especially if the aim is to study ice deposits in craters and such like. Organic Measurements on the Lunar Surface: Planned and Unplanned Experiments

IDEAL LOCATION FOR RADIO AND INFRARED TELESCOPES TO OBSERVE SPACE

The far side is the best place to build radio telescopes anywhere near to Earth.

• It is radio dark, shielded from Earth by the Moon. Even in the most remote places on Earth you don't get away from radio interference. Radio astronomers have particular wavelengths that are supposed to be allocated to them. The rest of the spectrum is crowded with signals
• Long wave telescopes could be built simply by rolling out lines of cable across the surface
• Let's you observe the unexplored frequencies below 100 MHz - these are blocked by the ionosphere on Earth. This would open up the last unexplored frequency regime in our radio observations of the universe. They would use a long baseline interferometry to simulate a larger telescope.
• Later on, we could build vast Arecibo dish type radio telescopes in natural craters on the Moon. Frank Drake once calculated that it should be possible to create Arecibo type telescopes with a diameter of 30 km or more on the Moon (see abstract on page 91) - Arecibo is 305 meters in diameter.

There are challenges as well of course. If observing during the lunar night (so least interference from the Sun) then it's both cold and without solar power. Either you use RTGs or power storage for the lunar night.

The Moon is also good for optical telescopes. See Lunar Telescope Facility Final Design Report (MIT, 2007)

• Far side is optically dark for much of the lunar month, with no stray light entering the telescope either from the Earth or the Sun
• Stable base for large base line interferometry. Easier to keep widely separated telescopes at the same orientation to each other than in free space because the ground is stable. It is possible to do this with free flying optical telescopes but easier on the Moon.
• No fuel for station keeping to keep the telescopes in position. The moon does that for you.
• Great for infrared passively cooled telescopes in the craters of eternal night near the poles, for instance using liquid mirrors

Artist's impression of a large liquid mirror infrared telescope on the Moon, passively cooled near the lunar poles. By a useful coincidence, a slowly spinning liquid in a gravitational field naturally forms an optically perfect parabolic shaped mirror. Because there is no atmosphere, the optics would be better than for a liquid mirror telescope on Earth, indeed it would be diffraction limited (i.e. the best that is theoretically possible).

These telescopes are so simple to build and lightweight that it should be feasible to build a 4-8 meter liquid mirror telescope soon after a lunar base is established, and larger mirrors later on, with 100 meter telescopes well within reach. Other advantages for the Moon include low shipping mass, ease of assembly, and low maintenance.

A lunar infrared liquid mirror telescope would have a mirror like this

which keeps its natural perfect parabolic shape through rotating the disk slowly. The photo shows the six meter diameter Large Zenith Telescope in Canada. On the Moon, this mirror would be made of ionic fluids (salts in a liquid state) rather than mercury, due to their very low vapour pressure which would let them work in the vacuum conditions of the Moon.

If the telescope is anywhere except at the poles, it will scan a circular strip around the sky. Also because the Moon's axis wobbles with a period of 18.6 years, it actually sees a fair bit more than that circular strip. Most of the lunar libration we see from Earth is due to changes in viewing geometry because of the Moons irregular orbit, it's slant relative to the Earth's equator, the varying distance from the Moon etc.

(Click to watch on YouTube)

Shows how the Moon librates as seen from the Earth. Most of this is due to changes in the viewing geometry as seen from Earth.

However, there is also a small variation due to actual motion of the Moon's axis. The Moon's axis precesses, and traces out a circle in the sky of radius 1.5 degrees every 18.6 years. That's quite a lot in astronomy - the Moon as seen from Earth spans only half a degree, so if we can use this unit of the Moon's apparent diameter as seen from Earth- we can see three Moon's worth above of sky above and below the track of the telescope depending on the position in that 18.6 year cycle.

(Earth does have a subtle effect also - nutation - it's precession period is 26,000 years but the the position of its axis varies by an extra half a degree over that same 18.6 year time period as the Moon, enough to be useful for liquid telescopes here as well.)

That's also why the peaks of sunlight at the poles have periods of darkness, but only for a few days of the year. This is actually an advantage for polar infrared telescopes on the Moon. For more details, see Liquid Mirror Telescopes on the Moon (NASA news, 2008). A liquid mirror infrared telescope on the Moon 20 meters in diameter could detect objects a hundred times fainter than the ones visible to the planned James Web Space Telescope. All the materials for a 20 meter telescope would amount to only a few tons, a single load on a heavy boost rocket. A hundred meter diameter telescope could detect objects a thousand times fainter than for the James Webb. Though not steerable, this would be especially useful for detecting faint red shift galaxies from the early universe, and supernovae from these early times, .

The dust can be dealt with, for instance by making a region around the telescope into glass, which is easy to do on the Moon as use of a microwave turns the regolith into glass as easily as you boil a kettle. Unlike Mars, there are no dust storms to deal with. Just electrostatic elevation of the dust. See Lunar Glass.

GEOLOGICALLY ACTIVE MOON

Another big surprise recently is that the Moon was recently geologically active, over the timescale of millions rather than billions of years. It might even be active today.

First there are the lobate scarps, fault lines that cut across small craters on the Moon. Small craters tend to be geologically young as they get obliterated by later craters. So these faults are thought to be young, possibly as young as a few hundred million years.

The Apollo seismometers recorded moon quakes, and though most are probably due to impacts, tides, and day / night temperature changes on the Moon, it leaves the possibility open that perhaps the Moon is still active today. These scarps may be a sign of it.

See NASA's LRO Reveals 'Incredible Shrinking Moon'

Then, they also found graben - trenches formed when the crust pulls apart. And these are as young as 50 million years old. That is so young that it suggests the Moon is still active.

Graben on far side of the Moon found by Lunar Reconnaissance Orbiter

This was a big surprise as the lobate scarps suggested that the Moon was shrinking. So how can it be expanding as well?

(click to watch on Youtube)

Then, there's this strange feature on the Moon, unlike most of the terrain there, the Ina depression.

Ina depression as imaged by LRO. It's 2.9×1.9 km and 64 m deep. Higher resolution available here. It is one of four similar features around the Imbrium basin.

First discovered by Apollo 15 in 1971, it's now known to be one of many such. They are small features, less than 500 meters across, and seem to be widespread on the near side of the Moon. Some may be as young as 100 million years old. Some of the younger features may be as young as 50 million years old. This means you can't rule out the possibility of future eruptions.

(click to watch on Youtube)

The LRO team found 70 "Irregular Mare Patches" (IMPs) from 100 meters to 5 km in diameter. To find out more, see Volcanoes erupted on the Moon within the past 100 million years which also has a link to the full Nature article which you can read through their article sharing initiative.

Whatever it is, it seems to be geologically recent, as there are few really small craters, and larger features have sharp edges and haven't been degraded. This suggests an age of only millions, rather than billions of years. It's spectra shows it to be bluer than the surrounding terrain (slightly) - a spectral signature consistent with freshly exposed Mare materials. Schultz et al interpret it as mare exposed by a sudden outgassing from the interior blowing away more than 12 meters thickness of overlying an regolith or pyrolastic material (rubble like in texture, result of fountaining lava in the past).

The Moon's surface layers are not just depleted in water compared to Earth, it's also depleted in the more volatile metals potassium and sodium too (by comparison with the less volatile zinc). One recent idea is that when two protoplanets collided to form the early Earth, and the debris condensed to form the Moon, volatile rich layers may have condensed first, then dry layers accumulated on top of them (links also to the original Nature paper) . That paper is mainly used to explain why the surface layers are so dry. They weren't able to determine if the interior would have volatiles in it, but it's possible.

So, it's not necessarily as dry all the way to the center as it is on the surface. Perhaps it has water and other volatiles deep down. If so, these outgassing ideas may be easier to accept.

Then you have the Transient Lunar Phenomena. Moon observers over the years have often noticed short term brightening of the Moon's surface, especially in the Aristarchus plateau. A small bright patch will appear, then disappear, just a bit too slowly to be just a flash from an impact on the Moon. Our eyes are very good at picking up such things, but also easily fooled. So it's a controversial observation. But they may be the result of outgassing, again, if they are real.

So anyway, Arlin Crotts of Columbia University noticed a correlation of the TLPs with sites where argon and radon gas is detected. This can't come from the solar wind and must be outgassing from below the surface. The usual explanation is that it's the result of slow leaks from radioactive decay from below the surface. He thinks that there may also be explosive outbursts of gas which may lead to the TLPs.

He developed these ideas in a series of papers called "Lunar outgassing, transient phenomena, and the return to the Moon", where he also suggests that the Moon may have ice some meters below the surface replenished from below

• I: Existing data
• II: Predictions and Tests for Outgassing/Regolith Interactions
• III: Observational and Experimental Techniques

In the third paper he proposes using ground penetrating radar in orbit around the Moon to search for subsurface ice. He also suggests various ways to monitor the Moon from orbit searching for outflowing gas as well as attempting to observe the TLPs directly.

LUNAR SWIRLS

Then there's this curious phenomenon, the Lunar Swirls. The most prominent of these is Reiner Gamma which is visible in a backyard telescope, but becomes much more impressive with high magnifications.

Lunar swirls in Reiner Gamma. The swirls are always associated with magnetic anomalies (a discovery made serendipitously in 1972 using two small satellites released into lunar orbit by Apollo 14 and Apollo 15 to study the Moon's magnetotail), and this is one of the strongest magnetic anomalies on the Moon. Though not all magnetic anomalies have swirls.

More swirls at Mare Marginis on the lunar far side.

The light coloured swirls in these photographs are not associated with any geological feature but seem to be laid on top of the surface, and they don't seem to have any noticeable thickness or have any effect on the topography.

• Perhaps the magnetic field deflects the solar wind away so keeping these parts of the surface lighter coloured.
• Or perhaps they are a result of plasma from a comet impact removing part of the surface regolith, and the magnetic field may be a frozen remnant of the magnetic field of the comet itself, recorded into the iron it melted during the impact.
• Or perhaps it's made from the electrostatically levitated dust you get as the terminator line between the dark and light side of the Moon passes over its surface. Perhaps this dust accumulates preferentially in the lunar swirls. The finest lunar dust is bright, which makes this hypothesis plausible. Recent research on the electrostatic dust levitation.

Whatever they are, they are intriguing and who doesn't love a mystery? Some of the magnetic anomalies on the Moon might also be tracers of iron nickel / platinum asteroid impacts, if so do the swirls tell us anything about them? Or are they something else, nothing to do with any metal asteroid impacts (e.g. for the comet hypothesis)?

EXPECT THE UNEXPECTED

On top of that you also have the unexpected. What I've described so far are things we can expect, or know to search for and investigate, on basis of what we know so far. But usually we get surprises when we explore new places in the solar system. And though in some ways the Moon is well understood, in other ways it is barely explored at all on the surface, never mind below the surface. It wasn't that long ago that ice on the Moon was a big surprise to many astronomers. It may have many other surprises in store too. We have spent hardly any time exploring the Moon so far, with only one expedition with a geologist on it. Imagine if we had given up on Antarctica as "done" after the first few expeditions that succeeded in landing a human on the continent?

IS THERE A FORTUNE TO BE MADE ON MARS, THE MOON, OR ANYWHERE ELSE IN SPACE?

This is material from the sections in Case for Moon First from Commercial value for Mars onwards. It was also published in Forbes Magazine as "Is There A Fortune To Be Made On Mars?".

COMMERCIAL VALUE FOR MARS

The debate about going back to the Moon focuses on the commercial value of the Moon, for exports to LEO, to Earth or to other locations in outer space, see The Moon is resource rich. But what about Mars, does it have any commercial value for exports to Earth or anywhere else? This is relevant whether or not we send humans to the Mars surface, so let's forget about planetary protection issues for this section. If there is anything of great commercial value on Mars then either

• You mine it telerobotically or robotically, humans remain in orbit around Mars - nobody visits the surface, Instead, they live in orbital colonies and control robot-like avatars on the surface. (For more on this, see my To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos)
• Or you mine it with humans on the surface

Elon Musk has said several times that he doesn't think there will be anything material from Mars that would be worth transporting back to Earth.

"I don't think it's going to be economical to mine things on Mars and then transport them back to Earth because the transport costs would overwhelm the value of whatever you mined, but there will likely be a lot of mining on Mars that's useful for a Mars base, but it's unlikely to be transferred back to Earth. I think the economic exchange between a Mars base and Earth would be mostly in the form of intellectual property"

Elon Musk interview on the future of energy and transport - and more quotes like this from him.

Robert Zubrin covers this in more detail:

"Another alternative is that Mars could pay for itself by transporting back ideas. Just as the labor shortage prevalent in colonial and 19th century America drove the creation of Yankee Ingenuity's flood of inventions, so the conditions of extreme labor shortage combined with a technological culture and the unacceptability of impractical legislative constraints against innovation will tend to drive Martian ingenuity to produce wave after wave of invention in energy production, automation and robotics, biotechnology, and other areas. These inventions, licensed on Earth, could finance Mars even as they revolutionize and advance terrestrial living standards as forcefully as 19th Century American invention changed Europe and ultimately the rest of the world as well."

Elon Musk is skeptical about space mining generally thinking it probably won't be possible to export from the asteroids -

"I'm not convinced there's a case for taking something, say, platinum, that is found in an asteroid and bringing it back to Earth."

Of course many think that this will be possible. Myself I just don't know, I've heard the arguments on both sides and remain on the fence here.

Anyway Elon Musk doesn't go into any more detail about the case for or against material exports. Robert Zubrin however has discussed this in a paper "The Economic Viability of Mars Colonization " in the Journal of the British Interplanetary Society from 1995, and later on in the Interplanetary Commerce section of Case for Mars. He first outlines the need for exports to make a Mars colony viable:

"A frequent objection raised against scenarios for the human settlement and terraforming of Mars is that while such projects may be technologically feasible, there is no possible way that they can be paid for. On the surface, the arguments given supporting this position appear to many to be cogent, in that Mars is distant, difficult to access, possesses a hostile environment and has no apparent resources of economic value to export. These arguments appear to be ironclad, yet it must be pointed out that they were also presented in the past as convincing reasons for the utter impracticality of the European settlement of North America and Australia."

..."While the Exploration and Base building phases can and probably must be carried out on the basis of outright government funding, during the Settlement phase economics comes to the fore. That is, while a Mars base of even a few hundred people can potentially be supported out of pocket by governmental expenditures, a Martian society of hundreds of thousands clearly cannot be. To be viable, a real Martian civilization must be either completely autarchic (very unlikely until the far future) or be able to produce some kind of export that allows it to pay for the imports it requires."

..."Mars is the best target for colonization in the solar system because it has by far the greatest potential for self-sufficiency. Nevertheless, even with optimistic extrapolation of robotic manufacturing techniques, Mars will not have the division of labor required to make it fully self-sufficient until its population numbers in the millions. It will thus for a long time be necessary, and forever desirable, for Mars to be able to pay for import of specialized manufactured goods from Earth. These goods can be fairly limited in mass, as only small portions (by weight) of even very high-tech goods are actually complex. Nevertheless, these smaller sophisticated items will have to be paid for, and their cost will be greatly increased by the high costs of Earth-launch and interplanetary transport. What can Mars possibly export back to Earth in return?"
(emphasis mine)

So according to his ideas, the Mars colony is supported on the basis of outright government funding for the early stages of exploration and base building. He thinks that in these early stages you need something over and above ISRU (In Situ Resource Uitilization) for a commercial case unless the base is autarchic - a word which usually refers to individual liberty and governing oneself - but in this context I think he must mean, producing everything it needs, independent of Earth.

So it is rather similar to Elon Musk's idea except that in Elon Musk’s vision, the settlement is supported by private funding from Earth in the early stages rather than government funding.

Zubrin then discusses the possibility of ores on Mars, and we'll come back to this later in this section:

..."Mars may have concentrated mineral ores, with much greater concentrations of ores of precious metals readily available than is currently the case on Earth due to the fact that the terrestrial ores have been heavily scavenged by humans for the past 5000 years. It has been shown that if concentrated supplies of metals of equal or greater value than silver (i.e. silver, germanium, hafnium, lanthanum, cerium, rhenium, samarium, gallium, gadolinium, gold, palladium, iridium, rubidium, platinum, rhodium, europium, etc.) were available on Mars, they could potentially be transported back to Earth at high profit by using reusable Mars-surface based single stage to orbit vehicles to deliver the cargoes to Mars orbit, and then transporting them back to Earth using either cheap expendable chemical stages manufactured on Mars or reusable cycling solar sail powered interplanetary spacecraft. The existence of such Martian precious metal ores, however, is still hypothetical."

In his section on Interplanetary Commerce in “Case for Mars” page 239 and following he also suggests deuterium as an export. I'll look at that below, between the section on geological products and the section on fuel exports.

He then goes on to suggest that Mars may play a crucial role for supply of ores and other exports to the asteroid belt once we have humans living there. He suggests Phobos and Deimos may also be valuable as a staging post on the way to the asteroid belt. Which may be true, but that's a rather later stage. I'm interested here in the earlier stages before we have large numbers of humans in the asteroid belt.

Do correct me if anyone knows of any other papers with detailed discussions of possible exports from Mars. That's all I've been able to find so far.

However it is discussed a fair bit online in places like Reddit, and the various Mars forums and spaceflight forums, and enthusiasts have suggested many other ways that they think a Mars colony could become profitable. So what I present here is based on that, as well as some thoughts of my own.

So, let's look at this a bit more closely, is there anything physical that could be worth exporting, (apart from the science value of the search for life and the information returned). Also is there anything worth exporting at a reasonably early stage such as the first few decades of a human exploration of Mars either on the surface or telerobotically from orbit?

• Samples of Mars dust and rock. The first samples could be worth billions of dollars per kilogram to start with, at least that's how much they plan to spend for Curiosity's successor's sample return

However, the price would go down quickly as we get more samples from Mars of the order of tons of material. You'd only return as much as was needed for the scientific research you need to do due to the high price of return of material from Mars.

Also, individuals might want to buy Mars rocks at high prices, but only for as long as they are rare. This would be like supporting a lunar mission by returning and selling Moon rocks. The first few rocks could be valuable to collectors, and if they were issued with a certificate of authenticity as the first rocks to be returned from Mars or the Moon maybe the first few rocks would retain their value. But longer term, how many people would want to buy into something of continually reducing value?

• There are also the products of past life. On the surface - all of that has been pretty much completely destroyed by cosmic radiation. But - could there be deposits set down by ancient life below the surface, like our oil and gas and oil shale deposits.

You'd think they must be rare or we would have spotted them on the surface. There's no sign at all of outcrops of oil shale. But on the other hand - cosmic radiation is very damaging. Would there be anything left of a surface oil shale deposit after billions of years?

It's an exponential process so you get very rapid reductions. Every 650 million years you get a 1000 fold reduction in the concentrations of small organic molecules such as amino acids on the surface because of cosmic radiation. So that's a million fold reduction every 1.3 billion years.

Cosmic radiation has little effect over time periods of years, decades, centuries or millennia. But over time periods of hundreds of millions of years the effects are huge. After 1.3 billion years, a thousand tons of amino acids gets reduced to a kilogram, with the rest converted mainly to gases like carbon dioxide, water vapour, methane and ammonia. After 2.6 billion years it's down to a microgram (millionth of a gram) and after 3.9 billion years you are down to less than a picogram (a millionth of a microgram) of your original thousand tons deposit.

So, I don't think absence of these deposits on the surface, at least ones easy to see from satellites, really shows that they don't exist below the surface. There could be millions of tons of organics from past life ten meters below the surface, and our rovers so far would probably not spot a thing. The organics of course also have to be there in the first place (surely likely to be patchy, in some places more than in others) and buried quickly - if it took several hundred million years to bury them, much of the organics would be gone also.

Oil itself is surely not worth the trouble of mining to return to Earth. But if there was some unique biological product on Mars that we don't have on Earth - which you could mine to find there, maybe that could be worth returning to Earth.

• Ordinary Earth life on Mars, e.g. vegetables, fruit, decorative flowers or whatever. Perhaps Mars could be a "garden planet" to export food to orbit and space colonies.

Mars could potentially be competitive with Earth for export of food for use on spacecraft and other space colonies due to the much lower launch cost, provided that the costs of growing the crops on Mars are also comparable to Earth's (quite a big if in the early stages).

But what about greenhouses in space? It would also have to compete with those. This would require it to be much easier to build a greenhouse on the surface than in space. Since it's a near vacuum and also has such huge diurnal swings in temperature, I'm not sure that it has much by way of advantages over, say, Phobos or Deimos, or indeed the Moon which has much less delta v than Mars. Even for export to Mars orbit, it could be as economical or more so to export from the Moon for foodstuffs that can keep for months long transport journeys. See Greenhouse construction - comparison of the Moon and Mars in Case for Moon First.

It could be more economical to export from Mars to Mars orbit rather than from the Moon perhaps for food that can spoil quickly, though this is not a net export from the Mars system. Another thought, if the natural Mars gravity was an advantage, and for some reason, easier to use than artificial gravity, perhaps it could be worthwhile.

It could also be worth doing if conditions on Mars let you produce unusual food or decorative plants more easily. As an example, it could be worth doing, if you can grow rare flowers on Mars that are very expensive to grow elsewhere, or similarly unusual and tasty rare new food stuffs that grow best on Mars for some reason, perhaps genetically designed for Mars conditions. This is related to the next topic:

Continuing to

• Products of present day life. If Mars has interestingly different biology, maybe RNA based, maybe XNA, or not a DNA type chemical basis at all - you might find it worthwhile to grow Mars micro-organisms in greenhouses or special habitats on Mars designed to make conditions conducive for them. Then these might make products useful for Earth.
• Or genetically engineered biology that grows best in Mars conditions for some reason (actually responds well to the near vacuum, and extreme swings of temperature for instance)

Products you could export could include

• Medicines, if Mars life produces products of value for human health
• Spices and special foods - if the extraterrestrial biology is especially tasty and is safe to eat but can't be grown on Earth.
• Chemicals, e.g. if Mars life consists of XNA and the XNA is valuable, you could make large quantities on Mars to export to Earth. Similarly for any proteins, enzymes or other chemicals that are easier to make on Mars.
• Nano structures - makes products that are unusual and useful on the nanoscale.

For this to work there must be some reason they can't be grown on Earth

• Needs Mars conditions of near vacuum, huge temperature differences from day to night, high UV levels, or cosmic radiation and solar storms, and it's easier to grow on Mars than simulate Mars conditions on Earth. (I don't know why life might need solar storms or high levels of UV radiation but just added that for completeness, given that it may be a totally alien exobiology and we don't know what it can do or how it works in detail - those are sources of energy that could potentially be used by exobiology in theory).
• Can't be grown on Earth at all for safety reasons - e.g. photosynthetic life that's more efficient than any Earth based photosynthetic life, or perhaps it depends on symbiotic microbes that would be harmful to the environment of Earth if returned here. Perhaps what we return from Mars is the product of such microbes rather than the microbes themselves.
  
  E.g. if it is based on XNA then it might not be safe to set up an XNA based ecosystem on the Earth to grow these products, because of the risk of escape and competition with DNA based ecosystems, and if they are also very valuable that could be a reason for growing them on Mars.
  
• Can grow on Earth but easier to grow on Mars.

This case might also be another reason to be really careful not to contaminate Mars with Earth life, so that you can continue to grow the native Mars life there without interference from Earth life to make unique products that can only be produced easily from the native Mars life.

However, even if you can't grow the products safely on Earth, at some point you'd have the capability to grow them in Stanford Torus type habitats, biologically isolated from Earth and designed to mimic Mars conditions. Still, by the time that's feasible, export costs from Mars could go down at the same time that prices of such habitats go down, so keeping Mars competitive with them.

This does seem a potential early export that may continue to be commercially viable for quite some time, maybe even indefinitely. But it depends entirely on what we find as we search for life on Mars and also on how easy or safe it is to grow them on Earth, the Moon or elsewhere.

• Geological deposits With the dry ice, low atmospheric pressure, cosmic radiation, things will be different from Earth in some respects. For one clear example, its salt deposits are made up of sulfates and perchlorates rather than chlorides as on Earth. Not that those are worth returning, but could it have other more valuable deposits that we don't have on Earth, or rarely find here? Which leads to the next idea, could it have unique rare gemstones for instance?
• Opals from Mars. In 2013, the Mars Reconnaissance orbiter found evidence of large deposits of opals (hydrated silica) on Mars. Now most of that won't be gemstones, just deposits of silica modified by water. But could it have valuable gemstones there? Might the opals have markings unique to the way they formed on Mars? This orbital discovery was backed up by discovery of trace amounts of opal in a Mars meteorite in 2015.

Raw opal found in Andamooka South Australia - photo credit CR Peters

Mars is different from asteroids or the Moon here, so it could have unique deposits. It's the only place we know of with deposits formed in ancient seas billions of years ago and its past and present climate is unique too. It might have unique minerals of decorative value.

What about:

• Gold from Mars (or substitute platinum, or titanium, or whatever you think is especially valuable that you could find on Mars). For gold, perhaps the geological processes on Mars involving water in its past have concentrated deposits of precious metals just as they do on Earth? It's also surely had many iron / nickel asteroids hit the planet so may have deposits of platinum, gold etc. for similar reasoning to Dennis Wingo's reasoning for the Moon. Indeed more so, because it is closer to the asteroid belt so gets hit by them more often.
  
  Robert Zubrin's list here includes silver, germanium, hafnium, lanthanum, cerium, rhenium, samarium, gallium, gadolinium, gold, palladium, iridium, rubidium, platinum, rhodium, europium, etc.

Remember, that

• You have to do all the work to actually run the gold mine on Mars, and then send the gold into orbit.
• The price of gold is going to go down as it becomes available from space - or else the amount you can sell to Earth gets regulated to keep prices artificially high.
• If it is viable from Mars, it’s likely to be viable from other places too, particularly, robotic mining of asteroids may undercut you, and then you’d get less for the price of your gold than you spent on mining it, if robot mining of asteroids costs less.
• There may be much easier of access sources of platinum, gold etc. from the Moon if Dennis Wingo is right. If you can get the transport costs from the Moon down to almost zero by using Hoyt's cislunar transport system or similar (see Exporting materials from the Moon), it would be very hard to compete with that from Mars.
• If the mines are operated by humans on Mars, you have to pay for all the supplies to the miners on Mars which could amount to trillions of dollars a year, before you can turn a profit. For telerobotic or robotic mining you have to pay for the telerobot replacement, maintenance and repair and you also have to pay for all the equipment needed, drilling machinery etc. Will humans be able to compete with telerobots or robots operated from Earth with maybe just a small number of humans in situ?

So, in short, it has to be competitive with platinum, gold etc. mined elsewhere in the solar system, and you have to bear in mind that the prices you can get from Earth will surely go down, or else your exports are limited to keep the prices artificially high. On the other hand if the material you are mining is very valuable, and launch costs are low, perhaps the margin due to cost of export from Mars doesn't make such a big difference. E.g. suppose the launch costs a few hundred million dollars but you are returning tons of material, worth billions of dollars, perhaps it doesn't matter so much that a few percent of your product's price is due to transport. Maybe other elements of the price such as mining are somewhat less expensive than they are for asteroids?

However for this to work, there has to be a reason why other elements of the cost of mining are low. Asteroids and the Moon have the advantages of:

• Iron rich asteroids consist of pure metal, not oxidized.
• There may be easy robotic ways to extract it e.g. using gas carbonyls, no need for drilling as this turns the metal directly into gas
• Much lower delta v requirements than Mars for some of the NEOs, and in case of the Moon could even be zero delta v with Hoyt's cislunar tether transport system.

It seems unlikely that the thin Mars atmosphere would help much with mining operations. Would the Mars gravity help, or be a hindrance? And the large temperature swings from day to night, could they help in any way to make it easier to mine materials?

• Getting colonists to pay using their fee for the passage out to Mars and to subsidize exports by using the nearly empty vessels for the journey back.

Just to make this clear, this is not Elon Musk's idea. As we saw, he thinks the colony would pay for itself in the early stages mainly through sale of intellectual property rights to the Earth. And Robert Zubrin, as we saw, thinks it will be paid for in early stages through government funding. But it's a topic that gets discussed in the online forums. So let's have a look at it.

If you get colonists who pay in advance for their flight out to Mars - and they use the Mars Colonial Transporter - a 100 people at a time, if SpaceX succeed in producing that spaceship - then the spacecraft has to come back to Earth after every run to transport colonists to Mars, and would be able to take exports with it, which is essentially free transport. So there would be a multiplier effect there of the original passage fee.

However unless the products are already worth returning for one of the other reasons, then at most they could get back their original passage fee by selling the material. Otherwise you'd have a case for sending empty colonial transporter ships to Mars just to return the products.

So, you'd get exports, yes, for as long as the colony continues to expand rapidly. However, that's not a business case in the long term, as it's not going to be sustainable, as a way of supporting a colony there. Even if they can get their money for the flight back from the goods returned from Mars, they then have to support themselves on Mars indefinitely, not just pay for the flight out. And with increasing numbers of colonists on Mars, you'd need exponentially increasing numbers of colonists going out there to support them with the passage fees. If you get increasing numbers of spaceships sent there to send them their supplies, again you need to pay for that somehow.

So, I don't think relying on the nearly empty transporter as it returns to Earth as a way to support the colony is likely to work long term. It works only as long as you have exponentially increasing numbers of colonists going to Mars and nobody coming back or few people coming back.

• Paid for by retirees to Mars. This is another idea sometimes mentioned online - could a small colony be paid for by retirees? Perhaps some people might be willing to pay large amounts to “retire to Mars” and older people nearing the end of their lives are more likely to be able to afford such a trip using their life’s savings.

After the initial romance of being “the first settlers on Mars” is over, would there be such huge demand to retire to Mars with not so much by way of home comforts as Earth and far away from their friends, relatives and children? Many older people take a lot of interest in their younger relatives and might not want to retire to Mars on their own and leave them behind on Earth.

Also, presumably the idea is that they can pay for the colony because they won't live long when they get there. But it doesn't work like that. If you have survived, say, to 65, your total life expectancy is much higher than for someone who has only survived to 20, because everyone who died before 65 are removed from the population at that point. Quote from here:

• "A man reaching age 65 today can expect to live, on average, until age 84.3.
• "A woman turning age 65 today can expect to live, on average, until age 86.6."

That's an average, so many of the population will live more than 20 years after they reach their 65th birthday.

It continues like that. The life expectancy of an 85 year old woman isn't 1.6 years, it's 7 years. T he life expectancy of a 90 year old woman, if they live that long, is still 5 years. Then on top of that, they have to be healthy enough to get to Mars, so it would be biased towards healthier old people. Those figures are from the US social services life expectancy calculator.

Even if you only accept 90 year olds, you'll want super fit 90 year olds which mean they will probably live for more than five years, maybe a decade or more, especially with improvements in medicine.

So, we have to expect retired people to live for decades after they retire. They need more hospital care, and nursing than the general population. They may also get Alzheimer's, and are less strong, on average (some may be super fit of course, for their age, but still they are not quite as strong as a 20 year old, athletes "retire" at a young age). How could that work?

I can’t see the retirees migrating from Earth paying for their own requirements for the rest of their life. And if it is a mix of older and younger people, even more so, I can't see the retirees paying for the requirements of all the younger people for the rest of their life.

This again becomes a case where you would need exponentially increasing numbers of immigrants needed to pay for the colony, and exponential growth can’t continue for long. Of course retirees would have a place to play in a colony that can support itself financially, as in any society. Just saying that the one off sum of money that retirees pay to go to a space colony can't be used as an income to support it. It's just the same as for younger folk, they need some way to continue to support themselves once there. So we need to keep looking.

• Export of deuterium from Mars. In “Case for Mars” page 239 and following Robert Zubrin suggests deuterium as an export.

This is one of the main points in the International Commerce section in Case for Mars, and is also often mentioned in discussions, so I should go into it in some detail.

So first, let's look at the data on deuterium abundances in our solar system. Curiosity measured a deuterium to hydrogen ratio five times greater on Mars than in the Earth oceans, probably due to the loss of hydrogen from the upper atmosphere of Mars over billions of years. See Heavy hydrogen excess hints at Martian vapour loss. This is for near surface ice. The Mars meteorite studies also suggest another reservoir of water below the surface with a lower ratio of two to three times that for Earth’s oceans which probably comes from an earlier phase of Mars history. Meteoritic evidence for a previously unrecognized hydrogen reservoir on Mars.

Deuterium occurs naturally on Earth in water as 1 in 6,400 hydrogen atoms or 1 part in 3,200 by weight. On Mars it is one deuterium for every 1,284 hydrogen's. Though Mars has a higher deuterium to hydrogen ratio than Earth, it’s not the most abundant source of it in the solar system. Rather, Earth’s abundance is if anything rather low, compared with many sources although high compared to the concentrations in the Sun and Jupiter and hydrogen from the solar wind. The solar wind hydrogen trapped in the lunar regolith also has a very low deuterium concentration.

Venus has the highest deuterium / hydrogen ratio recorded in our solar system of 120 times Earth’s and so 24 times that on Mars in its atmosphere. Implications of the high DH ratio for the sources of water in Venus' atmosphere.

Most meteorites that hit Earth have close to terrestrial abundances of deuterium but some have very high levels. This meteorite has 13 times the abundance of Earth’s oceans, so more than twice the abundance for Mars (many types of rock contain hydrogen and so you can measure their deuterium concentrations, this is a chondrite meteorite ).

Antarctic Meteorite Lab Photo of Sample WSG 95300 - details about it here - the deuterium measurements for this meteorite are here: Deuterium enrichments in chondritic macromolecular material—Implications for the origin and evolution of organics, water and asteroids

(see table 2, the δD there is measured in parts per thousand relative to terrestrial abundances, so for instance δD +1000 for double terrestrial values)

Jupiter family comets have higher deuterium abundances than Earth, perhaps around three times terrestrial abundances as for Comet 67p from the Rosetta mission, though there is some question here about whether comet outgassing may somehow concentrate the deuterium and lead to over estimates of the abundances.

So is Mars the best extraterrestrial source for deuterium? And is it worth importing from space at all?

Currently the main use of deuterium is as a moderator in a nuclear reactor. You have the choice of enriching the uranium, and using ordinary water, which is the method used currently in many reactors, or of using ordinary unenriched uranium and heavy water, as is used in heavy water reactors such as the ones developed by India. That works because heavy water slows down neutrons without capturing them so permitting a chain reaction with a lower concentration of radioactive Uranium than light water which captures many of the neutrons.

However his quoted price of $10,000 per kilogram for deuterium seems a bit high. You can get 99.96% pure deuterium oxide for $1,000 per kg from Cambridge Isotopes. (Deuterium Oxide 100%) You can get 99% pure deuterium oxide for $721 per kg (Deuterium oxide 99%) Unless he’s referring to the price for pure deuterium separated from the oxygen?

99% pure deuterium oxide is sufficiently pure for the production of plutonium from uranium. Because of this application, the technology to produce heavy water is tightly regulated and the deuterium produced in a plant is tracked carefully. (For an example of how this is done, see "Selection of a safeguards approach for the Arroyito heavy water production plant" )

He says that the price of deuterium would go up if we develop deuterium / tritium fusion. I don’t really see that, since the main cost comes from extraction and there is no shortage of water to extract it from. Would a higher demand not just lead to us building more deuterium extraction plants, and a search for methods to reduce the costs using larger scale production facilities, for economies of scale, and other methods of generating it, which would reduce the price rather than increase it?

And what if some other form of fusion power turns out to be more efficient or have advantages over deuterium / tritium fusion? It’s a bit tricky arguing based on a technology we don’t have yet, and there are many possible ways of generating fusion power being explored at present.

He says deuterium would be a natural byproduct of electrolysis of water sourced on Mars, which would produce around one kilogram of deuterium for every six tonnes of water electrolyzed on Mars. However to do this then you have to add a deuterium / hydrogen separation stage to the hydrogen production plant. How easy is that? He doesn’t go into details of how it would work.

That 5 times enhancement over the deuterium in Earth’s oceans is still a long way from 100% concentration. It’s normally extracted by using many stages, and each time the amount of deuterium is increased. With only one atom in 1,284 consisting of deuterium you would still need to concentrate it many times over to reach 99% concentrations. For instance water electrolysis, one of the most effective methods of concentrating it, would increase the deuterium concentration 5 to 10 times each time it is used. The 5 times higher concentration on Mars would just save one stage of water electrolysis of many that would be needed. Though in practice electrolysis has such high energy costs it is best used only once for a final stage, for water that is already 50% D2O. The Argentinian plant uses methane as a feedstock because the hydrogen can be dissociated thermally from methane, much more easily than from water. Similarly for other techniques. There are many methods used to extract deuterium. Each of them requires many stages of concentration and I don’t see how an enhancement of 5 times in the feedstock would make a significant difference here.

So that then leads to the practicality of building and operating an extraction plant on Mars and providing the high power levels needed to extract the deuterium (the main reason for its high cost). If it needs vast amounts of electricity to do the separation, it’s not going to be worth doing I think. Also heavy water plants on Earth are large scale and massive structures. This is the heavy water plant in Argentina:

Heavy water plant near Arroyito, photograph by Frandres This plant produces most of the world’s deuterium, at a rate of 200 tons per year, and is powered by a nearby hydroelectric power station at Arroyito dam with a power output of 128 MW. (I'm not sure how much of that power output is used for the plant, do say if any of you know).

The equipment for extracting deuterium weighs 27,000 tons including the support structures and includes 250 heat exchangers, 240 pressure vessels, 90 gas compressors 13 reactors and 30 distillation columns. (Statistics from Arroyito Heavy Water Production Plant, Argentina)

Would the five times higher concentration of deuterium lead to more than a minor saving in the costs of the plant? And how would that offset all the difficulties of setting up and operating the plant with near vacuum conditions outside it, as well as transport costs for equipment that can’t be built on Mars?

Of course Mars is different in many ways and though most of them seem to be disadvantages for operating such a planet, could any of them be advantages, such major advantages that it makes it worthwhile to build and operate it on Mars? For instance, could the near vacuum of its atmosphere be made an advantage somehow? (E.g. for distillation).

On the face of it, there doesn’t seem to be a compelling commercial case for this. If there is, it needs to be spelt out in more detail.

Details here come from Heavy Water: A Manufacturers’ Guide for the Hydrogen Century and Future Trends in Heavy Water Production (1983) - has details of the Argentina plant. and heavy water.

• Export of fuel to Mars orbit and further afield

Some of the internet discussions talk about this as a business case. The main issue I see with supplying fuel from the Mars surface is, would it compete with fuel generated on Deimos or indeed on the Moon for astronauts in orbit around Mars? Also, is methane valuable enough as a fuel in space, to make it worthwhile to export hydrogen to the Mars surface to convert into methane and return to orbit, or to split the hydrogen from water on Mars and use it to make methane?

That leads to the next idea:

• Export of water from Deimos for use as fuel in LEO (if there is water ice on Deimos).

This is the premise of the Deimos Water Company outlined by David Kuck. The delta v back to Earth is much less than from the Mars surface, and you can produce your own fuel for the journey. It would have to compete with volatiles on the Moon if those exist and are easy to mine. I think it's hard to judge this at present as we don't know what the volatiles are like on the Moon. We know they exist but don't know how abundant they are locally, or how easy or hard they are to extract. And so far we don't yet know for sure if there are any volatiles on Deimos, although spectroscopically it resembles a type of asteroid that often has them.

Supposing Deimos and the Moon have volatiles equally easy to extract, then the Deimos volatiles would still be favourable for use on Deimos and Phobos and for export to the Mars surface. They would also be favourable for delivery to Mars orbits such as Mars capture orbit at a delta v of 0.57 km / sec from Deimos. So, it would make a lot of sense for a base on Deimos to supply fuel to the Mars system. But that's not a commercial case for colonization. As Zubrin says - you need something over and above ISRU for a commercial case for exports you sell to pay for the things you can't produce there.

So we need to look into whether this can be competitive with the Moon for supply to the Earth Moon system. For the Moon to LEO the delta v is 5.7 km / second, and a bit more if supplied from polar regions - while it's 4.87 km / sec for Deimos to LEO which would seem to favour Deimos. However that does not take account of Hoyt's cislunar tether transport which could make the delta v for supply from the Moon to LEO almost zero.

So, in summary, there do seem to be a number of potential exports from Mars even at quite an early stage, although this is mainly based on internet discussions with not much actually published on the topic in peer reviewed journals. But they all depend on future discoveries so we won't know if this is possible until we know more about Mars. A few of the potential exports, involving exobiology, might require us to keep Earth microbes out of Mars.

There may also be exports from Deimos, but that depends on how easy it is to extract the volatiles, and if the lunar volatiles are as easy to extract as the ones from Deimos, then it might be hard to put a business case for export from Deimos to the Earth / Moon system, though it may be very useful for volatiles for spacecraft in orbit around Mars, on its moons or on its surface. As for exports to the asteroid belt, the chances are that they will find a way to mine their own volatiles out there, so it seems an unlikely case to me for the special case of volatiles.

Here I'm using the delta v figures from Hop David's cartoon delta v map.

Here are some of the online discussions I looked at. Of course they are not always 100% accurate. This is just enthusiasts discussing the topic, some more knowledgeable than others, and it may also contain a fair bit of nonsense in some of the discussions, so you have to filter and look up details to see if what they say is correct. Anyway if you are interested in doing that, see for instance:

• Is Mars worth mining for Earth purposes? answers on Quora
• Exporting goods from Mars discussion on Nasaspaceflight.com
• Exports discussions on the New Mars Forums under Martian Politics and Economy, for instance: here, here and here.
• What about bringing back valuable resources from Mars to finance one way trips for humans? (Reddit)
• Elon Musk quotes on Reddit with discussion
• Discussion of Zubrin's paper on Reddit

Wikipedia also has a page on Space Trade, though there isn't much in it yet. Then there's Robert Zubrin's paper, already mentioned, and the Interplanetary Commerce section of Case for Mars.

That's about it, do let me know if you have more sources!

WOULD A SPACE COLONY SURVIVE WITH ONLY EXPORTS OF INTELLECTUAL PROPERTY TO PAY FOR IMPORTS?

As we saw, Elon Musk and Robert Zubrin both are skeptical about any possibility of material exports from Mars, at least in the early stages (though Zubrin thinks there might be a case for deuterium exports), and both think that a space colony could pay for imports solely through licensing of intellectual property to Earth. Robert Zubrin draws the analogy with the "Yankee Ingenuity's flood of inventions" which he says was due to a situation of acute labour shortage in the US in a technological culture, which would be paralleled on Mars. But how would that work in practice?

First, for US readers, I'd like to point out that this whole idea is based on a US perspective on inventions. I'm from the UK and we also talk about our country as the source of a flood of inventions, frequently. Here is an example.

"We're a nation of inventors, from the worldwide web to the electric vacuum cleaner - here's a rundown of our most influential innovations", intro to a list of the 50 greatest British Inventions from the UK in the Radio Times.

And putting aside national pride, which all countries have, surely for such a small country, we have indeed made many inventions here. We don't have the same narrative that it was due to a labour shortage, nor do we think of the US that way either. I'm not talking about historians here, but ordinary folk. Robert Zubrin's quote was the first I heard of this idea, which I assume from the way he put it, must be quite commonly accepted in the US. We just think that we are a nation of inventors, and leave it at that. We don't try to explain why.

At any rate if it's true of the US, surely it can't explain why we have so many inventions from the UK as we've never had a significant labour shortage. Indeed the opposite, here technology put many skilled people out of work leading to uprisings by working people during the industrial revolution followed by military repression

The leader of the Luddites - self employed weavers who feared getting put out of work by the newly introduced weaving technology of the late eighteenth and early nineteenth century, and replaced by less skilled workers. They destroyed industrial equipment in protest. Later on agricultural workers joined in, destroying threshing machines. The UK government responded by military action against them, executions, deportation, and they made destroying industrial machinery a capital offence. The US narrative that invention was the result of a labour shortage just doesn’t work when applied to UK inventions. It was almost the opposite, inventions caused a labour shortage here, at least of skilled workers

Let’s look at some of the metrics that measure the talent and creativity of a country. The rankings vary from year to year, but in 2015,

• For R&D investment, Israel is top, (4.4 percent) followed by Finland (3.84 percent), South Korea (3.74 percent), Sweden (3.38 percent), and Japan (3.26 percent).
• For patents, South Korea is top ( 3,606 patent applications per million people) followed by Japan (2,691), Singapore (1,878), Hong Kong (1,797) and the United States (1,644).
• For the percentage in the creative class (workers in science and technology and engineering; arts, culture, entertainment, and the media; business and management; and education, healthcare, and law), Luxembourg takes top spot with more than half (54 percent), and United States is a fair way down the list at (33 percent) rank.
• For education, South Korea takes the top spot with a 100 percent in universities, colleges etc (tertiary education), and the United States is second (94 percent) with Finland just behind in third (94 percent). When you add in tolerance (which makes your country more open to creative people from other countries and to the ideas of creative minorities in your own country) then Canada is top followed by Iceland, New Zealand, Australia, and the United Kingdom. The US is eleventh.

When you combine all these measures, the US comes second with only Australia ranked higher. So it doesn’t seem that being inventive is the most important attribute when it comes to becoming a leading technological nation. It is one of several factors. Availability of education, tolerance and openness to ideas also has a lot to do with it as well as the numbers of people in the creative classes in society. See list of the most creative countries and then for the detailed stats, Global Creativity Index.

Perhaps there is some correlation with labour shortage, for instance, Japan, second in the list of inventiveness by patent applications, is top in the list of countries facing acute skill shortages - but which way does it go? Does innovation lead to skills shortages or vice versa? The whole question is a complex one. Here is a survey of the literature from 2005 from the Department of Trade and Industry in the UK which looks at some of the drivers of innovation. The focus is more on trying to find ways to fill the gaps in skill shortages, and ways to encourage workers to get involved in innovation since innovation often comes from the less skilled workers - it also looks at different styles of innovation - the radical creativity that we may be most familiar with and incremental accumulation with a slow and steady pace of innovation.

I think it is hard to say for sure whether space colonies would be more innovative than countries on Earth on the basis of this information.

Also, the space colonists would be using many inventions from Earth, so surely they would have to pay many royalties in the other direction back to Earth? How could it be possible to set up a system where the Earth has to pay royalties to Mars and not vice versa?

And then - how also could it work, even if a space colonists did turn out to be much more inventive than Earth? The only people who would be able to earn foreign currency for imports to Mars would be the ones who make these inventions. But it's not enough to be inventors. They have to make their inventions into paying inventions also. And highly profitable inventions too, to pay for such items as spacesuits.

It’s best to think of spacesuits as more like mini spaceships than the suits of science fiction stories and movies, which are depicted as not much more complicated than wetsuits with aqualungs. They have to be pressurized to hold in atmosphere at a pressure of tons per square meter when surrounded by a vacuum, yet also flexible too with many joints, also able to withstand minute micrometeorites hitting at kilometers per second, and to keep the astronaut cool because the vacuum of space is a good insulator, like a vacuum flask. This makes them far more complex than any diving equipment.

A typical NASA spacesuit would probably cost about $2 million dollars to build from scratch - that’s as a recurring item, not including the initial design costs. It requires about 5,000 hours of work and would take someone who had all the necessary skills about two and a half years to build, given supply of all the parts and materials needed. I get those details from Space suit evolution (NASA). It’s possible that this could change with future designs. But that’s the current situation, and for the foreseeable near future.

I'm an inventor, and I have invented dozens of things (mainly games and software ideas) but I only earn dollars per day from them, and many have never been published in any form (attempted to publish some of them with no success).

Similarly I've written many original articles, but again, though I earn a bit from the kindle booklets, it's only a dollar or two a day, at present anyway. And that's not at all unusual. For instance I have many composer friends, but it is rare for them to earn a living entirely from composing.

As for composers, artists, writers, or other creative people, earning amounts that would let them buy multimillion dollar spacesuits for all their friends, and ship them to a space colony - well forget about it, unless the next Harry Potter is written on Mars. Even then, J. K. Rowling’s estimated wealth is 1 billion - enough to buy spacesuits for 500 people. She earns 23 million a year, enough to pay for 11.5 spacesuits a year. You’d need a lot of J. K. Rowling’s to support a large Mars colony.

Amongst all my friends and relatives here in UK, another country with a high proportion of inventors, then yes many of them are indeed innovative and creative and inventors in spirit. But I can't think of many that make a living from their inventions, especially just as intellectual property rights. It's the same also for software programmers - most independent shareware developers that I know, often authors of very inventive software, do it part time, and couldn't earn enough from it to support themselves or their families.

Only a few of all the people who invent things go on to make millions of dollars from their inventions, enough to pay for spacesuits and the like for all their friends and colleagues if they so wished. Even Elon Musk came close to bankruptcy once, in his worst year.

"We were running on fumes at that point," Musk says. "We had virtually no money... a fourth failure would have been absolutely game over. Done." Elon Musk in an interview with Scott Pelley, March 30 2014.

So there is a measure of luck there as well. SpaceX would not be here today if his fourth test flight had gone wrong.

So, if you had a million colonists, I don't think we can expect to have a million Elon Musk's. You might be lucky to have one. I think it is fair to say he is at least a one in a million success story. And however brilliant he is, would he earn enough just through intellectual property rights on Earth, managed remotely from, say, Mars, to pay for all the imports needed for a colony of a million people? Even a billion dollars a year of earnings is only $1000 per person which wouldn't get you far importing expensive components from Earth to Mars.

There's also the question of how that would work in practice. Is it going to be a communal system or even communist (in the good sense) where the inventor's earnings are used equally to support everyone? If so, where is the incentive for the inventor to not just invent, but to go to all the work to get their invention into production, or for entrepreneurs to join in with them? Or is it the case that the inventors who are successful are the only ones who earn anything in Earth currencies, and so are the only ones who can afford to import goods, and they then sell them on to the other colonists at any price they care to set in the local Mars currency? And what’s to stop them from emigrating to Earth once they become financially successful, especially since most of their earnings would accrue on Earth and the on the spot business decisions would be made on Earth, and the meetings with investors and manufacturers etc. would also be done there?

I'm not expert in politics or economics. I may well be missing something here. But it seems on the face of it to be quite a problematical way to support a colony. I'm interested in any thoughts on this - do say in comments on the Science20 articles or the kindle booklets pages or here.

On the face of it, at least, this seems a major advantage of the Moon, that you'd have many different revenue streams to pay for imports, at least potentially.

• Intellectual property rights and royalties of course, for any inventions and intellectual creations, the same as for Mars. If Robert Zubrin is right, you have the same labour shortage in a highly technological society, which he thinks should lead to creation of lots of valuable intellectual property in space.
• Exports of the volatiles - initially supply of volatiles to cislunar space - depending on how easy they are to extract
• Exports of precious metals - with the much lower delta v, then these just possibly might be commercially viable. Dennis Wingo thinks that the Moon may have valuable resources of platinum, gold etc. as a result of impacts of iron rich meteorites as well as the core of the giant impactor that created the south pole Aitken basin
• Manufacture of computer chips that need high grade vacuum, readily available on the Moon, higher grade than anything easily achieved on Earth.
• Export of solar power - solar panels should be easy to make on the lunar surface using in situ resources and the high vacuum - and some think there may be an economic case for exporting this solar power to Earth.
• A place to build large particle accelerators - with no need for cooling or evacuation of the chambers.
• Scientific research stations which would be funded from earth - hard to set up at the distance of Mars (though we may get them there eventually).
• Astronomer's radio telescopes on the far side and passively cooled infrared telescopes and liquid mirror telescopes in craters - paid for by Earth - they may be built from Earth but probably need at least some human presence on the Moon.
• Tourists as well. It's reasonably possible that you'll get wealthy tourists going for holidays on the Moon in the not so distant future. But who would go on a holiday to Mars, whether to the surface or to orbit or its moons, if it means you have to take two years or more out of your life to go there and back? Venus also seems too far away to have much tourist traffic in the near future. The Moon seems likely to get the lion's share of any space tourist industry beyond LEO in the near future, unless transport is speeded up hugely, and especially also given the much higher costs of a long mission to Mars or elsewhere in the solar system.

I haven't listed exports of Helium 3 for fusion here. Although it gets a lot of publicity, it's based on technology we don't have, and some experts think we will never have. Also, Crawford calculates (page 25) that manufacturing a square meter of solar panels on the lunar surface - which you can do by melting the indigenous silicon and using the high grade lunar vacuum to form panels in situ - would create as much power through solar power in seven years as you'd get from mining the same region for Helium 3 to a depth of three meters.

So, if mining for helium 3 is viable, this suggests that beaming solar power from the Moon back to Earth or to spacecraft in LEO would also be viable and a better business case than Helium 3. It may however be a useful byproduct of other mining operations on the Moon, for cryogenics, neutron detection, and MRI scanners, and possibly for fusion in the future. For details, see Helium 3 .

Of those, only the first, intellectual property, applies to Mars, at least in the early stages.

That is of course, apart from the ideas mentioned in the previous section, but they are none of them things we can count on right away, and some may depend on keeping Earth microbes out of Mars.

Also, if Mars geology could lead to unique gems such as the possible Mars opals of the previous section, then what about the Moon? Might it also have unique exports that can only form in the lunar conditions? For instance, could there be lunar gems?

Surprising discovery in 2008 - the near side of the Moon has large deposits of relatively pure chromite spinel, which is a gemstone on Earth. This was discovered from orbit. The moon rocks have small amounts of spinel mixed up in them, but this was a much stronger signal. Could the Moon have spinel gemstones? As with the Mars gems, if they exist, they probably wouldn't be worth the cost of returning to Earth unless they have something distinctive about them due to formation in lunar conditions.

Or might there be anything else unique to lunar geology that we might prize back on Earth?

MAINTENANCE COSTS

For a profitable colony, I think the main thing in the very long term is how easy it is to maintain habitats and equipment in the years and decades into the future. If habitats have to be replaced every few decades (as for the ISS), and spacesuits similarly, the long term costs are going to be very high even if the startup costs are reduced.

As an example, the ISS cost €100 billion so over $110 billion, see How much does it cost? with a design life of about three decades (though it may be extended), and normal maximum number of inhabitants six. That makes the cost about 600 million a year or so per inhabitant with most of that due to the limited design life of the ISS.

The projected cost for the Stanford Torus was over $200 billion in 1975 US dollars for ten thousand inhabitants. That’s around a trillion dollars in 2016 dollars (Inflation Calculator), or a hundred million dollars per inhabitant.

If we can find a way to pay for a habitat as a one off cost, for instance through government funding, private funding, or it pays for itself commercially (the Stanford Torus was going to be paid for by exports of solar power from space to Earth), then the main issue after that is how to maintain it.

If the habitats costs a few hundred thousand dollars a year per inhabitant, then still, only the very rich could live there even after the build costs are paid off, and no matter how much the initial build costs are reduced, unless its exports are very valuable.

Then, if you can build the same habitats on Earth, for instance in a desert or floating on the sea, with no cost for its breathable atmosphere or cosmic radiation, solar flare and micrometeorite shielding, the exports from space have to be very valuable to make the space colonies competitive.

If you can reduce the maintenance cost to say hundreds of dollars per year per person then space does have some advantages over Earth, with no storms or earthquakes (depending where you build), no weathering from rain, wind, etc. Then a “home in space” might become a viable long term prospect.

On the downside you have micrometeorites, cosmic radiation, need for spacesuits etc. Can the cost of those really be reduced so much, or the exports from space be so valuable, that they compete with costs of maintenance due to weathering of buildings on Earth?

In this way, an easy to maintain colony will need exports mainly to pay for luxuries, while a hard to maintain colony will need many high value exports just to survive.

REDUCING MAINTENANCE COSTS FOR SPACE HABITATS

The three things here of most importance I think are:

1. An envelope that is low maintenance to preserve the habitat - to keep in air, and protect against any external hazards such as cosmic radiation, solar flares and micrometeorites.

2. A closed system biosphere inside - we need this for any long term space habitat as the logistic requirements and expenses are just too high otherwise. The variation in maintenance costs here would be mainly due to variations in how you supply light and heat to the habitat, and whether you get leaks of gases, water, and other materials that need to be replenished from time to time.

3. Maintenance and resupply of equipment for essential needs, for instance space suits, environment control, solar cells

For 2, I know a lot is made of the CO₂ atmosphere for Mars but you don’t actually need much by way of in situ resource utilization. For instance if it is a reasonably closed system, you don’t need constant supply of water, CO₂, or nitrogen. You just need to be able to top up any losses that there may be in the system. Plants don’t need a constant supply of CO₂ to grow, they get the CO₂ from the exhaled air of the astronauts. The astronauts in turn get their food and oxygen from the plants. In a biologically closed system those numbers all add up. If you produce enough food from plants, you automatically produce enough oxygen too and the astronauts eating that food produces enough CO₂ for the plants to use in their next growth cycle, as the Russians proved in practice with their BIOS-3 experiments.

MAINTENANCE FOR HABITATS IN FREE SPACE AND CITY DOMES

The costs can be reduced if you have a single envelope enclosing a large area, for instance a domed city or a cave or a Stanford Torus or O’Neil Colony style spinning space habitat. That’s because it requires less mass per volume to enclose a larger volume (area of envelope goes up as the square of the radius and the volume enclosed as the cube). So the launch mass and cost per inhabitant of maintenance for the envelope will be much lower for a larger colony.

The Stanford Torus design has 20 tons per colonist, or five colonists for every hundred tons of structural mass, excluding the radiation shielding. The ISS weighs 400 tons for up to 6 astronauts typically. As a Stanford Torus habitat, that much mass could support 20 colonists,s, and later designs are more efficient per colonist, for instance flattened torus or multiple levels inside the torus.

Apart from the radiation shielding, there isn't much difference between the Stanford Torus and other habitats on the surface of a moon or planet. The radiation shielding for the Stanford Torus would be supplied by mass drivers fed by bulldozers on the Moon, so doesn't need to be launched from Earth.

See also Using the Moon to build habitats in free space

MAINTENANCE FOR LUNAR CAVES

The Moon scores over just about anywhere else for the early stages, because of the lunar caves - at least, if they are as large as the Grail data suggests. See Lunar caves. They may be up to kilometers in diameter and over 100 km long. That’s as much internal area as an O’Neil colony.

Somewhat smaller ones, of up to a few hundred meters across would be preferable because the mass of the atmosphere becomes significant for larger ones. If it is easy to convert them into a low maintenance envelope for the habitat, turning interior walls to glass perhaps, the maintenance costs might go right down. They would protect from cosmic radiation, solar flares, micrometeorites and hold in the atmosphere against the vacuum of space.

You might wonder about power requirements to produce food on the Moon with the 14 day lunar night. Robert Zubrin uses figures of 4 MW per acre for artificial sunlight in his Case for Mars (page 237) or about a kilowatt per square meter.

However the power requirements per habitant are far less than you might think as with efficient hydroponics, you only need 30 square meters per person, to provide 95% of their food and oxygen, from the BIOS-3 experiments. Also those figures for the power requirements to illuminate the crops must be for the older halogen lights. Modern LEDs are far more efficient and can be optimized to emit only the frequencies of light that are most useful for plant growth. The result is that you only need 100 watts per square meter or about a tenth of the figures in Case for Mars.

When you combine those lower power requirements per square meter with the small growing area needed per inhabitant from the BIOS-3 experiments, that makes it only 3 kilowatts per inhabitant, which you’d need for 12 hours a day. On the Moon you’d only need it during the lunar night (in the caves, you can use solar collectors on the surface and light pipes during the lunar day). That's 12 hours a day for the lunar night of 14 earth days, so 504 kWh in total. That’s a power level that could be supplied using solar cells during the lunar day then power storage such as fuel cells or batteries for the lunar night.

Alternatively you can lower the temperatures of the crops during the lunar night from 24 °C to 2.5-3 °C (which helps maintain plant vitality during darkness) and leave them in darkness, which results in edible crop yields reduced by 30 - 50%. So that would require up to double the growing area, or around 60 square meters per astronaut, and no need to supply extra illumination during the lunar night.

For more on this see the sections in Case for Moon First:

• Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions)
• Fertility of lunar and Mars soil
• Earth length day on Mars versus advantages of close to 24/7 solar power at the lunar poles

For need for artificial gravity in the lunar caves, see:

• What about gravity - isn't that a big advantage for Mars over the Moon?
• Artificial gravity on the moon to augment lunar gravity

VENUS CLOUD COLONIES - A SURPRISING LOW MAINTENANCE SOLUTION

However if you want to reduce maintenance to an absolute minimum in space habitats, well there is one other place that has far lower maintenance costs even than a lunar cave. It also has greatly reduced initial costs for the habitats as they are very low mass. It’s a surprising one to most of you perhaps. That’s Venus cloud colonies. So I’ll briefly mention those too.

Venus, just above the cloud top level, is in some ways the most habitable region in our solar system outside of Earth. The temperature and pressure there is the same as for Earth. There’s abundant sunlight, and clear skies. The atmosphere above you provides the mass equivalent of ten meters of water, shielding you from cosmic radiation and solar flares, also from micrometeorites - they are not an issue at all. Solar flares will cause large scale magnetic effects because Venus has no magnetic field to shield from them - but this is only an issue if you have kilometers long conductive cables - which are not likely to be needed.

Earth’s atmosphere is a lifting gas in the dense CO₂ of Venus’ atmosphere. And just as with a weather balloon or airship - the pressure is the same inside and outside the envelope. So an airship could be filled with Earth pressure atmosphere with just a thin envelope to hold it in. Even if it is damaged, the air would leak out only slowly and the Venus acid filled atmosphere would also percolate in slowly too. Unlike any other space habitat, it would not be an emergency that you have to respond to in seconds, but something you could repair over a timescale of minutes or hours or even longer.

Russian idea for a cloud colony in the upper atmosphere of Venus, proposed in 1970s. This illustration is from Aerostatical Manned Platforms in the Venus atmosphere - Technica Molodezhi TM - 9 1971

This makes the Venus atmosphere the place offworld with the lowest maintenance costs of anywhere, I think. Also its atmosphere has all the main chemicals for life. It has carbon, oxygen, hydrogen, nitrogen and sulfur in abundance. The concentrated sulfuric acid is a source of water (it dissociates naturally into water and SO2 in the Venus sulfuric acid cycle). You can make plastics, and you can grow trees and other plants. You could even build new habitats using mainly wood and plastics and some thin layer to protect against sulfuric acid and UV light. To protect suits, airships, cloud colonies etc. from the acid involves covering them with an acid resistant coating, such as teflon (suggested here on the basis of tests simulating Venus atmosphere conditions).

Instead of $2 million spacesuits, you have acid resistant suits, which eventually you’d make locally, and aqualung style air breathers. This is a major saving since spacesuits are so complicated, and components for them when they fail would be a large budget item in any space colony I think.

Venus also has gravity levels identical to Earth, so if full Earth gravity turns out to be best for human health, this is easily achieved in the Venus cloud colonies.

Its long day may seem a disadvantage, as its solar day is a very long 116.75 Earth days. However the upper atmosphere super rotates once every four Earth days in a steady jet-stream like flow which gives the cloud colonies a two Earth day “night” and a two day “day” which is much more acceptable.

The cloud colonies also score at an early stage because you can launch a much larger habitat to the cloud colonies for far less mass per inhabitant. Or much more living space for the same mass sent to Venus. This would be an inflatable habitat like the Bigelow Aerospace idea - but one that is as lightweight as an airship.

Much of this will seem unfamiliar and unlikely to many of my readers. The thing is that ideas for Mars have been worked out in considerable detail, by the Mars colonization enthusiasts and the Mars Society etc. We don’t have any similar advocacy group for Venus or even the Moon. So there’s a tendency to look at everything with “Mars spectacles” and see how the Mars solutions would work on Venus or the Moon. And not surprisingly you find out that the solutions devised for Mars work better on Mars than anywhere else. But once you start looking at these other places in their own right, then a different picture may emerge.

If you are interested in this idea and want to follow it up further, see my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine"?

So I think at least potentially Venus cloud colonies have the lowest maintenance requirements of all and might well hit that $100s per colonist per year figure at an early stage.

Still you need to pay back the initial build costs. The Stanford Torus was projected to take 22 years to build for 10,000 colonists at a cost of around a trillion in 2016 US dollars (see Building the Colony and Making It Prosper). A Venus colony wouldn't need anything like as much mass, for instance no regolith shielding is needed, you only need thin envelopes and there is no need to contain the pressure of an Earth atmosphere against a vacuum. The engineering is simpler as well. You could probably launch it all from Earth for a similar number of colonists over a similar timescale at a much lower cost than the Stanford Torus

But you still need some motivation for doing it. Even if it costs much less, and is easier to maintain once built, how can you do it if there are no profitable exports and they can't pay back the build costs? So let’s just look briefly at its commercial value for exports.

ORBITAL AIRSHIPS FOR VENUS AND MARS

This depends a lot on how easy it is to export from Venus. That’s why I don’t see this happening in the very near future, for as long as you need massive rockets to launch from the colonies to orbit similarly to the ones needed for Earth. However JP Aerospace are working slowly and steadily on their idea for orbital airships. Even in the near vacuum of the Earth’s upper atmosphere, hydrogen and helium float in the near vacuum of oxygen and nitrogen. And that’s even more so with the denser Venus CO₂ atmosphere. They accelerate using ion thrusters, slowly over several days, and meanwhile also rise higher and higher in the atmosphere. Eventually they break the speed of sound barrier - but by then they are so high it is an almost vacuum and it is not a problem.

Artist’s impression of orbital airship from JP Aerospace’s Airship to Orbit handout - this would be a very lightweight 6,000 foot airship which slowly accelerates to orbit from the upper atmospheric station using hybrid chemical and electrical propulsion over a period of several days

You need a staging post at a high level in the atmosphere for Venus or Earth where passengers and goods are transferred to a high altitude orbital airship which is much larger and lighter, designed for upper atmosphere operations. See my Projects To Get To Space As Easily As We Cross Oceans for an overview, also see their book: The Airship to Orbit Program

Note that this also applies to Mars too. Their orbital airships would be able to accelerate to orbit from the Mars surface with no need for an upper atmosphere staging post. If this is possible, then you could have both Venus and Mars as “garden planets” and Venus would score over Mars in that respect because the greenhouses would be far less substantial for a larger living area and much lower maintenance.

But the Moon also would have low cost exports because of its low delta v and because of Hoyt’s cislunar transport system which could reduce costs to almost zero (see my Exporting materials from the Moon)

Apart from this idea of a garden planet, it needs to be some product of the Venus atmosphere. Sulfuric acid is the obvious one, but not especially valuable. Might there be some really high value product? One possibility might be deuterium. As I mentioned in the discussion of Mars exports, Venus has a deuterium / hydrogen ratio 120 times Earth’s (and 24 times that of Mars) Implications of the high DH ratio for the sources of water in Venus' atmosphere. Instead of six tons of water electrolysis yielding one kilogram of deuterium, as is the case for Mars water, this would yield 24 kilograms of deuterium, or four kilograms per ton. However as for Mars, can we count on deuterium to be a valuable commodity in the future? And would the higher deuterium levels lead to more than a modest saving in the costs of extracting deuterium? Even with one atom in 54 consisting of deuterium, that’s still far from pure and would require many stages of whatever process is used. On the other hand unlike Mars, Venus does have abundant solar power, even more so than Earth, which may help. Still, as for Mars, this seems a bit of a stretch to me, unless some method is developed for making it much easier to extract deuterium quickly with minimal power requirements - but in that case costs would also be reduced hugely on Earth as well.

As for Mars, another possibility is products of indigenous life, as there is a small chance of life in the Venus clouds. There is indirect evidence in the form of asymmetrical microbe sized particles in the atmosphere and carbonyl sulfide, a clear sign of life here on Earth (though it could be created inorganically on Venus). See my: If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus.

Again, as for Mars there’s the possibility of growing plants if conditions in the Venus clouds let you produce unusual food or decorative plants more easily, for instance, rare flowers on Mars that are very expensive to grow elsewhere, or similarly unusual and tasty rare new food stuffs that grow best on in the Venus clouds for some reason, perhaps genetically designed for those conditions, since the environment of the Venus clouds would be hard to replicate on Earth.

As for Mars, we have to explore Venus first.

(click to show on Youtube)

Concept for a robotic airship called VAMP to explore Venus. It weighs only 450 kg, although its wingspan of 46 meters dwarfs the space shuttle . It would inflate while still in orbit around Venus attached to its mother ship, and then spiral down to the cloud tops in a slow motion re-entry that needs only minimal thermal protection. Incredible Technology: Inflatable Aircraft Could Cruise Venus Skies, details here Venus Atmospheric Maneuverable Platform It could explore the Venus atmosphere for years at the cloud tops, the same level that’s suggested for the Venus cloud colonies. Video discussion here.

At a later stage, we could send astronauts to explore the atmosphere using airships, then return to Earth as explored in NASA’s HAVOC concept study. See project home page. The return to orbit would be accomplished using something like the Pegasus air launched rocket. This is an internal study, so it's at an early stage at present.

(click to show on Youtube)

NASA Study Proposes Airships, Cloud Cities for Venus Exploration. Technical details here.

Perhaps it’s possible that we might discover something of high value as we explore and study the Venus clouds. However, we can’t count on it at present.

MARS AS AN ASTEROID MINING HUB FOR THE DISTANT FUTURE - OR SHOULD IT BE VENUS?

I mentioned earlier that Robert Zubrin talked about the Mars system as a base to support asteroid miners in the more distant future. He thinks that Deimos and Phobos might be especially useful here. So, let’s just look at this for the more distant future. Intuitively, Mars is closer to the asteroid belt so you’d think, surely it’s the best place to support asteroid miners? However, the situation is not as clear as you might think.

First, asteroid mining is likely to start with NEOs - we get many asteroids that do close flybys of Earth, Venus or Mars, some with orbital periods close to the Earth or Venus year. The ones that do low delta v flybys of Earth seem the most likely ones for early mining operations after the Moon. We have dozens of large NEOs to mine first, kilometers in diameter. It’s not likely we’ll exhaust those any time soon, and this also has the extra benefit that we are removing asteroids that have the potential to hit Earth at some point in the maybe distant future. For NEOs ranked in various ways including commercial value, see astrorank.

We can also mine the Moon for asteroid resources - the Moon has been hit by many asteroids in the past, so whatever materials you have in asteroids are probably also on the Moon or in it. It's mainly a question of how accessible those materials are. There's some evidence suggesting that the Moon may have rich surface deposits of platinum (and so also of other metals) from the metallic core of the 100 km asteroid that created the Aitken basin as well as other iron rich asteroids and other asteroids of other compositions in the past (see Metals). The Apollo missions only explored a small part of the Moon and a few spots within that region and didn't travel far from their landing sites. Also they did nowhere near to a thorough survey of the places they did visit. They just didn't have the time for that, and only had a geologist there for the last mission. As for later investigations, you can only do so much with the few orbital missions we've had since then.

Then, when we do have humans in the asteroid belt, it's a lot of delta v to go from one asteroid to another and going via Mars doesn't help except on rare occasions. Most of the time, it will take much more delta v to get to an asteroid via Mars than a direct route, and the same is true for travel to / from Earth. Mars is only useful when you have an energy efficient trajectory that takes you from Mars to the asteroid, for instance via Hohmann transfer.

However, Geoffrey Landis has made a rather surprising observation here in his Colonization of Venus. See Accessibility of Asteroids from Venus in this paper. Even though Venus is closer to the Sun than Earth, because of Venus's faster orbit, the flight time to Ceres or Vesta is actually less from Venus than from Earth or Mars via Hohmann transfer. So Venus actually has advantages as a main asteroid belt mining hub over Mars, a bit counterintuitively. The transfer time is less and you have more opportunities also to get there because of Venus’s shorter year of 225 days instead of Mars’s 687 days, which is three times longer. So you’d get several opportunities to visit an asteroid from Venus for every single opportunity to visit it from Mars, though of course the delta v required is greater.

Hop David suggested the idea of Asteroid Cyclers to work in the same way as a Mars cycler, to cycle materials between Earth orbit and "railroad towns" colonies in the asteroid belt where the mining goes on. The same idea could be used to cycle materials between Venus and the asteroid belt.

Hop David has also suggested that Venus, and Mars would be good places to park large asteroids for mining operations - if it's too hazardous to risk parking them in Earth orbits. The Case For Asteroids. Of course this is for asteroids that already do close flybys of those planets.

Venus cyclers, like the more famous Mars "Aldrin cyclers", can get passengers from Earth to Venus by shuttling them to large spacecraft in permanent orbits that takes them back and forth between Earth and Venus over and over. The result is a somewhat shorter journey time than for Mars cyclers, and you can travel to Venus frequently, every 1..6 years instead of more than 2 years between visits. See his Case for Venus. If this happened, the cloud colonies could be useful for supplies to the asteroid miners, in return for supply of metals and other asteroid derived resources to the colonies.

So - though Mars might well be a useful staging post later on if we have a lot of people in the asteroid belt - the case is not as clear cut as you might think, and Venus might be as useful as Mars, even for supplies to the asteroid belt, depending on your priorities and the exact future situation. In the shorter term it might well be useful for mining asteroids that do close flybys of Venus, captured into Venus orbit temporarily for the purposes of mining.

CONCLUSIONS - IMPLICATIONS FOR INTERPLANETARY COMMERCE

So in short my conclusion is that the Moon is far superior over Mars in this respect and I am skeptical of the idea that a Mars colony could pay for itself via intellectual property. I just don’t see why the flow of intellectual property of commercial value has to be from Mars to Earth rather than vice versa or most likely both ways and I don’t find Robert Zubrin’s labour shortage argument in favour of that at all compelling.

Also it depends on not just inventing things but having the commercial talent to spot how to make the invention financially viable and the persistence and luck to take an invention all the way through to success. Why should Mars colonists be much better at this than anyone else? I don’t get it.

By contrast, I think a Lunar base could potentially be of commercial value, mainly because it has a low escape velocity and is so close to Earth and always at the same distance - especially so if something like Hoyt’s cislunar tether transport system is in place reducing transport costs almost to zero. It is also close enough for tourism to be a major industry eventually.

Mars I think will be the province of government sponsored or philanthropic explorations for some time - like Antarctica, where the return is not financial but scientific knowledge or just interest / excitement. I think that the initial stages of lunar exploration are also likely to be supported in a similar way - but that there is some possibility there of commercial value entering into the mix as well.

And I think we should explore Mars from orbit until we have a good understanding of surface conditions and especially not introduce Earth life to the planet. We could exploit it commercially from orbit through telerobotics, but that would depend on finding something there of commercial value to export. And I think conceivably there might be commercial exports from Mars in the future. Especially if the Mars biology produces some unique valuable biological product that can’t be made anywhere else - that might be worth exporting. But there’s currently nothing we know of that could be worth the cost of export from Mars, and whether there will be in the future, only the future can tell.

Longer term, Venus cloud colonies also seem of special interest. I suggest they are the least maintenance of all offworld habitats outside of Earth, so would need less income per habitant than any other space colony, but even so, it isn’t so easy to find a commercial case for more than an Antarctic style habitat maintained because of its science value and perhaps some tourism, because of the high costs of exports to orbit. Long term, if the orbital airships work out and reduce export costs to orbit almost to zero, perhaps Venus could be a place to grow food for export with the lowest mass and least maintenance greenhouses anywhere in the solar system outside of Earth. Orbital airships would also make Mars more commercially viable too. However, orbital airships would make it easier for Earth to export to space as well so both would have to compete with Earth, and of course the Moon.

In the more distant future both Mars and Venus could become a mining hubs for the asteroids, perhaps with asteroids parked in orbit around the planets for mining. In such a future, Venus, perhaps surprisingly, has some advantages over Mars for ease of access to the asteroid belt in terms of faster journey times and more frequent opportunities for travel.

ADVANTAGES OF THE MOON OVER MARS - SHORT SUMMARY

The idea is to go to the Moon first before we decide whether to go to Mars, but while researching this book I've found many advantages of the Moon suggesting it's a far better place for humans anyway.

• Potential commercial value - potentially, lunar colonies could be supported by export of water and other materials from the Moon. Hoyt's cislunar tether system could give us a way to export materials from the Moon's surface to LEO with no fuel at all. Or we could mine water and use that as fuel for our spacecrafts to use to send other exports to Earth. It's harder to make a case for near term commercial value for exports from the Mars surface to Earth. The only near future commercial value I've seen is mining ice from it's outermost moon, Deimos if it has ice. See also Commercial value for Mars and Would a space colony survive with only intellectual property to pay for imports? above.
  
  The commercial value of the Moon seems similar to the asteroids, with addition of some materials not available in near earth asteroids - such as titanium, aluminium, and especially the water ice in such pure form if it's available.
  
• Stays at the same distance from Earth, easy to get to and from at any time - this could give the Moon an advantage over asteroid mining, since typically Near Earth Asteroids can send their exports on minimal delta v transfer orbits only every decade or so.
• Lifeboats to get back to Earth within three days, from anywhere on the Moon
• Ideal for tourism - for both those reasons and more. Once we have regular transport to the Moon, you'd be able to go there and back within a week, or for a longer visit, in a fortnight. And you'd be safer than anywhere further afield because of the lifeboats. And the light gravity, including sports like human flight, and possibilities of lunar railways, no dust storms or other weather issues at all, possibility of vast underground caves, has many advantages for tourism, but most of all,it's proximity to Earth. Yet, it's a landscape you can actually stand on. And on the near side you'd see Earth in the sky. You could experience lunar eclipses when the entire landscape goes red. On the far side, experience an unfamiliar world, walking on land you can't see from Earth, and with Earth not even in sight anywhere.
• Very little lunar ascent fuel needed to get back to Earth - no need for a "Moon One" like "Mars one", if you can get to the Moon, it's not that hard to get back again.
• If the volatiles are easy to extract and exist in the quantities the preliminary results suggest, then water, nitrogen, carbon dioxide may be as accessible as on Mars, with perhaps enough to support millions of space settlers eventually.
• Some unique advantages that Mars doesn't have such as the regolith, easy to turn into glass using microwaves, high grade vacuum for chip and solar panel manufacture.
• Dust is probably less hazardous than for Mars, and is a known hazard, and with no dust storms or winds. It can be turned to glass and a region around a base or landing strip kept dust free, which is impossible on Mars
• Much of the research done for humans on Mars can be applied directly to the Moon.
• Scientifically interesting in many ways. Including biological interest for organics in the ice at the poles and search for meteorites from early Earth, Mars and perhaps even Venus
• Near constant sunlight at the peaks of (almost) eternal light for solar power and a steady temperature for all except a few days in mid winter
• Potential for huge caves, if we are lucky, these could be as large as a few kilometers in diameter and over 100 km long, similar in habitat area to an O'Neil cylinder.
• No concerns about introducing Earth life, with classification of category II, and plenty of space to explore to see effects of other forms of contamination by humans around the habitats
• What we learn there will help us as we do longer duration missions further afield
• Expect the unexpected.

The main advantage of Mars, as far as a colony of hundreds of thousands or a million, is the somewhat higher gravity. But we don't know if Mars gravity is okay for human health, or indeed maybe lunar gravity is just fine. Or in both cases we may need to augment the local gravity. I cover this in What about gravity - isn't that a big advantage for Mars over the Moon? and Artificial gravity on the moon to augment lunar gravity. in Case for Moon First.

The Mars surface also has some natural protection from solar storms due to its atmosphere, also Phobos's Stickney crater has advantages there too, however there's more risk for radiation on the journey to Mars or in orbit. In both cases it's possible to take precautions to prevent the worst effects. See Solar storms and radiation shielding - Moon and Mars in Case for Moon First.

BACKUP ON THE MOON - SEED BANKS, LIBRARIES AND A SMALL COLONY

This is an idea from this book by William Burroughs, which had a lot of mentions for a year or two then seems to have been forgotten.

I find this suggestion much more plausible than Elon Musk's ideas of a backup on Mars. The idea is not to try to set up a colony that would survive the implausible complete destruction of Earth to the point that Mars is more habitable than Earth. Rather, it's more like a seed bank, like the Norwegian seed bank, but one on the Moon, combined with a library of human knowledge, and a small group of people who could sustain themselves there for a century or two if necessary until the time comes to return to Earth. And they could even restore humanity to Earth if they became extinct here (though this seems very implausible to me for the reason mentioned in Case for Moon First that humans with stone age technology can survive almost anywhere on Earth from the cold of the Arctic to the hottest deserts, so we are amongst the least likely species to go extinct).

So anyway - this is an idea that's much more within our reach in the near future, to set up a colony on the Moon with sufficient supplies of the things they can't make on the Moon to last them for a century or two. Together with enough "lifeboat" spaceships to return to Earth easily, spacesuits to last them out / repairs for them etc. We don't need to make it totally self sustaining, just self sustaining enough for a small community of, say a dozen or a couple of dozen people to last for a few centuries.

And - I don't actually think we will lose all our technology on Earth, I think looking at past history that even when civilizations end, the technology doesn't vanish completely. Since the iron age, the world never lost the ability to smelt iron even though many small tribes can't do it. It never lost writing after it was invented. It never lost simple maths ideas like how to count, addition etc.

As it is now, I think we will never lose the place notation, fractions (quite a late development in maths), zero, and very very unlikely to lose algebra or calculus. Only a certain percentage can differentiate or integrate, but you don't need everyone to be able to do it to have a few teachers to pass it on to the next generation.

In medicine, we won't forget anesthetic, the microbe theory of diseases, the need for surgeons to wash their hands before surgery and nurses to keep clean, or vaccination,

I don't think we'll lose the ability to build airplanes either. Or bicycles. Both are really easy to do with a small amount of technology and mainly based on knowing it is possible and if we have engineering - even if we forgot most of it, we can reinvent it quickly.

The only remaining photograph of the original Colditz Glider made by British prisoners of war using bits of wood and wiring, including bed slats and floor boards. They designed it using details from a 1939 book Aircraft Design which they found in the prison library of Colditz.

And here is a video of the flight of its modern replica (no people on board, remote controlled for safety).

(click to watch on Youtube)

So, future humans could build a glider like this just based on a single book surviving from our era on the topic plus ingenuity and understanding of technology. You don't need very advanced tools to build a successful glider.

Probably similarly, our descendants in some such post disaster Earth won't forget the principle of the jet engine, or the rocket, even though they were innovative ideas at the time. Probably they would remember what a transistor is and the idea of an integrated chip even if we lose ability to make computers. They are not likely to forget how internal combustion works, or indeed steam engines. They would surely retain the knowledge of telephones, and of radio transmission. Once you know how to do it, it's easy to generate and receive radio waves, or to build a simple electric motor or generator.

Here for instance is how to build a simple "foxhole radio". You can alternatively use a crystal such as galena in place of the pencil and razor blade, and make one of the early crystal radios. You can use anything that rectifies the alternating current from the radio waves.

(click to watch on Youtube)

This is so simple, that it's surely unlikely that we'll lose the capability to listen to radio in some crude way or another so long as there are people around who understand some of the basic concepts of how radio works and are reasonably good with their hands, and we have access to wire and other basic components.

So long as some "higher education" continues, each generation teaching ideas to the next, or some library of books remains somewhere on Earth to explain these concepts to us, these ideas will surely not be forgotten. But we could lose the organization of society. Again I think we are probably in many ways much more robust than previous societies but looking forward, what if there was some "perfect storm" of nuclear war say, followed by a super volcano eruption, then maybe a giant impact just as we are recovering?

Possible "one-two punch" for dinosaur extinction. The Chicxulub impact surely was a significant factor in the extinction of the dinosaurs. Probably the earlier supervolcanoes of the Deccan Traps helped push the dinosaurs into a situation which made them more vulnerable - this artist's impression shows the Deccan Traps. That double whammy was separated by 100,000 years.

However a recent idea is that the asteroid impact itself could have triggered volcanic eruptions from the Deccan traps.

(click to watch on Youtube)

Could some closely spaced double whammy cause problems for Earth civilization? (Credit: Zina Deretsky for the National Space Foundation)

Even then I find it implausible that there wouldn't be some pocket of technology left. For instance after a global nuclear war, if we ever came to it, then the nuclear free zones such as Australia, New Zealand etc. would surely be spared and they have the entire Pacific ocean around them.

In this diagram, blue stands for nuclear weapon free zones. In these zones, even if the territories are owned by nuclear weapon states they have agreed not to station weapons there. This makes them very unlikely to be targets in a nuclear war. As you see, apart from a few islands, pretty much the entire southern hemisphere is free of weapons. Red stands for nuclear weapons states,, orange for territories where US nuclear weapons are stationed under a sharing agreement, and yellow for none of the above, but still subject to the non proliferation treaty. After a global nuclear war, should it ever come to that most of the Southern hemisphere would probably be unscathed and retain its technology.

This for instance is the South Pacific nuclear free zone, established by the treaty of Rotaronga. No testing, stationing or use of nuclear weapons permitted anywhere in this zone.

And with technology, even with a nuclear winter, or an asteroid impact winter, surely many would survive and carry through our knowledge and libraries into the future. After all we are not like the dinosaurs. With the most primitive of tools and our intelligence we can survive many things that would kill us easily without technology. It's only because of our technology of course, but that makes a huge difference. You don't get many great apes living in the Arctic or the Sahara desert.

But still, you could imagine that if we are unlucky and encounter turbulent political and environmental conditions, that we might lose some of our most important libraries, seed collections etc. If the billions of dollars semiconductor fabrication plants stop making computer chips, many of our modern machines depend on their output and could no longer be built. We could also lose the ability to send rockets into space, maybe even lose ability to make jet engines for a while. In that future, without computers and the internet, after living in a world increasingly dependent on it, perhaps we would indeed forget some of the vast store of our present day knowledge, even if we surely wouldn't lose it all.

All that knowledge could be preserved on the Moon so then later when we restore the ability to get into space we go up there and find it. Or else, if there's a small colony there, they can educate us via radio and come back to Earth when conditions are suitable here.

With present day technology, if we lose the capability to launch rockets into space, anyone living in a lunar colony could still return to Earth. That would be easy, using its "lifeboat" spacecraft which could be permanently kept next to the colony. But this would be a one way trip as there would be no way to return to the Moon without new large booster rockets to get them back into space again, along with a dedicated space launch pad and the staff and equipment to man it.

This may well change with future developments, if so, perhaps members of a lunar colony could return to Earth, refuel, pick up supplies, do what they can to help residents here, and go back to the Moon using just their own spacecraft without dependence on Earth. But until then, they would be dependent on supplies sent to the Moon in advance, to last them as long as needed for anything they can't make in situ on the Moon.

We don't actually need a permanent presence of humans on the Moon to preserve DNA and knowledge there. Some of the lunar caves probably have an internal steady temperature of around -20 °C (see page 5 of this paper). This is similar to the −18 °C for the Svalbard Global Seed Vault and should be perfect for an off world seed bank.

Entrance to the Norwegian Svalbard seed vault, photo by Bjoertvedt. The seeds need to be kept refrigerated, which is done using locally mined coal. A similar seed vault in a lunar cave would keep the seeds at the right temperatures by passive cooling.

As for preserving our knowledge, well we could leave physical written texts on the Moon, or we could leave engraved glass or diamond, or DNA, or radiation resistant glass DVDs amongst the various suggestions.

There is a Bible on the Moon already, perhaps it would be readable in the distant future?

“A Moment of Reflection” painting by Ed Hengeveld depicts Apollo 15 Commander Dave Scott placing a red leather Bible on the console of the lunar rover before departing the moon.

There's a company wants to send a Torah to the Moon as a hand written scroll preserved in an airtight compartment. They asked SpaceIL to take it to the Moon but were refused, but eventually surely texts like this will be taken to the Moon. The idea is to preserve some of our most ancient texts as a project for preservation of culture. If they succeed, they would send other ancient texts later on such as the Hindu Vedas and the I Ching

Lunar Mission One have as their goal to send a spacecraft to the lunar poles to drill into the ice - and when the mission is over, to bury a capsule with both DNA from human hairs, personal messages, and a digital archive of Earth's knowledge.

Lunar mission one, artist's impression. The aim is to drill and obtain an ice core to return to Earth. But before it returns to Earth, it will put a digital archive in the hold buried deep below the surface at the lunar poles. See "A time capsule on the Moon".

The Part time scientists, one of the teams left in the Lunar X Prize challenge, as a partner with, are planning to take a disk with part of Wikipedia on it to the moon as Wikipedia to the Moon.

Back in 2004, Transorbital, a California based company was first to get permission from the US government to launch to the Moon, planned to use a decommissioned Russian ICBM for the task. They also had the idea to set up a server on the Moon for digital backup services for companies worried about security of data on the Earth after September 11. These plans don't seem to have come to anything though they did send a small satellite into orbit around Earth.

KEO is a more ambitious plan to send data into orbit in glass radiation resistant DVDs into an orbit that will decay and return the satellite to Earth 50,000 years later as a kind of a space time capsule. The idea is that everyone on Earth can store a four page message for the future. The DVD could be played on a DVD player - but of course in the future they wouldn't have our players so it includes information on how to build the player.

The KEO glass DVD is a bit like the idea of the Voyager golden record though in their case the record is in audio format like one of the old analogue records but more durable. We could send a copy of both to the Moon.

A copy of the Voyager record on display at the Udvar-Hazy Center in Washington Dulles International Airport

Photographs of the cover and contents, of the actual record sent on Voyager

See Contents of the Voyager Golden Record to find out what is on it.

As it gets easier to send missions to the Moon, I think it is certain that we will leave various physical and digital data repositories on the Moon. The idea of the lunar backup is a more extensive version of the same. We could build a seed vault / library / small human colony with supplies sufficient for at least a century or two on the Moon with no imports from Earth.

Examples of what it could preserve digitally include:

• all the books in our libraries - at least the ones out of copyright or if the authors gave permission, including of course the entire Gutenberg collection of 50,000 free ebooks. And I wonder if a special case could be made for Google Books to be released unrestricted in its entirety with the understanding that it can't be accessed until after the copyright period is over? I can't imagine that any authors would complain about that. Maybe indeed publishers also would be willing to donate digital copies of their books on this understanding?
• all the academic journals (public access ones, and others as well if they had permission),
• huge archives of astronomical images etc. - our larger archives are copied using DVDs anyway, faster than sending them over the internet, so it would just be a case of sending a copy to the Moon.
• visual records of our art, sculptures, etc.
• audio recordings of music, sound, speeches
• video recordings of films, movies, TV shows
• The whole of Youtube - no reason why not in some near future with lots of storage available on the Moon. Or any other such repository.
• the Internet Archive, which has amongst its objectives, to keep a historical record of the internet for future generations.

Physically it could preserve

• seeds and DNA samples in perfect conditions for preservation long term, and any other small valuable physical objects that we can send to the Moon.

And it could have human caretakers

• with a small population of caretakers, with plenty of supplies, it could also act as a "backup" which could help and educate people on Earth remotely via radio in the case of some future chaos which leads us to losing some of our capabilities and libraries on Earth. The caretakers would consult their many archives up there for information needed, a bit like the librarians in Asimov's Foundation series.

It does have advantages over repositories on Earth, as the lunar caves provide perfect conditions for passive preservation of seeds and anything else that is best preserved in cold conditions in a geologically stable environment. The best analogue we have here might be in Antarctica, if we could arrange some backup in a cave excavated into the cold and stable conditions of some of the Antarctic mountains. Temperatures at the surface on the high Antarctic plateau seldom rise above 20 C, though they can dip down to below 60 C. So there may be places there also where you could build a passively cooled seed vault, which would cost far less than a lunar seed vault and be easier to supply. But you couldn't duplicate the lunar vacuum and very long term stability even over geological time scales. Also you would have to cope with the six months long Antarctic "night" for anything that requires solar power, while on the Moon it's a maximum of only 14 days of darkness, and less if close to the poles.

Of course, eventually, you would duplicate your lunar repositories. Especially you would have more than one copy of the most important information and materials on the Moon. You would need that as precautions in the remote chance that it gets a direct hit from an asteroid hitting the Moon. That's about the only thing that could happen there, and would be exceedingly rare, especially if it is located in a cave protected beneath the surface from most asteroids.

Future Humanity Archives on the Moon - Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

Artist's impression of a vault on the Moon.

It's also scaleable. You can start off by just sending archives to the Moon which can't be read from Earth, but are there for future lunar visitors to discover. The next stage would be archives that you can read remotely from Earth, like the lunar server backup idea. And later, you could have a telerobotic facility on the Moon which can be controlled remotely from Earth, getting the telerobots to look up things that are stored there, even recover seeds and "post" them back to Earth. And then finally, presence of a small group of human "caretakers" normally rotating in and out every few years if they want to - but with sufficient supplies to last them out for centuries on the Moon if needed, with closed system recycling.

I'm not sure if this is really needed for a backup. Can we not do the same on Earth, as we are doing already with the likes of the Svalbard seed vault? Even set up an identical self sufficient small colony underground in Antarctica?

But it's going to happen anyway, at least in a limited way, and the passive cooling is a great advantage for seeds, which are also light things to send into space. It would also provide perfect conditions for long term preservation, for billions of years into the future, even for future civilizations on Earth. And, in my view, it seems a lot more practical than ideas of a "backup on Mars", because it is so much closer to Earth, more accessible if we ever need it, far easier for its human inhabitants to get back to Earth, and easier to communicate with via radio.

TERRAFORMING AND PARATERRAFORMING

Terraforming Mars or the Moon is a far off future idea and it's surely not going to make any difference whether we start now, or a few centuries from now, if it is practical at all. A later start might mean we find better ways to do it, so that we finish sooner. But is it possible at all and should we attempt it? Surprisingly, the Moon can be terraformed too, and has some advantages over Mars in this respect, not that the Moon or Mars seems likely to be either easy to terraform, nor easy to keep terraformed.

THE DREAM OF TERRAFORMING MARS

It is not at all clear that we can terraform Mars, and if it is possible, with current technology, it's a thousands of years, or perhaps a 100,000 year long megatechnology project. There are so many questions. How sure can we be that we will continue such a project, when it is likely to cost billions of dollars a year and need support from Earth for thousands of years?

The Mars trilogy is science fiction, and the optimistic real world estimate from the Mars Society takes a thousand years to a stage where trees can grow but no animals or birds yet, and humans need aqualung like closed system breathing kits to get around. And it is based on assumptions about the amount of dry ice on Mars which are not yet confirmed, and doubts have been cast about how much dry ice still remains there.

Do we have the scientific understanding needed for it? We have never terraformed a planet, and with all our technology on Earth, we find it hard to just keep the CO₂ levels on Earth from rising by tens of parts per million. Would Mars unterraform as easily as it terraformed or go to some undesirable end state. Is terraforming Mars possible at all?

What about accidental planet transformations, where lifeforms we didn't mean to introduce change the climate in unexpected ways? And Mars gets much less light than Earth, so an Earth atmosphere would not be warm enough for without planet scale thin film space mirrors to double the amount of light reaching Mars, or industrial levels of production of artificial greenhouse gases, with many nuclear power stations to supply power, and cubic kilometers of fluorite ore mined per century to make the gases.

Are we confident that this is what our descendants a thousand years from now will want us to do for them? Will they be pleased that we started the project so soon and made Mars just as they wanted it, or will they be frustrated by our failed projects, and lament the pristine Mars they would wish to be able to study and possibly transform for themselves?

The main thing is that I think at our stage, as a young technological civilization, we need to look very carefully, with open eyes, before we close off possible futures that we maybe don't understand the potential of yet. While things like creating new space habitats is opening futures rather than closing them, so I think that's the better way to go.

In terms or habitable area, space habitats have got more potential actually. The reasoning of the 1970s is still valid - that we don't need to look any further than materials from the asteroids and the Moon.

Some asteroids consist almost entirely of pure metals including iron, nickel, and heavier metals such as the industrially useful platinum, and gold (a small asteroid 452 by 1011 meters across, to take an example, was estimated to contain 90 million tons of platinum). These metals may also be capable of being extracted without physical mining using carbon monoxide at low temperatures of 50–60 °C to convert metals to gas (second half of the Mond process with no need to extract the pure metal first), and perhaps directly converted back to metal parts again from the extracted metal carbonyls in 3D printers operating at the higher temperatures of 220–250 °C.

Other asteroids have organics, volatiles, and everything that we need. The conclusions of the 1970s are still valid, that there is enough material in the asteroid belt to eventually build the equivalent of a thousand times the surface area of the Earth. So there's far more potential for settlement in the asteroid belt than there is on either Earth or Mars, measured according to the available land area. And what's more, you can choose whatever climate and even atmosphere, and gravity level that you like for the habitats.

Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

Then, you can complete a Stanford torus on a timescale of decades, not thousands or hundreds of thousands of years. It's rather like terraforming, but on a very small scale, an experiment that's within our capabilities and on a timescale that we can manage.

The things we learn by doing that could help us to avoid some of the pitfalls, if we ever decide the time has come to do a much larger planet transforming project. If a Stanford Torus atmosphere goes bad, or some nasty lifeform takes it over, becomes a pest and you can't do anything, or some microbe becomes a nuisance, or if things get out of balance - you can at the worst vent the atmosphere, sterilize the soil, and start again. So it is biologically reversible. Biological mistakes can be solved. Or if there's an excess of methane, or hydrogen sulfide or some other gas in the atmosphere, well the entire atmosphere of the habitat is small enough so you can scrub the problem gas out of it.

Or if there is something fundamentally wrong with the whole design and approach, you can try a new approach with a different space habitat with a different design. If for instance some of the materials that make up your habitat react with the atmosphere in adverse ways (like the concrete in the Biosphere 2 reacting in a way that indirectly removed oxygen from the habitat), you can even build a habitat mark II using different materials. All these are things that would be very hard to impossible to fix for a planet. So it makes sense at our stage to start with the smaller biospheres of a space habitat in free space, or a city dome on the Moon, or a biosphere in lunar caves.


Trouble With Terraforming Mars

Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

To Terraform Mars with Present Technology - Far into Realms of Magical Thinking - Opinion Piece

Why Nukes Can't Terraform Mars - Pack Less Punch Than A Comet Collision
Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents
How Valuable is Pristine Mars for Humanity - Opinion Piece?

TERRAFORMING THE MOON

Longer term, some have suggested we could terraform the Moon. Surprisingly, we can - for a few thousand years. It loses its atmosphere quickly. Geoffery Landis looked into this.

• The Moon loses its hydrogen and helium very quickly, on a timescale of fifteen minutes on the sunlit side, just because the gravity isn't strong enough to hold them in place.
• It loses heavier gases like nitrogen and oxygen over time periods of thousands of years, longer than most civilizations last.
• When the atmosphere is very thin, it loses nitrogen and oxygen much more quickly, as a result of the atoms getting ionized and then swept away by electric fields associated with the solar wind. They get lost in about 100 days. But if we can thicken the atmosphere up enough, this mechanism is no longer significant (the number of atoms ionized is the same or even increased a bit, but it's a tiny fraction of the atmosphere), and then the atmosphere will last for thousands of years.

See his Air Pollution on the Moon for details. His main focus there is the adverse effect of the atmosphere created as a byproduct of industry on the Moon, leading to degradation of the valuable high vacuum there. But he does briefly touch on terraforming at the end.

Then Gregory Benford, the hard science fiction writer, thinks we can terraform it, in this rather intriguing article: A Terraformed Moon Would Be an Awful Lot Like Florida.

He envisions hitting the Moon with a hundred comets the size of Halley's comet - at the same time spinning it up so it has a day of 60 hours.(I'm not sure of his calculation, it seems that the number of comets needed is more like 10,000, see below).

Presumably you would keep adding new comets to it - but if he is right that 100 comets are enough - then (assuming the atmosphere can last for, say, 10,000 years) just adding one new comet a century would keep it going - and you could do that without harm to the citizens of the Moon by breaking the comet up into tiny pieces before it impacts onto the lunar atmosphere. So what would it look like if we could terraform it? Well here is an artist's impression of a terraformed Moon.

Terraformed Moon by Exospace on deviantART

I prefer this to ideas for terraforming Mars, because there is no life on the Moon to be impacted by it. Also the Moon is close to Earth, and it is clear that it has to maintain a high technology to keep it terraformed, so if both Earth and Moon have high technology, they can work together.

Once you accept that any attempt at terraforming Mars requires megatechnology such as thin film mirrors in space the size of a planet, or hundreds of power stations making greenhouse gases, which you have to sustain indefinitely to keep it terraformed - well - the Moon isn’t that different, is it?

The Moon is at a fixed distance, and we can set up some easy way of getting back and forth with space elevators or space tether systems such as Hoyt’s cislunar tether system which allows very easy transport to and from the Earth with no fuel at all - it uses the gravity gradient from the Moon to Earth as a source of power and is powered by moving materials “downhill” from the Moon to Earth.

And use the same factories - and exchange materials from one to the other easily etc. It is similar to the idea of a very huge Stanford Torus, to terraform the Moon. It turns a lifeless though very large region into a habitable area.

With Mars, then if it can be terraformed, it's on the thousands of years timescale, with lots to go wrong. It may seem like a new earth but if you can terraform it as quickly as that, it needs mega technology to stay terraformed, and it might unterraform as quickly as it terraformed.

• PLANTS HAVE TO WORK SIX TIMES HARDER - TO MAINTAIN THE OXYGEN IN AN ATMOSPHERE SIX TIMES THE MASS PER SQUARE METER

On Mars the plants have to work roughly three times harder than on Earth so it needs about three times as much oxygen to achieve the same partial pressure on the surface. For the Moon the plants have to work six times harder, but they get about double the levels of sunlight they get on Mars for photosynthesis.

If you work it out in detail, there is very little between them in this respect

Detailed calculation: to achieve an Earth normal 10 tons per square meter of atmosphere pressure in lunar gravity - you need 60 tons per square meter of gas in mass. The plants need to maintain 12.7 tons of oxygen per square meter (2.095*9.807/1.622) instead of the 2..095 tons per square meter of oxygen for Earth. On Mars they need to maintain 5.54 tons per square meter (2.095*9.807/3.711). So they need to create 2.29 times as much oxygen on the Moon. Earth (and so the Moon) gets 2.25 times as much sunlight as Mars. So there isn't much in it.

• CHECKING THE CALCULATION - 100 COMETS - OR 10,000 COMETS??

Let's check his Halley's comet calculation, calculation indented and I'll include all the steps in detail to make it easy to check.

With a radius of the Moon of 1737.4 km I make the surface area of the Moon 4×π×1737.4² = 37,932,328 square kms. For an Earth pressure atmosphere we need (9.807/1.622)×10 tons per square meter, or around 60 tons per square meter, and multiply also by 10⁶ for the number of square meters per square kilometer, that's 37,932,328 × 10⁶×60 = 2.28×10¹⁵ tons or 2.28 quadrillion tons. Halley's comet is 242.5 billion tons. 

So you would need around 2.28 quadrillion/242.5 billion or 9,402 of Halley's comet.

Geoffrey Landis assumes a one psi atmosphere, of pure oxygen, at the Armstrong limit so about 6.9% and works out the total mass needed as two hundred trillion tons or about 825 Halley comets..But he works that out as 50 to 100 Halley comets so must be assuming a larger mass for Halley's comet of two to four trillion tons - it's an early paper from 1990 so I think he is just using older data for Halley's comet.

So, I think Gregory Benford's 100 Halley comets probably comes from Geoffrey Landis's paper. - it means 100 comets the size of Halley but with an older figure for the mass of Halley. And both are assuming the thinnest atmosphere a human can breathe without the moisture lining their lungs boiling.

Approaching it another way, our 2.28 quadrillion tons of atmosphere for an Earth normal atmosphere corresponds to around 2.28 million cubic kilometers of ice assuming average density of 1 (there are a billion tons to a cubic kilometer of water). Or assuming a density of 0.532 tons / cubic meter (same as comet 67p) that's 4.3 million cubic kilometers

So solving for radius, then you get 
π×r³×4/3 =4.3 ×10⁶
So r = cube root(4.3×10⁶×3/(4×π))
= 100 km approx.

So in short, it seems that you need more like 10,000 copies of Halley's comet, or you could hit the Moon with a comet of about 200 km in diameter - or larger if the density is less than 0.532, less if it is more than 0.532. If you did that, you'd have enough material for an instant atmosphere. That is if it is all potential atmosphere, but of course a lot would be water, perhaps 80% which you'd need to convert to atmosphere somehow, perhaps split the hydrogen and oxygen to create an oxygen atmosphere.

If you aim is to make a CO₂ atmosphere, then assuming it is 80% water, then you'd need 

r = cube root(5×3.83×10⁶×3/(4×π))

= 166 km approx. Or 332 km in diameter.

If the aim is nitrogen, with 0.5% of the comet made of nitrogen, and needing 78% of the atmosphere as nitrogen, you are talking about cube root((100/0.5)×0.78×3.83×10⁶×3/(4×π)) or 522 kim in radius, so about 1044 km in diameter. There would then be plenty of water, and carbon dioxide.

Once we can move large comets easily from the outer to the inner soar system, this could be possible selecting a large comet of a suitable composition. You'd have a lot of water as well which would be useful.

• BUFFER GAS - SUCH AS NITROGEN, NEEDED FOR A BREATHABLE ATMOSPHERE

Then - for a breathable atmosphere - then you need to have a buffer gas, which on Earth is nitrogen (CO₂ is poisonous to humans in large concentrations).  You can have a thinner pure oxygen atmosphere with no buffer gas, but this is a fire risk (as we found out in practice with the Apollo 1 disaster), so not likely to be used for terraforming or large scale habitats, though it is used for spacesuits as it reduces the pressure inside the suit so makes them more flexible and easier to use and the fire risk can be managed in a spacesuit by using fireproof materials. It's not really feasible though to make a terraformed Moon in its entirety fire resistant.

So  most of that weight needs to be nitrogen - unless you have some alternative buffer gas. Halley's comet has hardly any ammonia (NH₃). As for Kuiper belt objects, their interior composition is highly varied from rocky all the way to solid ice, 
The compositions of Kuiper belt objects - but I can't find much about the ammonia and nitrogen abundances inside the objects (rather than on the surface). There are some meteorites also that are  rich in nitrates. But finding enough nitrogen might be a problem if that's our buffer gas, seems to me. Titan has a dense nitrogen atmosphere, and is larger than our Moon and has an Earth pressure atmosphere so it has enough nitrogen, it's just in the wrong place and inaccessible from Earth. It doesn't seem practical to transport its atmosphere to the Moon. Also, it's unique and interesting in its own right, as the only moon of its type in our solar system.

So, it seems that we depend on comets. If a comet is only 0.4% ammonia, or nitrogen etc., you need nearly 200 times as many comets for the nitrogen, so two million copies of Halley's comet. Or a comet 6.3 times larger turning our 200 km comet into a 1,260 km diameter dwarf planet.

Maybe you have to hunt around - there are lots of Kuiper belt objects, we only have discovered a tiny fraction of them and maybe one of them has lots of nitrogen? For all we know, maybe when we expand the search, maybe we find tens of thousands of nitrogen rich Kuiper belt objects the size of Halley's Comet - far too small to spot from Earth with existing telescopes? We can't really do a decent calculation here unless someone has a good idea of a source with a well known nitrogen rich composition.

If we find one, then we have to move it into the inner solar system and hit the Moon (gently) before the nitrogen rich ammonia (or more difficult, nitrogen ice) gets a chance to evaporate. If it is a large body and we move it into the inner solar system quickly, this seems feasible, without going into details of the calculation. It would be a bit like storing ice through the summer which they used to do in high latitudes before freezers and refrigerators. Or for that matter, with the level of technology we are imagining here, we could just cover the comet with a reflective layer to keep it cool for the journey.

• REACTION OF OXYGEN WITH THE LUNAR CRUST

Another problem - if you want an oxygen rich atmosphere - well the Earth had reduced iron and it took millions of years to oxidize it before we managed to get an oxygen rich atmosphere. Basically, all the reduced iron has to rust. A process that has already happened on Earth, and on Mars (it's the reason the surface is red) - but not yet on the Moon.

So - same is likely to happen on the Moon. Maybe we can speed it up but for a long time all the oxygen we create will get absorbed by the lunar crust through chemical reactions, as happened on Earth in the early stages of the Great Oxygenation Event

"The upper few kilometers of the lunar surface contain several times 10¹⁸ kg of iron(II) which in the presence of water would readily react with oxygen to form iron(III). Such an amount of iron(II) could easily absorb all of the oxygen in the Earth atmosphere.
 
"A large fraction of the Moons crust consists of oxides of calcium, magnesium, and iron(II), which in the presence of water would react to form hydroxides that would (partly) dissolve in the forming seas to create a poisonously alkaline fluid, with pH 10--11. If enough oxygen were available to oxidize the dissolved iron(II) hydroxides, insoluble iron(III) hydroxides would precipitate on the sea floors and shores, creating vast quantities of slightly poisonous, orange mud. Such reactions would be violent and fast in the upper part of the crust, but their rate would decrease with increasing depth. The oxidizing, hydration, and other processes would continue for ages. In the meantime oxygen and other pressures would not be stable. Most of important all: the absorption of such enormous amounts of oxygen, water, by the upper part of the crust of the Moon would make the rocks expand by perhaps as much as ten percent or more. One can wonder if such expansion would be a tranquil process. It could create strong quakes for possibly many thousands of years. "
from: An Atmosphere for the Moon

So, the upshot of all this is, the terraforming the Moon may well be possible with future technology which may not be that far away, especially with nuclear fusion or such like. But I think it may be a little harder than Greg Benford suggests in his article.

Terraformed lunar far side by Ittiz.

If I've got the figures right here, you need 10,000 Halley comets, or a giant comet 200 km in diameter. If you need to supply nitrogen as a buffer gas from comets with the same composition as Halley, you need two million copies of Halley's comet, or a larger dwarf planet perhaps up to 1,260 km diameter depending on how much nitrogen it has in its composition. After that, you would have many issues with reaction of the water with the dry lunar surface. And then the plants would have to work six times harder than on Earth to produce the same partial pressure of oxygen.

Do be sure to correct me if I have made any mistakes here!

Of course many of these issues would also turn up for Mars, and if you compare it with Mars it doesn't seem so bad.

Mars would need similarly huge amounts of nitrogen for instance, less per surface area but more in absolute terms.

Comparison of mass of nitrogen needed for Mars and the Moon: the Mars surface area is 144.8 km² and for the Moon, 37.9 km². To get the same atmospheric pressure, the Moon has to have 2.29 times as much mass per square meter than Mars. So the amount of mass needed for Mars is 144.8/(37.9* 2.29) so Mars needs 1.67 times the mass for the Moon. Or about 3.34 million Halley comets to supply it with nitrogen, unless it is available indigenously.

The plants have to work six times harder just as for the Moon. The atmosphere lasts longer on Mars, but it's not a permanent feature without megaengineering - it will disappear over millions of years timescales. On Mars you need to have global mirrors or greenhouse gases and still supply some volatiles with comets, for the Moon you don't need to compensate for reduced sunlight but need a constant input of volatiles.

New carbon cycles have to be set in place in both cases to return carbon to the atmosphere and these have to be based on novel principles as Earth's cycles won't work in the same way, especially the long term conversion of limestone back to CO₂ as a result of subduction due to continental drift won't work on the Moon or Mars. As for the Moon, Mars also has deserts which are extremely dry and will take up much of the water if water is added to the planet (though it doesn't have the problem of oxygen reacting with the surface materials). And so on.

So - the Moon might not be so bad if you compare it to Mars, maybe you could terraform it a bit faster as a smaller object needing less total mass, and it doesn't need any supplemented sunlight or greenhouse gases. But it seems an impractically mega project even so with present day technology.

To last longer than a few thousand years, it would need constant maintenance in the form of extra volatiles from comets, which you could do safely by breaking up the comets into small chunks and sending those to hit the lunar atmosphere.But the same is true for ideas of terraforming Mars they need constant maintenance in the form of orbiting mirrors or greenhouse gases and need some resupply of volatiles as well to keep it terraformed (if it worked).

In the case of Mars constant maintenance is needed because the planet is too cold to remain habitable without orbiting mirrors or greenhouse gases, while in the cases of the Moon it is because its gravity can't hold onto its atmosphere. Mars loses its atmosphere also, but on much longer timescales.

However, it's not really that much different.

The timescales are similar too for creating the atmospheres. To create an oxygen rich atmosphere on Mars means sequestering out all the carbon assuming there is enough CO₂ to make an Earth density atmosphere which most think there isn't (at most enough for 10%) and that process would take around 100,000 years using photosynthesis, as a result of which Mars would of course cool down even further without CO₂ to warm it up so need more greenhouse gases or orbital mirrors.

I'm not suggesting we terraform either. I don't think we are anywhere near the stage where it makes much sense to attempt terraforming, a trillion dollars a year project that you have to commit to for thousands of years, whether it is for the Moon or for Mars or anywhere else. We find it hard to commit to a space project for a few decades and a few billion dollars a year. That's apart from planetary protection issues. And as well, we just don't know anything like enough about how ecosystems work, getting unpleasant surprises with "toy ecosystems" the size of Biosphere II, and not able to make even tiny changes to the atmosphere of Earth. If we could make a 0.01% change in the amount of CO₂ in the atmosphere the global warming crisis would be over right away.

However, just as a matter of the physics. the Moon can in principle be terraformed though with many issues you'd have to sort out. But the same is true for Mars. I don't see them as that much different actually. Not with present day ideas of terraforming. We would need to understand this all in a lot more detail than we do now to see which is best, if either can be terraformed in practice. For more on this see: Terraforming and paraterraforming

Terraforming the Moon is another of those topics that doesn't seem to have a lot of attention in the academic literature. But apart from Greg Benford's article, here are forum discussions which are a good source of ideas, though of course not peer reviewed:

• Issues concerning the very difficult terraformation of Luna [Archive]
• Terraforming the Moon - Your opinion, please (Page 1) / Terraformation / New Mars Forums

And then the An Atmosphere for the Moon and there's the Universe Today's HOW DO WE TERRAFORM THE MOON?

However there's another solution,

PARATERRAFORMING THE MOON

Paraterraforming means covering the surface with habitats, eventually domed cities and eventually the entire surface covered in habitats. Eventually they could merge together to make a kind of a sky to hold the atmosphere in - with lots of partitions for safety.

That needs far less atmosphere - because instead of 60 tons of mass per square meter to supply atmospheric pressure - you just have as much air as is needed to fill your greenhouses, which may have heights measured in meters. Even if the greenhouses are a hundred meters high, that's 122.5 kilograms per square meter instead of 60 tons per square meter of air, a huge saving (using density of air of 1.225 kg / m2)

Can a complex closed ecosystem work with a shallower atmosphere like that? They thought it might with Biosphere II but it proved harder than expected, still there doesn't seem to be any major reasons why not. You still have the problem also of oxygen and chemical reactions. But maybe at the same time that you enclose them from above, you can insulate them from the subsurface as well so they don't lose their oxygen through chemical reactions with the lunar soil. One way to do that would be to turn the surface into glass, though you might instead want to use bulldozers to remove a few meters depth of regolith, turn the layer below into glass then replace the regolith to use as soil. This is quite reminiscent of Biosphere II where one of the main reasons it failed was because of chemical reactions involving the concrete the habitat was made of.

This has many advantages

• You need much less by way of air, because it only needs to cover the surface to a depth of meters rather than kilometers.
• You ned much less water, because you can use lined ponds, and generally, insulate surface habitats from the ground, rather than just pour the water onto the surface which is covered to considerable depth with dry regolith over dry rock. Drier even than the Sahara sands.
• Insulating habitats from the ground would also prevent those issues of moon quakes.
• You can reduce losses of water and air. As I said above, a 1 kg loss per person per day would be a thousand tons lost per day. That could be reduced to almost zero by using ionic fluids based liquid airlocks, or other techniques to make sure that almost no water or air is lost when you go in and out, and much improved recycling. Same also if the entire surface is covered in
• If you have really good technology here, to keep air and water inside your habitats, you may even be able to retain parts of the Moon as hard vacuum for factories and processes that need vacuum, or the pristine original surfaces for scientific study.
• You can start by living in the caves, which if they are as large as theory suggests, may be huge O'Neil colony sized habitats already set up for you. Then move up to domed cities in craters. There's a natural progression there on the Moon which could gradually lead to paraterraforming in the future.

I can't find an illustration of a paraterraformed Moon but the same idea can be used for large asteroids and small moons too, if you can tolerate the low gravity. Here is an artist's impression of a paraterraformed Phobos by Ittiz

WE ARE LIKE THE EARLY ANTARCTIC EXPLORERS

I've touched on this comparison a few times already but let's follow it through a bit further this time. We are like the first explorers to get to Antarctica.

To colonize space right now - Mars, the Moon or anywhere - is like Shackleton saying “Oh, we managed to survive a winter here, huddled under a boat and hunting seals, amazing, let’s colonize Antarctica”.

Those remaining on Elephant Island in Antarctica waving farewell to Shackleton and his five crew as he set off in the James Caird boat to find rescue in South Georgia. They managed to survive a winter in Antarctica, but they didn’t say “Oh great, let’s colonize Antarctica :) “ Ernest Shackleton and the Endurance expedition, The voyage of the James Caird, Elephant Island

(they weren’t first to overwinter, that honour goes to the Southern Cross Expedition)

Norway's most significant historic site in Antarctica Southern Cross expedition - first to over winter in Antarctica Southern Cross Expedition

Mars may look more habitable than Antarctica but that’s mainly because it is so dry. If it did have enough water, the whole planet would be covered in a thick sheet of ice. It is very very cold there, especially at night when the air gets so cold that some of it starts to freeze out as dry ice, even in equatorial regions, for many nights of the year forming the ice / CO₂ frosts photographed by Viking.

Actually Antarctica is far more habitable than either Mars or the Moon. It has a breathable atmosphere to start with, it's hard to beat that, no radiation problems, and you don’t have to hold in the air in against an outwards pressure of several tons per square meter. That is why space habitats like the ISS are such massive feats of engineering, with living quarters made up of tubes with rounded ends, or spheres, or in future perhaps, donut shaped space settlements. Everything has to be contained in rounded surfaces of strong materials.

Also they usually have few or tiny windows, because it is a challenge to make a transparent pane to withstand several tons per square meter of outwards pressure. If the interior is pressurized to Earth sea level equivalent, as for the ISS, then it has nearly a ton pressing outwards on a tiny window 30 cms by 30 cms (ten tons per square meter). It's much the same on Mars also, as there's only 1% difference in the amount of outwards pressure on the walls of a habitat on the Moon and on Mars, when pressurized to Earth sea level equivalent.

Even the summit of Mount Everest is far far more habitable than Mars.

Compared to the surface of Mars, the summit of Mt .Everest is a paradise and would be a wonderful place to grow your tomatoes compared to Mars. You can almost breathe the air, with only supplemental oxygen, you don’t have to wear a spacesuit, your lungs won’t be damaged irreversibly with the water lining them boiling, you just need to warm it up and supply a bit of extra oxygen, easy peasy :). Well not really but compared to Mars it is.

If Shackleton's party had had that as their main objective, to "live off the land" they might well have succeeded for a while, killing seals and penguins for food and using their fat for fuel to keep warm, a bit like high tech Inuit. After all Shackleton's party did survive an Antarctic winter while he and five companions set off to find rescue for them in a small open boat. However with early twentieth century technology, however enthusiastic the first settlers were; soon, surely their children would decide they'd had enough, and want to come home, and not continue with the harsh difficult way of life their parents had chosen.

Instead of attempting this, the early Antarctic explorers did their exploring, and scientific study, and then came back to their warm comfortable homes at the end of each expedition. For year after year they continued to explore, built temporary bases, then more comfortable ones, and now we have many bases in Antarctica that are occupied all the year round (though with fewer people there in the Antarctic winter).

So, I think it's the same with space. First, we don't need to colonize space for the human interest. As with Antarctica, there will be plenty of interest with scientific exploration, adventure and tourism. Then, if we go into space to try to colonize right now, we will never succeed. Instead people will just get discouraged.

It is just too hard. Would you colonize a mountain plateau 30 kilometers into the atmosphere, more than three times the height of Mount Everest? At that height you'd have the same atmospheric pressure as Mars, but it would be much more hospitable in other ways. Perhaps it could be done, but why live in a place where everything is so difficult to do? Unless you have some very strong reason for living there, people just wouldn't set up home in a place like that, Not once the novelty wore off. But you'd go there for adventures, you'd have scientists studying, and so on.

There is no sign at all that we have come to an end of the science we can do in Antarctica, or that people will lose interest in going there as tourists, or going on adventures there. Yet, we are nowhere near to starting up a true colony there, and nobody has that as an objective at present.

Maybe eventually we will find a way to live in such harsh conditions as the Moon easily. Maybe we can do this with 3D printers, and biologically closed systems, living in the lunar caves, and eventually on a paraterraformed Moon. Maybe this will lead us to find a way towards a more sustainable future here on Earth as well. Still, if we achieve the ability to do this easily, the technology would still work much better on Earth than in space. You can use much the same design for your colony - except that you can drop all the cosmic radiation shielding, forget about airlocks and spacesuits, and just have windows and doors, instead, which you can walk out of, into a breathable atmosphere.

Put that habitat almost anywhere on Earth, in a desert, or floating on the sea - and that would be a far easier place to live than any space habitat. So if those self sustained habitats became possible in the future, even as a spinoff from space habitat research, I think most of them would be built on Earth rather than in space, at least to start with. Let's meet this miraculous future when it happens, if it does. Meanwhile it's not realistic to send colonists into space in the hope that 3D printers will save the day. They are likely to die waiting for the technology to be invented.

So, in this vision of the future, we have humans in space, but they aren't there to colonize. They are there rather to explore and study, and for adventure and so on, for many of the same reasons we have people in Antarctica. In the near future anyway. I think this is just being realistic; choosing a future that is within our reach rather than a rather beguiling far future science fiction fantasy that is probably centuries or even thousands of years out of reach, such as a terraformed Mars.

In this way, without that imperative that we have to colonize as quickly as possible, and turn everything into the nearest to a pale imitation of Earth as we can manage - then we can have a more open ended future. It gives us space to consider other possibilities; or at least, to look at them. For instance, in one possible future we could introduce Earth life to Mars, accidentally and irreversibly, perhaps from a crashed human occupied spaceship, creating a new geological era on Mars. But we don't have to do that. There is no need quite yet to make such an irrevocable decision for ourselves and all future civilizations on Earth.

Let's study Mars carefully from orbit first. There is no hurry, and this would be a fascinating exploration using telepresence, once we can get humans there. Maybe we will know enough in the future to make such far reaching decisions, but meanwhile, let's keep our options open for the future.