NASA Near Earth Asteroids and Comets Sentry Table - As words instead of techy numbers

Dates for harmless objects
Clickable name for harmless objects (to click on for more info)
Show number of asteroids removed in each past year - and number of asteroids left in the table for each future year (number of possible impacts)

Search for asteroid or comet

If the object you searched for isn't in the removed list or the table, this means its harmless. If it is a real Near Earth Object, you can click the "page at ESA" link to find out more. Normally it will say "not in risk list".

Note - some removed asteroids aren't in either list (Example, 1999 AN10) - perhaps because they were removed before the online sentry table was started?

Roll dice to get an idea how unlikely this is

Calculate diameter from brightness and albedo

(back to Search for asteroid or comet)

This is only relevant if the albedo isn't known - CNEOS assumes 14% if not known. For details of how this works see the CNEOS albedo calculator

Absolute magnitude:
(a measure of how bright it would be at the same distance from Earth as the sun, see absolute magnitude - a bigger number here means a dimmer asteroid)

Albedo %
(How dark or shiny it is, range 0% to 100%, most asteroids 3% to 22% rarely 1% to 90%)

Calculated diameter for this albedo = meters.

Most asteroids are either
1. carbonaceous 0.03-0.09
2. silicaceous 0.1-0.22
3. metallic 0.1 - 0.18

What is a Potentially Hazardous Asteroid (PHA)?

(back to Search for asteroid or comet)

You may have come here after reading a story in the press about a PHA. Despite the name MOST OF THESE ARE NOT HAZARDOUS TO US. Most can't hit us for centuries or millennia. Also they can't "change course" and hit us. It is just a tech term for an asteroid that comes within a certain distance of Earth's orbit. Most will eventually hit the Sun or Jupiter or be ejected from the solar system after many flybys of other planets. A few percent of them will hit Earth or other planets some time in the next several tens of millions of years. See PHA below.

How effects and average wait time to next asteroid of this size are calculated

(back to Search for asteroid or comet)

The paragraph about tsunamis in the search result is, based on Near and far-field hazards of asteroid impacts in oceans" March 2019 in Acta Astronautica.. For my summary of relevant parts of that paper see: More details about the splosh, and Asteroid Generated Tsunami: Summary of NASA/NOAA Workshop. The modeling is tricky with few complete examples worked out yet, so this is just a rough idea.

The links for the effects for the asteroid impacts take you to the Imperial College London / Purdue University Impact Effects calculator.

The section on global effects is based on an article by Michael Paine which is from 2004, but I can't find much more recent that gives a suitable summary.

The estimated time before the next impact of this size on average is based on table 2.1, page 26, of the 2018 Report of the Near-Earth Object Science Definition Team.

If anyone reading this knows of better sources to use for any of this information please let me know (support@robertinventor.com). Thanks!

Sentry table - Options - Search for asteroid or comet - More table options - About - Torino scale - Removed - Removed options - Did you know - Convert AU - Support - Contact

More table Options

(see also the Options below the table)

Max number of objects to list: (0 to show all)
Far future objects

Hide objects with first possible impact after date
Hide objects smaller than size in meters
NOTE: most diameters shown in the sentry table are estimates, based on the average brightness of asteroids when lit up by sunlight (their "albedo").

Add - HARMLESS
Palermo values (how it compares with the background rate)
Sort by maximum palermo value for single impact
(when unselected,, sorts by combined palermo value for all its possible impacts through to 2100)

OK Cancel message for the randomized warnings

Sentry table - Options - Search for asteroid or comet - More table options - About - Torino scale - Removed - Removed options - Did you know - Convert AU - Support - Contact

About this table

It is using the same data as the more geeky NASA Sentry table which this page accesses using their published Sentry API.

I just present it in a way that is more friendly for less geeky readers. For instance, instead of just giving a "0" for the Torino value, I say in words what it means, "No hazard" and added "- HARMLESS" just to help make it really clear and be easy to see on the page. For the ones with no Torino value, instead of leaving it blank I label them as FAR FUTURE. For the Palermo value I translate it into a number that shows how less likely it is than the background rate.

This is what the geeky Sentry table says if you know how to read it.

I do this to help the many not very techy people who find the official version scary because they read the sensationalist press stories about "NASA Asteroid warnings" and don't see anything to contradict them because it is just a mass of numbers for them.

I've also added details of how many dice you’d need to throw and get them all as sixes to be equivalent to this happening. I find this works well when explaining the table to non techy scared people.

I've also taken the opportunity to do a unified search for the Search for asteroid or comet that shows you whether an object is in the table, or removed, and if it is neither of those, provides a link you can use to look for its JPL Small-Body database page as well as the list of observations in the IAU Minor Planet center. That's to help for the fake asteroid warnings such as for instance the Daily Express warning that Vesta was "approaching Earth"! I've also added extra links e.g. to show the size of crater, fireball, air burst etc on Bing Maps via the Imperial College London / Purdue University interactive project.

Torino scale

How to use the techy numbers in the Sentry table: Entries are ordered with the highest risk, if any at the top of the page. You will see a number in the final column under Torino scale. This refers to the number in this table.

It's usually 0 (NO HAZARD) or 1 (NORMAL). Up to level 4 it's only of interest to astronomers - there is nothing merits attention of the public. A warning at level 4 it only merits public attention if the encounter is less than a decade away.

What I did with this page is just to convert those numbers into words in the last column of the table.

Far future entries - beyond 100 years - Bennu and 1950 DA

The top two entries (as of writing this, March, 2022) are Bennu and 1950 DA. The NASA table is sorted by the Palermo scale which includes very low probability far future events. Bennu and 1950 DA both have a blank in the Torino scale column in the techy table because the Torino scale is only used for impacts in the next 100 years.

Bennu has only 1 chance in 2.5 million for an impact in 2178. This means that it's a near certainty we prove it misses as we get closer to the date. In the remote chance it's still in the list we have lots of time to deflect it. We might even mine it away to nothing and use it to make radiation shielding for space habitats or to mine for minerals or both.

The reason Bennu is right at the top of the list is because it's quite big, at 490 meters in diameter. It is very rare for an object that big to hit Earth.

At that size Bennu is similar in effect to a particularly destructive hurricane, and with plenty of time for everyone to evacuate in the very remote chance that we don't prove it misses before 2178, find it is going to hit, and decide not to deflect it.

The Palermo scale shows how much less likely the calculated chance is than the background rate for impacts for undiscovered objects.

Bennu is 39 times less likely than the background rate for impacts between now and 2178. That's the highest palermo value in the list (as of writing this) so it's at the top of the list. But objects this large are very rare and only hit every few tens to hundreds of thousands of years. So the background rate is very low. So even 39 times less likely than the background rate means very very unlikely.

That's why it's at the top of the list even though we calculate only a 1 in 2.6 million chance it can hit based on the measurements so far.

Similarly 1950 DA, which is larger at 1.3 km has a very minute chance for 2880 but again near certainty that at some point in the next few centuries we prove it misses. It's likely hundreds of thousands to millions of years to the next impact at this size

Removed Near Earth Asteroids and Comets

Removed options

Clickable name
Add e m o j n
(adds extra links below each object, for page on ESA, Minor planets center observations, orbit at JPL, JPL page and NEOdys-2 page)

Did you know

If the Egyptians had build an asteroid detection telescope back at the time of the Great Pyramid of Giza, over 4,500 years ago - it would still be waiting for its first asteroid to hit a city

A miss is a miss

I am often contacted by people who are scared that an asteroid might hit Earth thousands of miles away or harm them by just flying past a million kilometers away in space. But for an asteroid a miss is a miss.

It is like a passing train. It doesn't matter if you are standing just next to the line or a hundred miles away. So long as you are not standing on the track the train can't harm you.

It is the same if it hits Earth. It doesn't matter if you are a thousand miles from the impact point or ten miles. So long as you are far enough away to not be hit by it directly or affected by the fireball, it does nothing to you. Even the bigger ones with global effects - it means things like a few months cooler than normal (at the lowest end of the range) because of dust in the atmosphere, not that they could kill you from a distance.

Earth is a tiny target in the vastness of space. Asteroids pass at a great distance many times a week. From time to time you get "Sensationalist press pick a random harmless asteroid". There is no reason to run a story about it at all, perhaps they had a slow news day. When they run out of other stories they seem to just say "Oh well let's take a random asteroid - those always get lots of clicks and views"

It is normal for the "chance" of an asteroid hitting to increase until eventually it is removed

NOTE: if the chance of impact seems to increase, this is normal and doesn't mean it is going to hit. It is just the chance based on observations so far and it is normal for it to increase as more observations are made until the object is removed meaning there never was any possibility of a hit.

Here are their graphics. Initially 10%. I think the position of the asteroid in these figures is the posistion of closest approach. It isn’t shown to scale, compared to Earth. It’s hugely magnified otherwise you wouldn’t see it as it would be a tiny fraction of a pixel.

Earth will still be within the error margin for the flyby. But the Earth is now within a much smaller error margin, which covers a much smaller region of space.

For simplicity let's suppose the region of space it could fly through is circular - or an ellipse , stretched out circle in shape and it's reduced by the same amount in all directions. Then the area is reduced 4-fold so the "chance" of impact increases 4-fold.

Earth is still within this area since in reality it passes by at 300,000 km. So it now has a ten times smaller area of space to pass through than before when we knew the flyby only accurate to a million kilometers.

So it is still in the risk list and its "chance" based on the observations so far has increased ten-fold.

At this point they abruptly they remove it from the list because in reality it flies past at 300,000 km so now they know it misses by at least 50,000 km.

So you expect chance to go up as they refine measurements then suddenly fall to zero once they know the flyby accurately enough.

Then the chance of an impact may increase a million-fold before it is finally removed.

In this case we might not know for some time because it takes a very long observation arc to get it accurate to within 1000 km.

With a long enough arc eventually we get it accurate to within 10s of kms or less and we'd know, so long as we are able to keep observing it.

We do occasionally get atmosphere grazing asteroids, so as we continue to bring online our telescopes and space telescopes that can detect fainter and fainter objects, it is not impossible that we predict an asteroid that grazes Earth's atmosphere in advance, especially a small fireball sized one like this one.

It can also get removed suddenly by a prediscovery observation which someone spotted from say a decade ago which gives a much longer observation arc.

It's also possible to get more complex patterns like that, for instance if there are many possible flybys and whether it hits later in the century depends on exactly where it flies past earlier in the century. That's because each flyby changes the direction slightly and so the orbits and flybys for the rest of the century depending on where it goes past in that first flyby. Apophis for instance had to fly through a small "keyhole" in 2029 to hit in 2036. They quickly proved it couldn't hit in 2029 but it took a while to show it would also miss in 2036, then after that there was a small chance for 2060, and then finally they removed it altogether.

What is a potentially hazardous asteroid (PHA) or comet?

Short summary: it means it has an orbit that comes within a certain distance of Earth's orbit. It does not mean it is a real hazard. For instance 2010 TK7 (150 to 500 meters in diameter) orbits permanently a third of an orbit ahead of Earth. It is no danger to us because it is always a third of an orbit ahead of us. However, because it is constantly tracing the same orbit as us, it gets this classification PHA. As that example shows a PHA doesn't even need to come close to Earth, just close to its orbit.

There are many others that sometimes cross our orbit but always at great distances from Earth and they also are PHA’s but no threat to us.

Most usually they have a few percent chance of hitting us some time in the next several tens of million of years but can’t hit us right now.

If it is not in the Sentry table it can’t hit us this century and most likely can’t hit us for many centuries.

Whenever they do close flybys of Earth this deflects them into slightly different orbits. It is really hard to hit Earth because it is so tiny, so they usually will end up being deflected away into other orbits that are far from Earth. Earth clears its orbit over a period of a few tens of millions of years, most of them being sent elsewhere.

Most of them will eventually hit the sun after many flybys of several planets, or hit Jupiter or be ejected from the solar system.

A few percent eventually hit other planets and that includes some that will eventually hit Earth, most likely thousands or millions of years into our future.

Most come from the asteroid belt originally, and a few, less than one percent, come from beyond Jupiter after first getting deflected by Jupiter into short period Jupiter family comets.

The sensationalist press often run stories about PHA’s. If it is not in the sentry table then it is not a risk.

Then click through to the ESA page and if they have it, it will say “not in risk list”

But if they don’t have it (as in this case) you can click through to the JPL page and they will usually have it, this is mainly just to check you entered it in correctly - if you got the name right and it isn’t in the Sentry table or removed objects list then it was never considered to be any risk to Earth.

What is a NEO?

Meanwhile a NEO just means an object whose perihelion (closest point to the sun) is less than 1.3 au, i.e. 1.3 times Earth's orbital radius. Most are not even PHA's.

According to one estimate, the largest undiscovered NEO is likely to be about 3.5 km.

The table is complete from 10 km upwards (the size of asteroid that ended the dinosaur era). There are only four asteroids and four comets at this size.

The only other NEO of 10 km across or larger that seems to have a chance of hitting Earth even thousands of years into the future is the asteroid 433 Eros (16.84 km).

It could hit us some time after a million years from now, with a 5% chance, but is not likely to hit us before 100,000 years from now.

How does the Sentry table work?

The Sentry table is automated. New objects go their automatically as soon as they are identified as a NEO. The name consists of the year it was first observed, e.g. 2007 then a code of two letters and a number. The first letter gives the half month in which it was found, e.g. A is Jan 1-15, B is Jan 16 - 31 and so on. So F is Mar 16 - 31. Then the last bit, say, T3 in 2007 FT3, is assigned in a cycling system, first asteroid in that half month gets letter A, through to 25th gets letter Z (omitting the letter I), then it goes through A1 to Z1, and so on.

When they get the first observations, often they don't know if it is an asteroid or not (might just be a data glitch). If so, it goes into the Scout table where it gets a temporary name. As soon as it is confirmed to be a NEO it is then transferred to the Sentry table if there is any risk, or just to the JPL and ESA tables. When an object is first discovered then the orbit is not well known, and there are many possible simulated orbits ("virtual orbits"). Only one of them is the real orbit but which it is isn't known at this stage. The impacts in the risk list are for simulated orbits.

When an object is removed from the table it's usually because the orbit is better known and the simulated orbits that would impact can be ruled out.

The calculations also take account of all the ways asteroids and comets can minutely shift in their orbits.

Smaller asteroids can change orbit over years to decades due to minute effects such as absorbing sunlight and re-radiating it as heat in a different direction as it spins. In this case many of the virtual orbits can be removed once we know the amount of this effect better.

Comets can change orbit slightly due to outgassing and jets. Again this leads to many virtual orbits and once we observe what a comet does during a flyby of the sun we can then remove many virtual orbits for the next time around.

The usual reason an object gets removed is just because its orbit is better known.

Why small asteroid tsunamis don't travel far

The reason that tsunamis from earthquakes travel so far is because a large area of the floor of the ocean moves up or down suddenly. An earthquake can shift an area a hundred kilometers across or more. The resulting tsunami typically has a wavelength of 100 km or more, and that's how they travel so far inland, as this vast wave surges in towards a coast. The wavelength is also typically more than twenty times the depth of the sea, which lets them travel far without dissipating.

This gives the wave far more energy than a small asteroid can deliver. The short wavelength waves from a smaller asteroid splosh are only hundreds of meters or kilometers in diameter. These dissipate quickly, can't travel far across the sea, easily break up if they approach shallower water, and can’t travel far inland because the trough is only a short way behind the crest.

For all except the larger asteroid impacts, any waves dissipate rapidly. You get a big splosh that can shoot up to several kilometers into the atmosphere, but almost no waves. The crater that forms in the sea rapidly fills back inwards on itself, and any waves that ripple outwards don't travel far.

The transient crater in the water depends on the angle and speed, but as a very rough guide you are talking about a crater of a few kilometers wide for an impactor of up to 500 meters in width, and the depth depending on the angle. However a stony asteroid of 200 meters diameter or less would break up in the atmosphere and have almost no effect because the airburst energy is largely reflected by the sea. So for small asteroids, any tsunami risk is only for iron asteroids.

If the wavelength is less than a tenth of the ocean depth there is very rapid dissipation (inversely proportional to distance traveled). If the wavelength is more than 20 times the ocean depth then there is almost no dissipation. Earthquake tsunamis are typically 100 km or more, and so fall into this category of ones with almost no dissipation.

So you have those three things for a small splosh iron meteorite, the waves because they are much shorter in wavelength than the depth of the sea, disperse a fair bit before they get to the shore. Then whatever is left because it is short wavelength is likely to break as it bunches up, and whatever is left after that because it is short wavelength doesn't travel far up onto the land before it is caught up by the trough following behind and stops.

So there isn't much risk of tsunami in most cases, but that small splosh would be spectacular to watch from the air at a safe distance. In a simulation of a 85 meter impact of an iron meteorite, the splash curtain mainly fell back into the transient crater, but the central rebound jet rose several kilometers into the atmosphere and the first rebound wave was 200 meters high before breaking. About 1% of the impact energy was turned into a one meter high tsunami but with short wavelengths, which when it came on shore would be not much different from a storm surge.

Debunks for individual objects

These are some of the stories that have scared enough people so that I needed to do a special debunk for them. If you enter any of these objects into the search box under Search for asteroid or comet, then it will add a link to my debunk

Convert AU to kilometers, miles, light minutes and light seconds

1 au was originally defined as the average distance from the sun to the Earth. It's now defined as 149597870.7 kilometers exactly. See Astronomical unit (Wikipedia). You can use this calculator to convert a number in au into kilometers, miles or light minutes and seconds.

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This page developed by Robert Walker - any questions, suggestions, mistakes (however minor) or bug reports contact me via support@robertinventor.com

No Hazard (White Zone)	0	The likelihood of a collision is zero, or is so low as to be effectively zero. Also applies to small objects such as meteors and bodies that burn up in the atmosphere as well as infrequent meteorite falls that rarely cause damage.
Normal (Green Zone)	1	A routine discovery in which a pass near the Earth is predicted that poses no unusual level of danger. • Current calculations show the chance of collision is extremely unlikely with no cause for public attention or public concern. •New telescopic observations very likely will lead to re-assignment to Level 0, "NO HAZARD".
Meriting Attention by Astronomers (Yellow Zone)	2	A discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to re-assignment to Level 0, \"NO HAZARD\".
	3	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of localized destruction. Most likely, new telescopic observations will lead to re-assignment to Level 0, \"NO HAZARD\". Attention by public and by public officials is merited if the encounter is less than a decade away.
	4	A close encounter, meriting attention by astronomers. Current calculations give a 1% or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to re-assignment to Level 0, \"NO HAZARD\". Attention by public and by public officials is merited if the encounter is less than a decade away.
Threatening (Orange Zone)	5	A close encounter posing a serious, but still uncertain threat of regional devastation. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than a decade away, governmental contingency planning may be warranted.
	6	A close encounter by a large object posing a serious but still uncertain threat of a global catastrophe. Critical attention by astronomers is needed to determine conclusively whether or not a collision will occur. If the encounter is less than three decades away, governmental contingency planning may be warranted.
	7	A very close encounter by a large object, which if occurring this century, poses an unprecedented but still uncertain threat of a global catastrophe. For such a threat in this century, international contingency planning is warranted, especially to determine urgently and conclusively whether or not a collision will occur.
Certain Collisions (Red Zone)	8	A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offshore. Such events occur on average between once per 50 years and once per several 1000 years.
	9	A collision is certain, capable of causing unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.
	10	A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

NASA's Near Earth Asteroids and Comets Sentry Table

Options

Search for asteroid or comet

Roll dice to get an idea how unlikely this is

Calculate diameter from brightness and albedo

What is a Potentially Hazardous Asteroid (PHA)?

How effects and average wait time to next asteroid of this size are calculated

More table Options

About this table

Torino scale

Far future entries - beyond 100 years - Bennu and 1950 DA

Removed Near Earth Asteroids and Comets

Removed options

Did you know

A miss is a miss

It is normal for the "chance" of an asteroid hitting to increase until eventually it is removed

What is a potentially hazardous asteroid (PHA) or comet?

What is a NEO?

How does the Sentry table work?

Why small asteroid tsunamis don't travel far

Debunks for individual objects

Convert AU to kilometers, miles, light minutes and light seconds

Support for the easily scared

SEVEN TIPS FOR DEALING WITH DOOMSDAY FEARS

FACEBOOK SUPPORT GROUP

OUR PETITIONS

Contact