This is a question that is frequently asked on Quora, with a different date each time. We get a fair number of quite worried people asking this question, in all seriousness, concerned that Earth is about to be hit by a giant impactor. Sometimes they have read sensational stories by online papers that should know later.

It is easy to keep up to date with potential impact dates by visiting this page, automatically updated for the Sentinel program: Current Impact Risks. Just look and see if there are any entries coloured orange or red. Then look for the predicted date of impact. So far this has never happened.

If (as is usual) all the entries are coloured blue, white or green then there is no confirmed impact threat for the next 100 years It's quite common for an asteroid to reach level 1, green briefly.

If any entry is yellow, then it merits attention by astronomers. It may also be of significance to the public if the potential impact is less than a decade away - but chances are it will soon be reassigned to level 0. This has happened in the past. The Near-Earth Asteroid 2004 MN4 briefly reached level 4 in 2004 so well into the yellow section, setting a record, but was soon reassigned first to level 1 and then by 2006, to level 0. See 99942 Apophis

Small Objects	Torino Scale Colors
Estimated diameter 50 meters or less.	0	1	2,3,4	5,6,7	8,9,10

Eventually they are bound to predict some very small asteroid that will hit the Earth as we are hit by meter scale objects all the time.

You usually get these scare stories soon after an asteroid is discovered. That's because with only a few observations, you haven't got enough data to work out its trajectory accurately, and for a while it may seem to have a low probability of hitting Earth at a particular date, perhaps after a couple of close flybys. Later on as you constrain the orbit with more observations you find out that it is less likely to hit Earth. Then, almost always, finally you find it has no chance at all of hitting us.

There's one exception there, one of the objects (29075) 1950 DA , large enough to do serious damage possibly even global in its effects (1.1 to 1.4 kms in diameter) had a 1 in 300 chance of impacting earth in 2880. After the orbit was refined, the risk went down. However, it still has a 1 in 20,000 chance of hitting Earth. But not an immediate worry as this event is nearly nine centuries from now, 865 years from now. It's not included in the Sentry tables because they only list potential impacts for the next century.

Another object (89959) 2002 NT7 a little smaller but large enough to have potentially global effects, had a risk of 1 in a million of hitting Earth in February 2019, for a while, but it is now known it will miss.

Another object 2013 TV₁₃₅ when first discovered, with only one week of observations, had a 1 in 63,000 chance of hitting Earth in 2032, leading to headlines such as Massive asteroid that could hit earth in 2032 with force 50 times greater than biggest nuclear bomb, it's now known it will miss by 0.76 AU on that date (three quarters of the distance to the sun).

Details: Near-Earth object

The journalists often use spectacular images like this to illustrate the articles:

or

To put it in perspective, if you had an image of the Earth with 1600 pixels resolution for its diameter, like a high resolution computer screen, then these asteroids would be a little over 1 pixel in diameter. So you can understand why they don't use realistic images - you wouldn't be able to see the impactor at all.

The dramatic images you see of huge impactors almost moon sized hitting the Earth are artist drawings of the impacts that happened in the early solar system. Chances of one of those is too tiny to consider and we'd know about it long in advance if there was anything. For instance Mercury being deflected from its orbit by the perturbations of Jupiter - may happen about half a billion years from now and if it did head for Earth it would be devastating - but not something we need to worry about. Long enough for humans to evolve a second time from primitive microbes.

This is a more accurate image, where they show it glowing but don't show the impactor at all, and close up image of the Earth

That's 2013 TV₁₃₅ the one that we now know will be over three quarters of the distance to the sun away from Earth on that date.

That's a good example of the way the probabilities progress after discovery.

The thing is that when an asteroid is first discovered, and especially if it does close flybys of Earth, then it is very hard to estimate its long term trajectory. Just small changes in position for one of those flybys could lead all your projections to be out from then on.

So - it's fairly common for an asteroid to have a chance of hitting the Earth when it is first discovered. But that risk normally goes away when the orbit is refined with more observations.

RISK OF DINOSAUR EXTINCTION TYPE EVENTS

There definitely is a risk of an object this size hitting Earth, but you get a "dinosaur killer" type event only every few tens of millions of years.

So if you ask for a best guess about when we are likely to be hit by an asteroid that large, then the answer is, probably ten million years from now upwards.

Since there are far more of the smaller impacts, I expect the first successful prediction of an impact on the Earth would be something like the Russian meteorite - an impactor maybe of the order of up to 100 meters diameter or so, and you tell people to stay indoors and avoid windows in the target region or at worst might have to evacuate some place.

Impact craters as large as this are likely to be created every 10 to 100 years

Diameter about 45 meters and this one may have been created by an incoming iron meteoroid only 1.3 meters in diameter some time in the last few thousand years - one of the very few small impact craters known on the Earth. Pristine Impact Crater Discovered in Egypt Desert (first discovered on google maps :) ).

It's a challenge to track things as small as that. But as we get to smaller and smaller impactors, then eventually we are bound to predict a small impact like this.

So just by probability, the most likely first confirmed predicted impact would be a small impactor just because they are so common.

And it's likely to be in an uninhabited area, of desert, mountains, tundra etc, because most of the Earth's surface is uninhabited.

FIRST SUCCESSFUL IMPACT PREDICTION

And as it happens we have managed to predict one impact. And as you'd expect as most likely, it was a small impact, into an uninhabited region (in this case the Sudan desert)

This is 2008 TC3

80 tonnes 4.1 meters in diameter, hit in the Sudan desert and about 600 meteorite fragments were recovered. It was detected only 19 hours before impact, but is first successful prediction of an impact before it happened.

It was used as a way to test the process of tracking NEOs on impact trajectory to Earth and many observations were made. Wikipedia (which is good on this topic area) summarizes it like this:

It was notable as the first such body to be observed and tracked prior to reaching Earth.[6] The process of detecting and tracking a near-Earth object, an effort sometimes referred to as Spaceguard, was put to the test. In total, 586 astrometric and almost as many photometric observations were performed by 27 amateur and professional observers in less than 19 hours and reported to the Minor Planet Center, which issued 25 Minor Planet Electronic Circulars with new orbit solutions in eleven hours as observations poured in. On October 7, 01:49 UTC,[9] the asteroid entered the shadow of the Earth, which made further observations impossible.

It would be a similar process if we found a big asteroid headed our way.

COLLABORATION PROCESS FOR A BIG ASTEROID

Amateurs and professionals would collaborate to make as many observations as possible to refine the orbit until we know exactly where it is headed and all we can find out about it. Astronomy is interesting as an area of science where amateurs are of great importance Modern telescopes owned by amateurs are very capable.

And while they don't have giant telescopes of tens of meters in diameter, they do have them up to a meter or so in diameter, and some a bit larger. And have the advantage that there are many amateurs to take observations when the pros are few in number by comparison.

BTW here is a telescope built by an amateur astronomer (truck driver) using a mirror from an old spy satellite he got hold of, that was nearly 2 meters in diameter (70 inches). He had to silver it himself. It is basically a giant "dobsonian" steered by hand I think.

At the time anyway, says he was the largest amateur built telescope in he world.

see: Utah truck driver builds worlds largest amateur telescope

Here is a list of some other large amateur telescopes larger than 1 meter in diameter: Largest Amateur Telescopes

Astronomy is one of the few areas of science where amateurs continue to make significant on going contributions through their own observations. There are so many events and objects to observe that the very few big telescopes can't possibly handle all the observations.

It would be absurd to track a newly discovered asteroid with say the Hubble space telescope and such like. They would be sure to rely on the amateur community as well as pro astronomers world wide.

Many of the smaller ones break up as they approach the Earth like this dramatic "fireball" type meteor. Photos were taken of the fireball from space

and 600 fragments recovered

I expect many more events like this before we get any predictions of a major impact.

We are bound, eventually, to have a prediction of an object of Tungaski sized or above. But it will probably be for many centuries or a thousand years into the future. Just by probabilities.

There are ways we can deflect an asteroid, especially given lots of time we could deflect even a kilometer scale one. Even by a measure as simple as painting one of its hemispheres white.

As for NASA somehow being able to predict a major impact that nobody else knows about, forget about it, it's an absurd idea :).

SUMMARY CONCLUSION

There is nothing currently in the list of Current Impact Risks that has any probability at all of hitting Earth in the next year. None at all. Nothing even big enough to be hazardous to a city or cause a tsunami.

We will get many smaller impacts up to say ten meters or so - we get those all the time. They usually burn up in the atmosphere causing a spectacular but harmless fireball, occasionally some remnant makes it to the ground.

There is a chance of larger impacts up to hundreds of meters, and a tiny chance of larger impacts than that because its work in progress. It will take a while before they have them all mapped out. The plan is to have nearly all mapped out by the 2020s down to a few hundred meters.

A world ending impact - big enough to create the Hellas basin on Mars or the Lunar Mares is not going to happen anyway. Nothing that big has hit us for billions of years, so the probability is about zero. We can tell this from the impact history of the inner solar system on the Moon and Mars where we can see it all clearly laid out plain to view. This is probably because Jupiter disrupts any large incoming objects from the outer solar system so that they break apart, or hit Jupiter or do close passages of the sun and break up there or hit the sun. And while we are not protected in the same way from the larger inner solar system objects like Ceres, Vesta etc, they are all now in stable orbits.

For dinosaur extinction scale impacts - i.e. that many humans could survive because of our technology but dinosaurs couldn't - chances are minute of that happening in the near future, say before 2200.

Impacts as big as that happen every few million years. So, the best guess, at this stage before they complete the survey is that we probably have to wait at least a few million years before we have an impact as big as that.

With thousands, or millions of years to prepare for the impact, we may be able to divert it quite easily. Even with a century or two we may find a way to divert it, or if worst comes to the worst, have many decades to prepare e.g. building shelters etc to protect us, or identifying areas of the world that are safe to migrate to until the impact is over.

The dinosaurs didn't have submarines or boats or the ability to build shelters or use fire proof materials or, if it comes to it, oxygen breathers to last out a firestorm.

Many creatures did survive the big Chicxulub crater impact, including mammals probably by burrowing underground, and fish, and birds - perhaps by flying out to sea. With our submarines and boats and ability to travel to anywhere in the world - even if you have to move the entire population of the world out to sea, or to Antarctica to survive it - we could do that, especially given a few centuries to prepare. We could also build up food supplies to last out the years of "nuclear winter" after the impact itself.

But given a long enough lead time, we can hope to divert it, with many ways this could be done. See Asteroid impact avoidance for some of the suggestions about how to do this.

RISK OF DINOSAUR EXTINCTION TYPE EVENTS

FIRST SUCCESSFUL IMPACT PREDICTION

COLLABORATION PROCESS FOR A BIG ASTEROID

SUMMARY CONCLUSION

See also

KINDLE EBOOK

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