First, real life is a bit different from the movies. The main reason NASA and other space agencies don't have it as a priority is because this is not a probable event, though it is understandable that movie goers might assume it is
We do get smaller asteroids about 10 km or so in diameter every hundred million years, so there is indeed a one in a million chance of one of those in the next century (that's a very tiny chance).
There's no significant chance of an asteroid the size of Texas hitting Earth however.
We know that, because, in the last over 3.5 billion years, there are no craters in the inner solar system large enough to be caused by such a huge asteroid.
There are several impacts that large from the early solar system during the late heavy bombardment. One of them also created the Hellas Basin on Mars
Then on the moon we have:
Notice that they are all from about the same age. I haven't mentioned Venus here, that's because Venus's surface is very young, the entire surface, by its cratering record. It has an unusual geology - we think the entire surface "turns over" every few hundred million years in a period of intense volcanism that happens from time to time. So any large craters on Venus were erased completely long ago.
This is the end of the "Late Heavy Bombardment" which is also when the larger lunar impact craters were formed.
Except we don't have anything quite that old on the Earth either, again because any really old craters would be erased.
For big impacts on the Earth, see Impact of 23-mile-wide asteroid boiled Earth's oceans 3.26 billion years ago (and another link, and scientific paper) which would be after it was tailing off. If anyone knows of any evidence of earlier large craters on Earth do say in the comments.
The details are still to be worked out here. For instance it is a top priority for the Moon, for geologists, to find the exact date of the Aitken basin. We can't do this from orbit, can only get relative datings that way - but need a spacecraft to land there to take a sample and analyse it. Sadly the Apollo datings don't give the information needed.
But at any rate, though we might be unsure of some of the exact dates on timescales of hundreds of millions of years, we do know they are ancient. That's because there are many later craters on top of them, it's clear that all the large craters in the inner solar system are very ancient, well over 3 billion years old.a
Something has to be protecting us. The best hypothesis is that it is Jupiter.
We saw this in action in 2010 with the impact of Shoemaker Levy into Jupiter.
First it got broken up into numerous smaller comets
Then these impacted into Jupiter
So Jupiter probably protects us in both these ways. Any comet coming in to the inner solar system is far more likely to pass within the gravitational influence of Jupiter than any other planet. Because it is so huge. The inner solar system planets are tiny, and it is also very unlikely to be in exactly the same plane as the planets in the inner solar system. Here is the orbit of Halley's comet.
As you see although strictly speaking it is an "Earth crosser" it crosses the Earth's orbit, it does it at a steep inclination and is never much closer to Earth than Venus, no matter where we are in our orbit.
A big comet on its first flyby of the inner solar system is incredibly unlikely to intersect the orbit of one of the inner planets, never mind hit the planet.
It only does that after many flybys of Jupiter which changes its orbit, flattens it out - but also tends to break it up through tidal disruption when it passes close to Jupiter from time to time. And many comets get deflected to either hit Jupiter or the sun.
There are no really huge Near Earth Asteroids either likely to hit us.
Even ones as small as 10 km across are easy to spot at some distance. Mars's outermost moon Deimos is that size, and can be spotted in good decent sized amateur telescopes.
So amateur astronauts could easily track objects this big right across the other side of the inner solar system.
Indeed the known Jovian trojans are about this size, so large telescopes can also spot something like this quite easily as far away as Jupiter's orbit.
433 Eros is the second largest Near Earth Asteroid 34.4×11.2×11.2 km. This is a bit bigger even than the dinosaurs meteorite - and it has a 50% chance of hitting Earth in the next 100 million to billion years.
But it's not a problem for us right now, it can only hit us if its orbit changes quite a bit first.
And - if any large asteroids do hit Earth - it is extremely unlikely that they do it on their first flyby of Earth. Again because Earth is such a tiny target.
They would do numerous flybys of Earth first. And that then is why we'd be able to deflect them quite easily. We'd have decades of warning, and for a really big one ten kilometers across or so, as big as Eros, likely to have thousands of years of warning.
The best time to deflect them is a long way before their next flyby of Earth. Just deflect by a fraction of a meter per second, a few years before they do their next flyby - and then they will miss the "key hole" small region of space that they need to pas through in order to hit the Earth next time around.
Also asteroid detection is not particularly done by NASA. It's an international partnership of numerous countries and telescopes.
Space telescopes are of little value for detecting asteroids as you need wide field telescopes, that photograph large parts of the sky, continuously, throughout the night. Hubble would be absolutely hopeless at that. Instead the first detection of asteroids is done by purpose built ground based Schmidt telescopes (Schmidt telescopes are ideal for clear sharp images over a huge field of view). It used to be done by amateur astronomers, but they have given up on it now because these purpose built asteroid detection instruments are so good at the job.
Then you need to do numerous follow up observations. Many of these are still done by the amateur astronomy community as once you've found your asteroid amongst the numerous faint stars in the sky - they don't need huge telescopes to follow them to see where they move - but rather intermediate sized ones which are affordable by keen amateurs. If you just want to obsere a particular asteroid, you focus on that one with an amateur telescope and plot exactly where it is. So, since there are so many of them, you need lots of telescope time to do this plotting, and that's where the amateurs score, because they can use their telescopes for as long as they like, any clear night, with no competition with other research programs.
Because it is such an international effort, with the amateur astronomers closely involved, there is no way that the process could be classified or hidden in any way. It is all out in the open and publicly shared with everyone.
And - then the very first asteroids we find that we predict to hit the EArth are almost certain to be far smaller, not 10 km, or 1 km in scale.
The large ones are possible. But for each one of those there are many smaller asteroids of the order of tens of meters in size, up to say 100 meters or so.
I'd be very surprised myself if the first successfully predicted asteroid to hit the Earth is larger than, say, 100 meters in diameter. And, probably detected a few decades before it hits.
Now an asteroid that size, it could easily hit us without warning, especially if it does in the next ten years, because we can only see it easily if it does a close flyby of Earth and we haven't been monitoring them for that long. We have actually detected an asteroid a couple of days before it hit the Earth, a very tiny one, first ever to be detected before it hit.
But by the mid 2020s we expect to have found nearly all of the NEAs of this size. So by then, it would be likely that we have decades of warning. And as ones that big hit the Earth every few centuries, chances are we find one that is due to hit us in the next few hundred years, and there is a distinct chance that we find one that large due to hit us in the next century.
So in short, the Armageddon movie is pretty much impossible. It's never happened to any of the inner solar system planets for the last over three billion years. And something that huge, you'd know about it for thousands of years in advance - that is unless it is on first pass through of the inner solar system or on very long period orbit - in which case it is highly unlikely to be in the same orbital plane as the Earth and almost no chance at all of hitting.
Smaller ones are possible, but the most likely ones are much smaller, a hundred meters or less. Perhaps up to 1 km but those are increasingly unlikely. They are serious events, which could destroy a city if they hit in the wrong place (though most likely to hit deserts or the seas as most of the Earth is still uninhabited). These are the ones that are the main focus of Sentinel.
A good example of the type of asteroid we are most likely to predict is 99942 Apophis diameter about 325 meters. At one point it had a 2.7% chance of hitting Earth, until we worked out its orbit more accurately. Now it's thought to have a chance of impact in April 12 2068 of 1 in 149,000. See 99942 Apophis (2004 MN4) Impact Risk. Asteroids this large impact Earth every 80,000 years on average. So it is around the upper end of the range of asteroids that we have a reasonably high chance of predicting in the near future.
We are likely to know about them many decades in advance. That is - unless we just discover them at the last minute with a couple of days to spare. If they are only found as they are approaching Earth, not much we can do at present except to evacuate the impact zone, which we can do.
But by the late 2020s the chance of that will become very small. And the most likely situation is that we have several decades of warning. And in that case, we have plenty of time to work out how to deflect them, or destroy them, and with a 100 meter asteroid that's not going to be a huge challenge, so long as we spot it at least one flyby before the predicted impact.
This is a significant and important natural disaster, needs at least as much attention as volcanic eruptions, tsunami, earthquakes etc. But it is rather unique as just about the only major natural disaster that we actually know how to prevent happening altogether.
While searching for these smaller asteroids, which are the main focus, we will also detect larger ones too. And it will take a fair while before we have close to 100% coverage especially as after we have plotted all the NEAs we then would need to start tracking the more distant short period Jupiter family comets.
If we did spot something really huge headed our way, with only a few years warning, with no previous flybys before it hits - then the best bet, apart from evacuating the impact region, is to use thousand megaton nuclear weapons to either deflect it or destroy it.
But that's such an unlikely scenario. The basic idea is worked out. But it is not NASA's priority or anyone's priority to develop actual missions to launch for such asteroids yet, and I don't think anyone suggests that we should do that. Though there are many theoretical papers that have worked out details of ways that we could deflect asteroids if needed.
Our priority right now is to detect the asteroids. Any deflection missions then would depend on what we find. Which most likely would start with gentle fraction of a meter per second nudges of a 100 meter asteroid due to hit us a few decades from now. Or, we might find there is nothing of any great significance due to hit Earth in the next century. That's entirely possible. May in that case deflect a few smaller ones of a few tens of meters across. The lowest end of the size range there - though they create impressive fireballs which can be as bright as the Moon or brighter - leave hardly any debris to hit the surface, a few meteorites, and typically no crater. We get hit by those regularly.
As we track smaller and smaller asteroids, we'll eventually be able to predict even the larger of these bolide events, up to ones that like the Russian meteorite will break windows and can cause some damage if they happen to pass through the atmosphere close to an inhabited area. Those happen so often that if we get down to that level of sensitivity, we are bound to be able to find a few that will impact Earth - and then - the main question would be whether it is significant enough to do anything about it, such as deflect it, or evacuate the impact zone - or just to ask the people to keep away from windows and look out for a bright fireball at the predicted time.
I think these movies are good in the sense that they do highlight the impact risk, which we need to do something about. But they do also tend to greatly exaggerate the size of the asteroids likely to hit Earth, for dramatic effect and artist's license doubtless. They tend to focus attention towards really huge ones when it is the medium to small ones that need most attention, the larger 10 km ones need some attention but will be discovered anyway as a part of the search for the smaller ones.
And we can, and are doing something about it, with detection the obvious way to spend funding for this risk, at this stage. For not that much funding, we can detect nearly all the near earth asteroids by the late 2020s. That's obviously got to be everyone's priority as the way to spend the available budget, and that is what they are doing, and the money stretches a long way as a lot of the tracking work particularly is done by volunteer amateur astronomers. If we do find any that are on a trajectory to hit Earth, small ones most likely, of course the priorities would change then. But the astronomers concerned would also probably get more funding as well, enough to mount space missions and so on to actually deflect them (space missions, even small scale ones, are still very expensive, and there are lots of competing projects, so you need a strong motivation before you can get funding for them).
You might enjoy this video on the subject of asteroid deflection:
and
The ESA is currently looking into plans for a mission to impact a meteorite to study effects of kinetic impact directly.
AIDA (mission)
The idea is to impact the smaller of two asteroids in a binary system - and then see the effect of the impact on its orbit around the larger asteroid. It would test our ability to deflect asteroids in this way by a kinetic impact. The impact would happen in 2022 during a close approach to Earth. Neither asteroid has any chance of hitting Earth, and there is no risk of the test deflecting either to hit Earth.
There are many papers on deflection strategies, and on the keyhole effect. And some of the ideas (not all of them) are summarized in the wikipedia article Asteroid impact avoidance which also has many links to the literature on this subject.
This 2006 study by NASA is a good overview:
2006 Near-Earth Object Survey and Deflection Study
See also: Robert Walker's answer to Is the act of drilling into an Earth-bound asteroid to blow it up with a nuclear weapon (as seen in Armageddon) technically feasible? If so, how much would it cost and how many nukes would you need? where I talk a bit more about ways of deflecting them.
And also What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015? where I talk about ways that we detect these asteroids and ways that we can detect more of them in the future and detect them a long time in advance.
We do get smaller asteroids about 10 km or so in diameter every hundred million years, so there is indeed a one in a million chance of one of those in the next century (that's a very tiny chance).
There's no significant chance of an asteroid the size of Texas hitting Earth however.
We know that, because, in the last over 3.5 billion years, there are no craters in the inner solar system large enough to be caused by such a huge asteroid.
There are several impacts that large from the early solar system during the late heavy bombardment. One of them also created the Hellas Basin on Mars
Those impacts undoubtedly happened in the past.
Impact from 3.8 billion years ago when large asteroid impacts were still common. 3D map of Mars
Then on the moon we have:
The Aitken basin at the lunar South pole. It's believed to be over 3.8 billion years but the exact date is hard to pin down. Impact of an asteroid perhaps 170 km in diameter.And on Mercury we have:
Caloris Basin Mercury, 3.8 to 3.9 billion years old
Notice that they are all from about the same age. I haven't mentioned Venus here, that's because Venus's surface is very young, the entire surface, by its cratering record. It has an unusual geology - we think the entire surface "turns over" every few hundred million years in a period of intense volcanism that happens from time to time. So any large craters on Venus were erased completely long ago.
This is the end of the "Late Heavy Bombardment" which is also when the larger lunar impact craters were formed.
Except we don't have anything quite that old on the Earth either, again because any really old craters would be erased.
For big impacts on the Earth, see Impact of 23-mile-wide asteroid boiled Earth's oceans 3.26 billion years ago (and another link, and scientific paper) which would be after it was tailing off. If anyone knows of any evidence of earlier large craters on Earth do say in the comments.
The details are still to be worked out here. For instance it is a top priority for the Moon, for geologists, to find the exact date of the Aitken basin. We can't do this from orbit, can only get relative datings that way - but need a spacecraft to land there to take a sample and analyse it. Sadly the Apollo datings don't give the information needed.
But at any rate, though we might be unsure of some of the exact dates on timescales of hundreds of millions of years, we do know they are ancient. That's because there are many later craters on top of them, it's clear that all the large craters in the inner solar system are very ancient, well over 3 billion years old.a
Something has to be protecting us. The best hypothesis is that it is Jupiter.
We saw this in action in 2010 with the impact of Shoemaker Levy into Jupiter.
First it got broken up into numerous smaller comets
Then these impacted into Jupiter
So Jupiter probably protects us in both these ways. Any comet coming in to the inner solar system is far more likely to pass within the gravitational influence of Jupiter than any other planet. Because it is so huge. The inner solar system planets are tiny, and it is also very unlikely to be in exactly the same plane as the planets in the inner solar system. Here is the orbit of Halley's comet.
As you see although strictly speaking it is an "Earth crosser" it crosses the Earth's orbit, it does it at a steep inclination and is never much closer to Earth than Venus, no matter where we are in our orbit.
A big comet on its first flyby of the inner solar system is incredibly unlikely to intersect the orbit of one of the inner planets, never mind hit the planet.
It only does that after many flybys of Jupiter which changes its orbit, flattens it out - but also tends to break it up through tidal disruption when it passes close to Jupiter from time to time. And many comets get deflected to either hit Jupiter or the sun.
There are no really huge Near Earth Asteroids either likely to hit us.
Even ones as small as 10 km across are easy to spot at some distance. Mars's outermost moon Deimos is that size, and can be spotted in good decent sized amateur telescopes.
Indeed the known Jovian trojans are about this size, so large telescopes can also spot something like this quite easily as far away as Jupiter's orbit.
433 Eros is the second largest Near Earth Asteroid 34.4×11.2×11.2 km. This is a bit bigger even than the dinosaurs meteorite - and it has a 50% chance of hitting Earth in the next 100 million to billion years.
But it's not a problem for us right now, it can only hit us if its orbit changes quite a bit first.
And - if any large asteroids do hit Earth - it is extremely unlikely that they do it on their first flyby of Earth. Again because Earth is such a tiny target.
They would do numerous flybys of Earth first. And that then is why we'd be able to deflect them quite easily. We'd have decades of warning, and for a really big one ten kilometers across or so, as big as Eros, likely to have thousands of years of warning.
The best time to deflect them is a long way before their next flyby of Earth. Just deflect by a fraction of a meter per second, a few years before they do their next flyby - and then they will miss the "key hole" small region of space that they need to pas through in order to hit the Earth next time around.
Also asteroid detection is not particularly done by NASA. It's an international partnership of numerous countries and telescopes.
Space telescopes are of little value for detecting asteroids as you need wide field telescopes, that photograph large parts of the sky, continuously, throughout the night. Hubble would be absolutely hopeless at that. Instead the first detection of asteroids is done by purpose built ground based Schmidt telescopes (Schmidt telescopes are ideal for clear sharp images over a huge field of view). It used to be done by amateur astronomers, but they have given up on it now because these purpose built asteroid detection instruments are so good at the job.
Then you need to do numerous follow up observations. Many of these are still done by the amateur astronomy community as once you've found your asteroid amongst the numerous faint stars in the sky - they don't need huge telescopes to follow them to see where they move - but rather intermediate sized ones which are affordable by keen amateurs. If you just want to obsere a particular asteroid, you focus on that one with an amateur telescope and plot exactly where it is. So, since there are so many of them, you need lots of telescope time to do this plotting, and that's where the amateurs score, because they can use their telescopes for as long as they like, any clear night, with no competition with other research programs.
Because it is such an international effort, with the amateur astronomers closely involved, there is no way that the process could be classified or hidden in any way. It is all out in the open and publicly shared with everyone.
And - then the very first asteroids we find that we predict to hit the EArth are almost certain to be far smaller, not 10 km, or 1 km in scale.
The large ones are possible. But for each one of those there are many smaller asteroids of the order of tens of meters in size, up to say 100 meters or so.
I'd be very surprised myself if the first successfully predicted asteroid to hit the Earth is larger than, say, 100 meters in diameter. And, probably detected a few decades before it hits.
Now an asteroid that size, it could easily hit us without warning, especially if it does in the next ten years, because we can only see it easily if it does a close flyby of Earth and we haven't been monitoring them for that long. We have actually detected an asteroid a couple of days before it hit the Earth, a very tiny one, first ever to be detected before it hit.
But by the mid 2020s we expect to have found nearly all of the NEAs of this size. So by then, it would be likely that we have decades of warning. And as ones that big hit the Earth every few centuries, chances are we find one that is due to hit us in the next few hundred years, and there is a distinct chance that we find one that large due to hit us in the next century.
So in short, the Armageddon movie is pretty much impossible. It's never happened to any of the inner solar system planets for the last over three billion years. And something that huge, you'd know about it for thousands of years in advance - that is unless it is on first pass through of the inner solar system or on very long period orbit - in which case it is highly unlikely to be in the same orbital plane as the Earth and almost no chance at all of hitting.
Smaller ones are possible, but the most likely ones are much smaller, a hundred meters or less. Perhaps up to 1 km but those are increasingly unlikely. They are serious events, which could destroy a city if they hit in the wrong place (though most likely to hit deserts or the seas as most of the Earth is still uninhabited). These are the ones that are the main focus of Sentinel.
A good example of the type of asteroid we are most likely to predict is 99942 Apophis diameter about 325 meters. At one point it had a 2.7% chance of hitting Earth, until we worked out its orbit more accurately. Now it's thought to have a chance of impact in April 12 2068 of 1 in 149,000. See 99942 Apophis (2004 MN4) Impact Risk. Asteroids this large impact Earth every 80,000 years on average. So it is around the upper end of the range of asteroids that we have a reasonably high chance of predicting in the near future.
We are likely to know about them many decades in advance. That is - unless we just discover them at the last minute with a couple of days to spare. If they are only found as they are approaching Earth, not much we can do at present except to evacuate the impact zone, which we can do.
But by the late 2020s the chance of that will become very small. And the most likely situation is that we have several decades of warning. And in that case, we have plenty of time to work out how to deflect them, or destroy them, and with a 100 meter asteroid that's not going to be a huge challenge, so long as we spot it at least one flyby before the predicted impact.
This is a significant and important natural disaster, needs at least as much attention as volcanic eruptions, tsunami, earthquakes etc. But it is rather unique as just about the only major natural disaster that we actually know how to prevent happening altogether.
While searching for these smaller asteroids, which are the main focus, we will also detect larger ones too. And it will take a fair while before we have close to 100% coverage especially as after we have plotted all the NEAs we then would need to start tracking the more distant short period Jupiter family comets.
If we did spot something really huge headed our way, with only a few years warning, with no previous flybys before it hits - then the best bet, apart from evacuating the impact region, is to use thousand megaton nuclear weapons to either deflect it or destroy it.
But that's such an unlikely scenario. The basic idea is worked out. But it is not NASA's priority or anyone's priority to develop actual missions to launch for such asteroids yet, and I don't think anyone suggests that we should do that. Though there are many theoretical papers that have worked out details of ways that we could deflect asteroids if needed.
Our priority right now is to detect the asteroids. Any deflection missions then would depend on what we find. Which most likely would start with gentle fraction of a meter per second nudges of a 100 meter asteroid due to hit us a few decades from now. Or, we might find there is nothing of any great significance due to hit Earth in the next century. That's entirely possible. May in that case deflect a few smaller ones of a few tens of meters across. The lowest end of the size range there - though they create impressive fireballs which can be as bright as the Moon or brighter - leave hardly any debris to hit the surface, a few meteorites, and typically no crater. We get hit by those regularly.
As we track smaller and smaller asteroids, we'll eventually be able to predict even the larger of these bolide events, up to ones that like the Russian meteorite will break windows and can cause some damage if they happen to pass through the atmosphere close to an inhabited area. Those happen so often that if we get down to that level of sensitivity, we are bound to be able to find a few that will impact Earth - and then - the main question would be whether it is significant enough to do anything about it, such as deflect it, or evacuate the impact zone - or just to ask the people to keep away from windows and look out for a bright fireball at the predicted time.
I think these movies are good in the sense that they do highlight the impact risk, which we need to do something about. But they do also tend to greatly exaggerate the size of the asteroids likely to hit Earth, for dramatic effect and artist's license doubtless. They tend to focus attention towards really huge ones when it is the medium to small ones that need most attention, the larger 10 km ones need some attention but will be discovered anyway as a part of the search for the smaller ones.
And we can, and are doing something about it, with detection the obvious way to spend funding for this risk, at this stage. For not that much funding, we can detect nearly all the near earth asteroids by the late 2020s. That's obviously got to be everyone's priority as the way to spend the available budget, and that is what they are doing, and the money stretches a long way as a lot of the tracking work particularly is done by volunteer amateur astronomers. If we do find any that are on a trajectory to hit Earth, small ones most likely, of course the priorities would change then. But the astronomers concerned would also probably get more funding as well, enough to mount space missions and so on to actually deflect them (space missions, even small scale ones, are still very expensive, and there are lots of competing projects, so you need a strong motivation before you can get funding for them).
You might enjoy this video on the subject of asteroid deflection:
and
The ESA is currently looking into plans for a mission to impact a meteorite to study effects of kinetic impact directly.
AIDA (mission)
The idea is to impact the smaller of two asteroids in a binary system - and then see the effect of the impact on its orbit around the larger asteroid. It would test our ability to deflect asteroids in this way by a kinetic impact. The impact would happen in 2022 during a close approach to Earth. Neither asteroid has any chance of hitting Earth, and there is no risk of the test deflecting either to hit Earth.
There are many papers on deflection strategies, and on the keyhole effect. And some of the ideas (not all of them) are summarized in the wikipedia article Asteroid impact avoidance which also has many links to the literature on this subject.
This 2006 study by NASA is a good overview:
2006 Near-Earth Object Survey and Deflection Study
See also: Robert Walker's answer to Is the act of drilling into an Earth-bound asteroid to blow it up with a nuclear weapon (as seen in Armageddon) technically feasible? If so, how much would it cost and how many nukes would you need? where I talk a bit more about ways of deflecting them.
And also What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015? where I talk about ways that we detect these asteroids and ways that we can detect more of them in the future and detect them a long time in advance.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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