Yes there's a big program underway to do this. By the 2020s we hope to spot almost all meteorites of 140 meters diameter or larger. The B612 foundation is working on a telescope that would fly nearer to the sun, similar orbit to Venus, looking outwards so it can spot really faint meteorites close to the sun. It would spot nearly all meteorites down to 40 meters and many down to 30 or 20 meters diameter.
If that was done we'd know about nearly all the Earth crossing meteorites. But still possible that there are long period comets, and the searches of the Near Earth Asteroids would need to be continued as we'd be sure to miss a few.
Eventually I don't see why we can't know the orbits of just about everything of 20 meters upwards, even smaller, as it gets easier to access space, in near vicinity of Earth.
That leaves long period comets. Most of them are likely to be in inclined orbits, and so not in the same plane as the Earth. And as well as that, Earth is a tiny target. In practice, then the inner solar system hasn't had any big impacts since over 3 billion years ago.
There were huge impacts up to a little over 3 billion years ago:
One of them also created the Hellas Basin on Mars
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)
We have big impacts of that age on all the inner planets.
But we don't get those any more.
The inner solar system is stable now over periods of tens of millions of years at least, up to hundreds of millions of years. Eventually half a billion years from now resonances of Jupiter with Mercury could lead Mercury to leave its orbit, even exit the solar system, which would be sure to lead to some chaos even Mercury could hit Earth, and other possible outcomes. But that's - low probability but also, a long way into the future. For a perspective on those timescales, see Robert Walker's answer to What are the odds that a viable population of humankind will escape to another planet before the earth becomes uninhabitable?
As for comets from the outer solar system, then Jupiter tends to break these up as happened to Comet Shoemaker Levy, through tidal effects
and then they may impact into Jupiter as well.
Those marks on Jupiter were from Comet Shoemaker–Levy 9 - if it wasn't for Jupiter then it would still be orbiting the solar system.
Jupiter either catches them, or breaks them up, or ejects them from the inner solar system, or it diverts them to hit the sun, or it may put them into a short period with its apehlion (furthest point from the sun) at around Jupiter's orbit.
The short period comets there are the main hazard as they can have a similar inclination to Earth, and they do many flybys of the inner solar system so have a better chance of hitting us. These count as "Near Earth Objects" and would be found eventually by the program that's searching for them.
So, there are lots of smaller objects like that. This shows the objects that passed within 20 million kilometers of the Earth in one year in 2007 - 2008
These are the main ones we need to be concerned about.
So - not the "world ending ones" - about zero chance of any of those because they just haven't happened in the inner solar system for 3 billion years.
But dinosaur extinction event type impacts could happen. Still, an impact like that which happens on average every 100 million years - then that means the chance of such an impact in the next century is one in a million.
The smaller city destroying or tsunami causing ones are the most important to track. If we do see one of those big ones, and see it is due to impact Earth, even if it is a million years away, still, it might be that we can do something about that also and put it into a safe orbit. And for the smaller ones kilometer scale and downwards, then - maybe eventually we will find one of those due to impact in the next few thousand years. And when you get to the 100s of meters down to 10s of meters - well the smallest ones hit the Earth all the time, so eventually the tiniest ones, we will predict as we get more observations in a few decades.
We can also spot even the smaller meteorites on their final approach to Earth, this can give up to a day of warning, possibly more, which could be useful to prepare people, tell them to stay indoors and keep away from windows, say, or even evacuate the impact zone. But it depends on what direction they come from and it is easy to miss the smaller meteorites as we saw with the Russian meteorite which nobody saw coming in advance. Eventually though we will have all those meteorites plotted too, sooner if the B612 foundation telescope gets built.
Read more: Is It True That An Asteroid Will Strike Earth On [Insert Date Here]? - Truth Behind Asteroid Scare Stories
(my article about this topic on Science20)
Robert Walker's answer to What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015?
If that was done we'd know about nearly all the Earth crossing meteorites. But still possible that there are long period comets, and the searches of the Near Earth Asteroids would need to be continued as we'd be sure to miss a few.
Eventually I don't see why we can't know the orbits of just about everything of 20 meters upwards, even smaller, as it gets easier to access space, in near vicinity of Earth.
That leaves long period comets. Most of them are likely to be in inclined orbits, and so not in the same plane as the Earth. And as well as that, Earth is a tiny target. In practice, then the inner solar system hasn't had any big impacts since over 3 billion years ago.
There were huge impacts up to a little over 3 billion years ago:
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
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)
We have big impacts of that age on all the inner planets.
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.
The Caloris basin on Mercury.
But we don't get those any more.
The inner solar system is stable now over periods of tens of millions of years at least, up to hundreds of millions of years. Eventually half a billion years from now resonances of Jupiter with Mercury could lead Mercury to leave its orbit, even exit the solar system, which would be sure to lead to some chaos even Mercury could hit Earth, and other possible outcomes. But that's - low probability but also, a long way into the future. For a perspective on those timescales, see Robert Walker's answer to What are the odds that a viable population of humankind will escape to another planet before the earth becomes uninhabitable?
As for comets from the outer solar system, then Jupiter tends to break these up as happened to Comet Shoemaker Levy, through tidal effects
Jupiter either catches them, or breaks them up, or ejects them from the inner solar system, or it diverts them to hit the sun, or it may put them into a short period with its apehlion (furthest point from the sun) at around Jupiter's orbit.
The short period comets there are the main hazard as they can have a similar inclination to Earth, and they do many flybys of the inner solar system so have a better chance of hitting us. These count as "Near Earth Objects" and would be found eventually by the program that's searching for them.
So, there are lots of smaller objects like that. This shows the objects that passed within 20 million kilometers of the Earth in one year in 2007 - 2008
The Animations Page. Note, SIZES NOT TO SCALE - the asteroids, and also Earth also, are far smaller than they appear to be in this animation. You wouldn't even see the Earth if drawn to scale at this resolution (it would need to be drawn to a resolution of 32,000 pixels to show Earth as a single pixel).
These are the main ones we need to be concerned about.
So - not the "world ending ones" - about zero chance of any of those because they just haven't happened in the inner solar system for 3 billion years.
But dinosaur extinction event type impacts could happen. Still, an impact like that which happens on average every 100 million years - then that means the chance of such an impact in the next century is one in a million.
The smaller city destroying or tsunami causing ones are the most important to track. If we do see one of those big ones, and see it is due to impact Earth, even if it is a million years away, still, it might be that we can do something about that also and put it into a safe orbit. And for the smaller ones kilometer scale and downwards, then - maybe eventually we will find one of those due to impact in the next few thousand years. And when you get to the 100s of meters down to 10s of meters - well the smallest ones hit the Earth all the time, so eventually the tiniest ones, we will predict as we get more observations in a few decades.
We can also spot even the smaller meteorites on their final approach to Earth, this can give up to a day of warning, possibly more, which could be useful to prepare people, tell them to stay indoors and keep away from windows, say, or even evacuate the impact zone. But it depends on what direction they come from and it is easy to miss the smaller meteorites as we saw with the Russian meteorite which nobody saw coming in advance. Eventually though we will have all those meteorites plotted too, sooner if the B612 foundation telescope gets built.
Read more: Is It True That An Asteroid Will Strike Earth On [Insert Date Here]? - Truth Behind Asteroid Scare Stories
(my article about this topic on Science20)
Robert Walker's answer to What are the chances of Asteroid 2012 TT5 hitting the Earth on September 24, 2015?
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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