As large as Texas? No chance at all. There has never been an impact that large for over 3 billion years in the entire inner solar system.
We can't tell with Venus because it's entire surface is very young. Something possibly a global volcanic upheaval, resurfaced it in is entirety a few hundred million years ago. That's probably because it is an Earth sized planet, very active inside, hot outside, and no continental drift so not easy for heat to escape - it builds up and eventually the whole surface turns over.
So Venus does have catastrophic planet resurfacing events every few hundred million years, we believe, but from internal processes. (No huge impact craters).
Apart from that, Mercury, Mars, our Moon and Earth itself show no evidence of large impacts in the last several billion years with impactors larger than about 10 km or so maybe up to 20 or 30 max. Similar size to the one that brought an end to the dinosaur era.
They have many very large craters, e.g. Hellas basin on Mars, Caloris basin on Mercury, and even larger features like the lunar mares, and even the entire northern hemisphere of Mars may be depressed by some very giant impact in the past. These were created by giant asteroids as large as in the movies or larger, but are all well over three billion years old.
Hellas Basin on Mars
- but those all date back to the late heavy bombardment, when our solar system was still settling down, mostly ended already about 3.8 billion years ago.
Since then, only had the 10 km or so, occasionally a bit larger.
These impacts would be easily survivable by some humans at least, with our technology. Birds, mammals, turtles, alligators - many creatures survived them and what a bird or turtle can survive, some humans with technology could survive also. They won't make us extinct.
But - they are also extremely unlikely, even those. The largest ones happen once every hundred million years, so that's a one in a million chance of it happening this century. A one in a million probability is usually thought of as "negligible".
They are definitely worth paying attention to, because so devastating even if so unlikely. But not likely. And they would do many flybys of Earth, almost certainly, before they hit us. So we'd get plenty of time to work out how to deflect them.
It's just incredibly unlikely that a 10 km asteroid will be discovered headed straight for Earth with only a few months notice, we are a tiny target and it is far more likely to miss us first time around and do dozens of flybys first.
As for Texas sized - well they would be visible beyond the orbit of Jupiter in the larger amateur telescopes. Amateurs can spot the two moons of Mars, and they are only city sized, ten kilometers or so scale.
And before they can be a really significant threat they have to be deflected into an orbit in the same plane as the inner solar system, so likely to do many flybys of Jupiter first, and in the process they'll almost certainly pass close enough so they hit Jupiter, or get deflected to hit the Sun, or deflected out of the solar system altogether, or get torn apart by tidal effects into smaller 10 km class asteroids. That's probably why there have been no large impacts anywhere in the inner solar system for several billion years.
The movies are great for dramatization. But the most important asteroids for us to try to spot right now are rather smaller. The aim is to try to find nearly all the asteroids in Near Earth Orbits down to perhaps as small as 20 meters diameter by the late 2020s. And since there are far more of those than there are 10 km or larger ones, and most of the larger ones already discovered - then I think it is a reasonable bet that the first successful prediction of an asteroid due to hit Earth large enough to form a crater will be perhaps 50 to 100 meters max in diameter. And the chances are that it does many flybys of Earth before it hits. That then makes it easy to deflect because each time it does a flyby, a tiny delta v some time before it reaches Earth, of only a fraction of a meter per second, can send it on a significantly different trajectory after is flyby.
So, if you ask me to guess, I'd guess that the first accurate newspaper story about an asteroid prediction will be "50 - 100 meter asteroid been predicted to hit Earth in 2100" or some such - long distance in the future, small asteroid. And eventually we might get "20 meter asteroid to hit Earth in 2040" or some such - and "Watch out for a bright fireball if you are in Siberia on 1st July" or whatever - likely to be over a remote unpopulated area because most of the Earth's surface is sea or unpopulated even today.
Far more likely than "10 km asteroid to hit Earth in six months - evacuate Australia and help store food to last through some years of poor crops worldwide" - that would be very surprising.
And "Texas sized asteroid to hit Earth within six months" - I just can't see that happening at all, because it seems from the cratering record that we are protected - perhaps by Jupiter and have been for the last over three billion years.
See also : How big would a meteorite have to be in order to be a "planet killer"?
Is NASA presently capable of fending off a collision if a meteor or any other celestial body were to strike the earth?
We can't tell with Venus because it's entire surface is very young. Something possibly a global volcanic upheaval, resurfaced it in is entirety a few hundred million years ago. That's probably because it is an Earth sized planet, very active inside, hot outside, and no continental drift so not easy for heat to escape - it builds up and eventually the whole surface turns over.
So Venus does have catastrophic planet resurfacing events every few hundred million years, we believe, but from internal processes. (No huge impact craters).
Apart from that, Mercury, Mars, our Moon and Earth itself show no evidence of large impacts in the last several billion years with impactors larger than about 10 km or so maybe up to 20 or 30 max. Similar size to the one that brought an end to the dinosaur era.
They have many very large craters, e.g. Hellas basin on Mars, Caloris basin on Mercury, and even larger features like the lunar mares, and even the entire northern hemisphere of Mars may be depressed by some very giant impact in the past. These were created by giant asteroids as large as in the movies or larger, but are all well over three billion years old.
Hellas Basin on Mars
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)
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 those all date back to the late heavy bombardment, when our solar system was still settling down, mostly ended already about 3.8 billion years ago.
Since then, only had the 10 km or so, occasionally a bit larger.
These impacts would be easily survivable by some humans at least, with our technology. Birds, mammals, turtles, alligators - many creatures survived them and what a bird or turtle can survive, some humans with technology could survive also. They won't make us extinct.
But - they are also extremely unlikely, even those. The largest ones happen once every hundred million years, so that's a one in a million chance of it happening this century. A one in a million probability is usually thought of as "negligible".
They are definitely worth paying attention to, because so devastating even if so unlikely. But not likely. And they would do many flybys of Earth, almost certainly, before they hit us. So we'd get plenty of time to work out how to deflect them.
It's just incredibly unlikely that a 10 km asteroid will be discovered headed straight for Earth with only a few months notice, we are a tiny target and it is far more likely to miss us first time around and do dozens of flybys first.
As for Texas sized - well they would be visible beyond the orbit of Jupiter in the larger amateur telescopes. Amateurs can spot the two moons of Mars, and they are only city sized, ten kilometers or so scale.
And before they can be a really significant threat they have to be deflected into an orbit in the same plane as the inner solar system, so likely to do many flybys of Jupiter first, and in the process they'll almost certainly pass close enough so they hit Jupiter, or get deflected to hit the Sun, or deflected out of the solar system altogether, or get torn apart by tidal effects into smaller 10 km class asteroids. That's probably why there have been no large impacts anywhere in the inner solar system for several billion years.
The movies are great for dramatization. But the most important asteroids for us to try to spot right now are rather smaller. The aim is to try to find nearly all the asteroids in Near Earth Orbits down to perhaps as small as 20 meters diameter by the late 2020s. And since there are far more of those than there are 10 km or larger ones, and most of the larger ones already discovered - then I think it is a reasonable bet that the first successful prediction of an asteroid due to hit Earth large enough to form a crater will be perhaps 50 to 100 meters max in diameter. And the chances are that it does many flybys of Earth before it hits. That then makes it easy to deflect because each time it does a flyby, a tiny delta v some time before it reaches Earth, of only a fraction of a meter per second, can send it on a significantly different trajectory after is flyby.
So, if you ask me to guess, I'd guess that the first accurate newspaper story about an asteroid prediction will be "50 - 100 meter asteroid been predicted to hit Earth in 2100" or some such - long distance in the future, small asteroid. And eventually we might get "20 meter asteroid to hit Earth in 2040" or some such - and "Watch out for a bright fireball if you are in Siberia on 1st July" or whatever - likely to be over a remote unpopulated area because most of the Earth's surface is sea or unpopulated even today.
Far more likely than "10 km asteroid to hit Earth in six months - evacuate Australia and help store food to last through some years of poor crops worldwide" - that would be very surprising.
And "Texas sized asteroid to hit Earth within six months" - I just can't see that happening at all, because it seems from the cratering record that we are protected - perhaps by Jupiter and have been for the last over three billion years.
See also : How big would a meteorite have to be in order to be a "planet killer"?
Is NASA presently capable of fending off a collision if a meteor or any other celestial body were to strike the earth?
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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