Depends how good your eyesight is. But Vesta is just barely visible to naked eye at its brightest. Vesta and Ceres: How to Spot the Solar System's Biggest Asteroids. Vesta is much smaller than Texas, about a third of its width.
So you'd spot it way out in the asteroid belt beyond Mars as a naked eye object in dark skies.
Anyway no asteroid this big has hit any of Earth, Mars, Venus (as far as we know - its surface record gets erased by massive lava flows every few hundred million years) Mercury or the Moon. Not for well over three billion years since the "late heavy bombardment" that formed the seas and largest craters on the Moon ended.
The reason may be that Jupiter breaks them up or they crash into Jupiter or the sun.
Such large objects must come into the inner solar system from time to time. But they are very very unlikely to have an orbit in exactly the same plane as one of the terrestrial planets, to the accuracy of the diameter of a planet. And extraordinarily unlikely to hit the planet on such an orbit.
They could do so after many repeated flybys of the inner solar system. But in the process, they are far more likely to pass close enough to the giant Jupiter to be deflected by its massive gravitational field, disrupted by its tidal forces, or to hit it.
So in short that Texas asteroid hitting Earth scenario is a movie trope that doesn't match the reality of our solar system. Hugely increased in size for movie thrills. A ten kilometer diameter asteroid is so small you can't see it on a high resolution image of the Earth, only about 1 pixel. So you can understand why they want to make it larger.
And probability increased also - there's only a one in a million chance of an asteroid impact as big as the dinosaur one in any given century. And we have already mapped out all of the asteroids of 10 km or larger out to Jupiter.
So there is no chance of an asteroid even ten kilometers in diameter sneaking up on us with less than a couple of decades or so warning (because it's "year" has to be measured in several decades to be not discovered yet). It rather looks as if we are in one of the 999,999 centuries out of every million that doesn't have a giant impact, on the basis of the information so far.
We are continuing to search for these ten kilometer diameter asteroids - not proved that there is no possibility of any such impact to the end of this century, not yet - but the main attention is now on even smaller asteroids.
The one kilometer diameter search out to Jupiter should be almost completed in the 2020s to at least 99%.
Smaller ones of 100 meters or so are beginning to become the main priority. Those are much more of a challenge and you'd need a space telescope to complete the survey quickly because of the difficulties involved in mapping ones that have most of their orbit between the Earth and the sun and spend a lot of time close to the daytime sun in the sky. Though they are not global in their effect, they can easily destroy any major city and devastate an entire medium sized country, or a US state. Or cause major Tsunamis. Still not likely - no recorded example of this happening throughout written history - but still - far more probable than the larger impacts and potentially very devastating, is worth the time and effort involved to search for them.
Especially as it is the one natural disaster we can actually prevent, given a decent warning like a few decades, to deflect it. Also time it to the minute so you can organize an evacuation if you can't deflect it, you know exactly when it will hit. We could easily map NEARLY ALL OF THESE down to 20 meters within six years, the ones that are in orbits close to Earth, for cost of a bit under half a billion dollars for the B612 space telescope if the funding was made available to do this one way or another.
See also my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them - as a science blog post
and also available as a kindle book
Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)
So you'd spot it way out in the asteroid belt beyond Mars as a naked eye object in dark skies.
Anyway no asteroid this big has hit any of Earth, Mars, Venus (as far as we know - its surface record gets erased by massive lava flows every few hundred million years) Mercury or the Moon. Not for well over three billion years since the "late heavy bombardment" that formed the seas and largest craters on the Moon ended.
The reason may be that Jupiter breaks them up or they crash into Jupiter or the sun.
Such large objects must come into the inner solar system from time to time. But they are very very unlikely to have an orbit in exactly the same plane as one of the terrestrial planets, to the accuracy of the diameter of a planet. And extraordinarily unlikely to hit the planet on such an orbit.
They could do so after many repeated flybys of the inner solar system. But in the process, they are far more likely to pass close enough to the giant Jupiter to be deflected by its massive gravitational field, disrupted by its tidal forces, or to hit it.
So in short that Texas asteroid hitting Earth scenario is a movie trope that doesn't match the reality of our solar system. Hugely increased in size for movie thrills. A ten kilometer diameter asteroid is so small you can't see it on a high resolution image of the Earth, only about 1 pixel. So you can understand why they want to make it larger.
And probability increased also - there's only a one in a million chance of an asteroid impact as big as the dinosaur one in any given century. And we have already mapped out all of the asteroids of 10 km or larger out to Jupiter.
So there is no chance of an asteroid even ten kilometers in diameter sneaking up on us with less than a couple of decades or so warning (because it's "year" has to be measured in several decades to be not discovered yet). It rather looks as if we are in one of the 999,999 centuries out of every million that doesn't have a giant impact, on the basis of the information so far.
We are continuing to search for these ten kilometer diameter asteroids - not proved that there is no possibility of any such impact to the end of this century, not yet - but the main attention is now on even smaller asteroids.
The one kilometer diameter search out to Jupiter should be almost completed in the 2020s to at least 99%.
Smaller ones of 100 meters or so are beginning to become the main priority. Those are much more of a challenge and you'd need a space telescope to complete the survey quickly because of the difficulties involved in mapping ones that have most of their orbit between the Earth and the sun and spend a lot of time close to the daytime sun in the sky. Though they are not global in their effect, they can easily destroy any major city and devastate an entire medium sized country, or a US state. Or cause major Tsunamis. Still not likely - no recorded example of this happening throughout written history - but still - far more probable than the larger impacts and potentially very devastating, is worth the time and effort involved to search for them.
Especially as it is the one natural disaster we can actually prevent, given a decent warning like a few decades, to deflect it. Also time it to the minute so you can organize an evacuation if you can't deflect it, you know exactly when it will hit. We could easily map NEARLY ALL OF THESE down to 20 meters within six years, the ones that are in orbits close to Earth, for cost of a bit under half a billion dollars for the B612 space telescope if the funding was made available to do this one way or another.
See also my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them - as a science blog post
and also available as a kindle book
Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)
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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