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Mars has far less water and a much thinner atmosphere than Earth... even assuming that once the atmosphere there was warmer and wetter, a hole as big as the one we're seeing in this simulation wouldn't erode away even over millions of years. If that happened to Mars, we'd have learned about it from the Mariner missions in the 70's.
Makes you think... even the impact crater in Antarctica - the one believed to have caused the Permian extinction - and the crater at the floor of the Gulf of Mexico don't begin to compare to this simulation's result. We're talking a major collision here, even on the scale of the solar system.
Thankfully, the number of objects in the solar system of a given size appears to vary inversely with the size of those objects. There are plenty of 1/2-mile sub-asteroidal objects, but comparatively few objects of even a 30-mile size, and only about three dozen exceeding 1000 miles. Moreover, the scale of space is kind of dizzying. To imagine the odds, imagine randomly firing BB's into a space the size of the Astrodome, and hitting a moving billiard ball somewhere in that space. Not only that, but you can only fire 10,000 BB's, and there are 40 other billard balls floating around, in addition to the one you're concerned with hitting. It could happen, but the odds are pretty low (or we'd be dead already). There are other factors involved as well, but most of those tend to decrease rather than increase the odds of such a collision. Not that it can't happen - certainly it could - but the odds are, well, remote.
Also, from the simulation, it should be painfully obvious that no underground shelter would be safe; the seismic event accompanying such an event would probably cause a cave-in, and the heat induction would almost certainly roast the shelter's inhabitants fairly quickly, regardless of their location on earth. A space station wouldn't even be safe because there's no way to know how the ejecta plume would behave, or where such a putative strike would be. As with all titanic events, if you're close enough to view it with the unaided eye, you're probably entirely too close.
Peace PsA
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