Why do all impact craters appear round as if they all come from strikes that are perpendicular to the surface of the impacted surface?
Shouldn't there be a mix of a few round and many different oval shapes to account for all of the strikes that are glancing blows instead of direct hits? It seems there should be many more indirect hits that produce oval impact craters rather than perfect perpendicular hits which make perfect impact circles. Thanks!
The short answer is that the energy involved in an impact is so huge that when the impactor hits the ground, it explodes like a bomb, rather than just denting the surface like a rock thrown into mud. Explosions are generally symmetric, so the resulting crater from most impacts is circular. Only very very shallow impacts form elliptical craters, but they do exist!
A more extensive answer to this question can be found at Scientific American.
Here is an interesting Planetary Society blog entry about a nice example of an elliptical crater on Mars.
Updated on July 18, 2015