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Why are impact craters always circular? © Getty Images

Why are impact craters always circular?

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If you throw a stone into mud at an angle you normally end up with a ‘crater’ that’s elliptical or elongated, but is it the same when it comes to impact craters?

Asked by: Tony Hersh, Newbury

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If you throw a stone into mud at an angle you normally end up with a ‘crater’ that’s elliptical or elongated. It’s natural to suppose the same would be true of a meteoroid hitting the Earth or another planet. But these kinds of impact craters are formed in an entirely different way to the ‘mechanical’ process of a stone hitting mud.

Meteoroids are moving at extremely high velocities (up to tens of kilometres per second). At the moment of impact this enormous kinetic energy is almost entirely converted into heat, which then vaporises the meteoroid instantly. It’s this ‘explosion’ and not the meteoroid itself that creates the impact crater. Since material is ejected equally in all directions, regardless of the direction of travel of the meteoroid, the resulting crater is circular. There can be exceptions to this but only if the impact occurs at an extremely shallow angle.


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