Craters on planets

[kwicky]

Why is it whenver craters are shown on a planet the hit is always round (asteroids are irregular in shape) and hit at a tangential angle (ie 90% to the surface)?

[dundurn]

They're not always round but they are more often than you'd think. See this link for some explanations and some good examples.

http://www.straightdope.com/mailbag/mcraters.h tm

[nucleardream]

Wouldn't the central tendency of the planet 's core directly attract stellar debris? The gravity is strongest from the core due to it representing the most mass (e.g. looking down at a spherical object has the most distance viewing it straight down as opposed to viewing it's edge). The asteroid would thus naturally hit the planet from a 90% angle due to this phenomenon. That would also explain the typical round shape of the crater.