We generally think of craters as being very old, but some are very new. As you may know, the moon is literally covered in pock-marks, each of which was formed when it was hit by an asteroid. But do you know when the most recent strike occurred?
The last asteroid, that we know of, to hit the moon did so only fifty years ago. It was formed in nineteen fifty-three. Just by luck, an amateur astronomer living in Oklahoma was photographing the moon one night, when blammo--he caught on film the most recent known collision of an asteroid with the lunar surface. Today, scientist estimate the explosion was on the order of five hundred thousand tons of TNT. The crater is about 200 feet deep, and the asteroid was probably about 66 feet wide.
Asteroids this size could hit the moon--or the Earth--as frequently as once every ten years. It may very well have happened on the moon since then...only no one was watching.