The moon is always getting hit.
Comets stream through our solar system in their own orbits that not on the same plane as the planets. The planets cross through these orbits that are strewn with debris known as meteors. Occasionally collisions occur of either whole comets (as happened with the Shoemaker-Levy Comet crashing into Jupiter) or large meteors, and a planet. This articles explains the origins, life and inevitable end of comets and meteors.
Facts About John Couch Adams and His Contributions.