A good and interesting question. Actually the Japanese probe Kaguya recently ran into the moon on June 10 and the NASA LCROSS's upper stage will do the same on Oct. 9. Ah, gotta love the particle physics way of experimentation, as my Modern Physics professor put it:
"Its like taking a Grandfather Clock and throwing rocks at it. Then figuring out how the clock works for how the rocks bounce off it and how the clock breaks."