LCROSS (for Lunar Crater Observation and Sensing Satellite) was a NASA space probe observing the Moon, particularly for water Ice in the polar regions, to scout out the possibility of using such water during future missions. It was launched in 2009 into a 37-day orbit around Earth, then crashed into the Moon the same year. At the end of its mission, the rocket stage (a Centaur) that put it in orbit was positioned to crash into the Moon minutes before the spacecraft so the spacecraft could observe and report on the results of the initial impact.
LCROSS was a companion of Lunar Reconnaissance Orbiter (LRO), the pair being two parts of a single NASA project.
Lunar Reconnaissance Orbiter (LRO)