The LCROSS satellite (Lunar CRater Observation and Sensing Satellite) will crash into the moon Friday morning at 6:31 Central Time. The mission is to determine the amount of water ice in the soil at the south pole of the moon. Here's how:
The LCROSS satellite is attached to the second stage rocket that launched it. Both are on a path that will crash them into the moon. The rocket will separate from the satellite and crash, sending rock and soil upward. Any water ice in the soil will vaporize into H ions and OH ions. These will be detected by the satellite before it crashes 4 minutes later.
You can watch the impact on NASA TV on the internet.