The smaller probe with five cameras and four other scientific instruments is now trailing behind a 2.2-ton empty rocket hull.
That hull will smack into the moon first Friday morning while the smaller probe measures the debris the big hull kicks up. Then the smaller probe, called LCROSS (EL-cross), which is short for Lunar Crater Observation and Sensing Satellite, will hit the moon four minutes later.
Cameras across the world and in space will look at the lunar dirt kicked up and search for some form of water in it.
On the Net
NASA's LCROSS mission: http://www.nasa.gov/lcross