Source - NASAtelevision on Sep 22, 2011:
Did our planet once have two moons? Some researchers say so. Moreover, the missing satellite might still be up there--splattered across the far side of the Moon. NASA's GRAIL mission could help confirm or refute the "two moon" hypothesis:
Companion article: GRAIL and the Mystery of the Missing Moon
Visit http://science.nasa.gov/ for more.