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. The Gravity Recovery and Interior Laboratory is a NASA lunar science mission, which uses high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow) were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10.   GRAIL A entered moon orbit on 31 December 2011 and GRAIL B followed on 1 January 2012. Wiki