High Quality Content by WIKIPEDIA articles! The Gravity Recovery and Interior Laboratory (GRAIL) is an American lunar science mission in NASA`s Discovery Program, which will use high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A and GRAIL B were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10. GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They will arrive at their orbits around the Moon 24 hours apart.