NASA's GRAIL (Gravity Recovery and Interior Laboratory) twin spacecraft await launch atop a Delta II rocket at Cape Canaveral Air Force Station, Florida. GRAIL is a lunar science mission in NASA's Discovery program. It will use high-quality gravitational field mapping of the Moon to determine its interior structure. The two spacecraft, GRAIL A and GRAIL B, were launched on September 10, 2011. 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.