The GRAIL was an lunar science mission in NASA's Discovery Program which used 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 September 10, 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 arrived at their orbits around the Moon 25 hours apart. The first probe entered orbit on December 31, 2011 and the second followed on January 1, 2012. The two spacecraft impacted the Lunar surface on December 17, 2012.
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