The Gravity Recovery and Interior Laboratory (GRAIL) mission utilizes the technique of twin spacecraft flying in formation with a known altitude above the lunar surface and known separation distance to investigate the gravity field of the moon in unprecedented detail. The technique utilizes radio links between the two spacecraft as well as radio links to stations on Earth. The mission also will answer longstanding questions about Earth's moon, including a possible inner core, and provide scientists with a better understanding of how Earth and other rocky planets in the solar system formed. GRAIL is a part of NASA's Discovery Program.