The Gravity Recovery And Interior Laboratory, or GRAIL, flew twin spacecraft in tandem around the Moon to precisely measure and map variations in the Moon's gravitational field. The mission was designed to provide the most accurate global gravity field to date for any planet, including Earth, to reveal differences in the density of the Moon's crust and mantle and help answer fundamental questions about the Moon's internal structure, thermal evolution, and history of collisions with asteroids. The aim was to map the Moon's gravity field so completely that future Moon vehicles can safely navigate anywhere on the Moon’s surface.