NASA’s GRAIL mission mapped the Moon’s gravity field using two spacecraft flying in formation in 2012.
GRAIL launched as two nearly identical spacecraft named Ebb and Flow. Their separation changed slightly as they passed over regions where lunar gravity was stronger or weaker. By measuring those changes precisely, scientists created an extremely detailed gravity map.
The mission’s name stands for Gravity Recovery and Interior Laboratory. Its results revealed previously unknown structures beneath the lunar surface, including ancient impact basins and crustal variations. The measurements also helped scientists estimate the Moon’s internal composition and thermal history.
Ebb and Flow intentionally impacted the Moon on December 17, 2012, after completing their primary science work. They were later named for pioneering astronaut Sally Ride. GRAIL is sometimes confused with Earth-orbiting GRACE, which studied changes in Earth’s gravity and water distribution.