Neil Armstrong and Buzz Aldrin landed far from where they had anticipated because, 50 years ago, we didn't have a complete understanding of the Moon's gravity. Recent maps of the Moon's gravity have taught us a lot about its overall shape, and have been invaluable for lunar exploration. Maria Zuber, principal investigator of NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, reflects on the twin spacecraft and their implications for future investigations.​