On this day in space! Sept. 10, 2011: NASA launches GRAIL probes to the moon to map lumpy lunar gravity

NASA launched the Gravity Recovery and Interior Laboratory (GRAIL) mission on Sept. 10, 2011, sending twin spacecraft—named Ebb and Flow—aboard a United Launch Alliance Delta II 7925‑H from Cape Canaveral. The pair entered lunar orbit in early 2012 and began a six‑month campaign of precise ranging measurements between the two probes, a technique that allowed scientists to map variations in the Moon’s gravitational field with unprecedented resolution. By charting subtle differences in gravity, the mission produced a high‑definition model of the lunar interior, revealing the thickness of the crust, the distribution of mass beneath the surface, and the presence of ancient impact basins and volcanic structures.

The gravity map generated by GRAIL is critical for both scientific inquiry and future exploration. Detailed knowledge of the Moon’s internal structure informs theories about its formation, thermal evolution, and the dynamics of the early Earth‑Moon system, while accurate gravity data improve navigation and landing safety for upcoming robotic and crewed missions. The mission’s legacy continues as researchers worldwide employ the GRAIL dataset to refine models of planetary formation and to support the planning of sustainable lunar infrastructure.