On September 10, 2011, NASA successfully launched the Gravity Recovery and Interior Laboratory (GRAIL) mission from Cape Canaveral Air Force Station in Florida. The twin spacecraft, named Ebb and Flow, were stacked atop a Delta II rocket with the objective of creating high-resolution maps of the Moon’s gravitational field to help scientists understand its internal composition.
The mission focused on detecting “mascons,” or mass concentrations, which are dense pockets of material beneath the lunar surface caused by ancient asteroid impacts. These anomalies create localized variations in gravity, making the Moon’s gravitational pull uneven. By measuring these fluctuations from an extremely low orbit, GRAIL provided unprecedented data on what lies beneath the Moon’s crust.
Ebb and Flow traveled in tandem, maintaining distances between 109 and 140 miles (175 to 225 km) while approaching as close as 30 miles (50 km) to the surface. Each probe carried three primary instruments: a Lunar Gravity Ranging System, a Radio Science Beacon, and the MoonKam (Moon Knowledge Acquired by Middle School Students) camera system. MoonKAM was a student-focused experiment developed by undergraduates at the University of California, San Diego, in partnership with the Sally Ride Science foundation to encourage youth engagement in science.
The names for the spacecraft were suggested by fourth-grade students at Emily Dickinson Elementary School in Bozeman, Montana, as part of a national naming competition. This partnership highlighted the educational component embedded within the $496 million mission.
Data from GRAIL confirmed long-held theories about lunar geology. Jay Melosh, a GRAIL co-investigator at Purdue University, stated that the findings proved lunar mascons were formed when large asteroids or comets struck the ancient Moon while its interior was significantly hotter than it is today.
After approximately nine months of mapping, NASA concluded the mission on December 17, 2012, as fuel reserves dwindled and the orbiters could no longer maintain their low trajectory. To prevent uncontrolled crashes that might damage historically significant sites such as Apollo landing zones, controllers guided both probes into a crater rim near the Moon’s north pole.
Twenty-nine dollars and change per kilobyte? I guess you could say GRAIL was a bargain for NASA.
Mascons still fascinate me. The idea that ancient impacts permanently changed the moon’s gravity field is wild.
Wait, they deliberately crashed them to protect Apollo sites? That is actually pretty thoughtful engineering.
I didn’t realize they mapped gravity from just thirty miles up. The precision must have been incredible.
Ebb and Flow are such perfect names for those twin probes. Loved the student involvement story.