The Gravity Recovery and Interior Laboratory (GRAIL) was a NASA mission utilizing twin spacecraft flying in formation around the Moon. By precisely measuring the changing distance between the two probes, scientists mapped the lunar gravitational field in unprecedented detail. This data revealed the internal structure and thermal history of the Moon.
Heard on the job
The GRAIL data showed us exactly where the mass concentrations are beneath the lunar basins.
Insider tip
The twin spacecraft were famously named Ebb and Flow by elementary school students before they were intentionally crashed into the lunar surface at the end of their mission.