The mission, start to impact

Two ships, one ruler

GRAIL did not photograph gravity. It felt it. As Ebb and Flow flew in formation, a mountain or a dense buried mass would tug the lead ship, then the trailing one. The gap between them stretched and shrank. A radio link measured that gap to within a single micrometer, thinner than a red blood cell.

Ebb and Flow exchanging radio signals in lunar orbit

175.000 km

Separation, ship to ship

0.000001 m

Smallest change detected

90%

Instrument software reused from the Earth-orbiting GRACE mission

Illustration: NASA / JPL-Caltech

Start to impact

Fifteen months from a Florida launch pad to a deliberate crash near the lunar north pole. Tap any moment to open it.

By the numbers

Every figure here comes from NASA and the mission record.

December 17, 2012 · 22:28 UTC

Out of fuel and low on orbit, Ebb and Flow were steered into a mountain near the Moon's north pole, seconds apart, at 3,760 miles per hour. The crash site was named for Sally Ride.

◂ EBBFLOW ▸

Their names were not chosen by scientists. Fourth-graders in Bozeman, Montana won a nationwide contest to name them. A mission built, from the very start, around kids who kept asking.