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.

In brief5 min read

Twin spacecraft Ebb and Flow launched in September 2011 and orbited the Moon in tight formation.
They mapped the Moon's gravity by tracking tiny changes in the distance between them, down to a micrometer, revealing a thin, cracked crust and the dense mascons buried under the maria.
Out of fuel, both were deliberately crashed into a mountain near the north pole in December 2012, at a site later named for Sally Ride.

Ebb and Flow exchanging radio signals in lunar orbit

~175 km

Average 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.

See all the figures

What GRAIL taught us

The gravity map was never the goal. It was the instrument. Reading it revised what we thought we knew about the Moon.

Crust

Thinner, and cracked through

GRAIL put the Moon's crust at roughly 34 to 43 km on average, thinner than earlier estimates, and so fractured by ancient impacts that it reads as measurably porous.Wieczorek et al., Science, 2013

Procellarum

Buried rifts under the storms

Beneath Oceanus Procellarum, gravity exposed long straight structures: ancient magma-filled dikes set in a giant rectangular pattern, a sign the early Moon expanded and split.Andrews-Hanna et al., Nature, 2014

Mascons

Why basins ring like bullseyes

GRAIL data plus modeling explained the bullseye gravity of the maria: dense mantle rebounding after a giant impact, then lava loading the center. The pattern now helps date basins.Melosh et al., Science, 2013

The people who showed up

GRAIL was never only scientists. Days before launch, the public who traveled to Kennedy Space Center gathered around the NASA insignia for a photo. This is the same mission that would soon hand its cameras to classrooms.

A large group of NASA Social attendees posed around the NASA insignia on the floor at Kennedy Space Center before the GRAIL launch in September 2011

NASA Social attendees at Kennedy Space Center, September 7, 2011. The marked face is me, in the crowd for the launch. Photo: NASA/Bill Ingalls, source · CC BY-NC-ND 2.0, used unmodified.

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.