How twin GRAILs use gravity to map the Moon’s interior

From ARS Tecnica:

This variation is reflected in the gravitational pull exerted by these mountains, though you’d never notice the difference. The difference in the thickness of tectonic plates produce a variation in Earth’s gravitational field, for example, but Earth-orbiting satellites typically don’t need to take that into account.

However, measuring these fluctuations can reveal a lot about the interior of a rocky world. The GRAIL mission (Gravity Recovery And Interior Laboratory), launched in 2011, is designed to map the details of the Moon’s composition with an eye toward understanding both its interior and its history. GRAIL’s design is based on an earlier Earth-orbiting mission known as GRACE (Gravity Recovery And Climate Experiment), a collaboration between the United States and Germany in 2002.

Both GRAIL and GRACE consist of two satellites orbiting in tandem. Each uses a laser to measure the distance to the other. The relative position of the satellites can map the fluctuations in the gravitational field to higher precision than a single instrument would be able to do.

Each GRAIL spaceship is about 200kg in mass and the size of a refrigerator. The craft follow a nearly polar orbit, looping around the Moon such that, as it rotates on its axis, GRAIL was able to take measurements of the entire body in swaths (akin to the segments of an orange).

As the two satellites—known as GRAIL-A (“Ebb”) and GRAIL-B (“Flow”)—fly over the Moon’s surface, they speed up or slow down relative to each other, as variations in the density of the lunar interior exert different gravitational pulls. The ranging instruments for measuring relative position pinpoint the distance between the craft to a matter of microns: a few millionths of a meter. Even slight differences in density due to lunar crust composition or interior variation can be measured this way

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