For most astrophysics purposes, we don’t have to worry about the details of the inner structure of planets and moons. However, if we want to reconstruct their full history, it helps to know all the variations in density and composition. The twin GRAIL spacecraft in orbit around the Moon are designed to do just that. By a bit of fancy formation flying, GRAIL-A (“Ebb”) and GRAIL-B (“Flow”) have provided the most detailed map yet of the Moon’s surface and interior, by measuring fluctuations in the gravitational pull. My editor John Timmer and I wrote a pair of stories (not named Ebb and Flow) describing the spacecraft and their discoveries. The Moon turns out to have had an even more violent past than previously suspected, including an impact so powerful it shattered the crust.

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). [Read more…]