Two spacecraft flying in formation measured the Moon's gravity precisely enough to weigh its crust, and found the far side carrying roughly twice as much of it.

GRAIL flew twin probes in 2012 and tracked the distance between them to a fraction of a micrometre a second. The answer came out thinner overall than anyone expected — an average in the thirties of kilometres — and lopsided, which is the same asymmetry that decides where the lava plains are.