AP Photo

If you're spending the money to put something in orbit around the Moon, you'll likely stuff it full of as many scientific instruments as it can fit. But now NASA has sent not one, but two spacecraft to the Moon with only a single scientific instrument. Despite the light load, the GRAIL mission turns out to have been a phenomenal success, as it's providing the clearest map of the composition of our closest neighbor. The results show a world shaped by a combination of impacts (some of which probably penetrated to the Moon's mantle) and the stretch marks of an early expansion in its size.