On this day in space! Sept. 10, 2011: NASA launches GRAIL probes to the moon to map lumpy lunar gravity

On Sept. 10, 2011, NASA's GRAIL spacecraft launched on a mission to the moon! GRAIL, which stands for Gravity Recovery and Interior Laboratory, was sent to the moon to map its gravitational fields. Scientists could study the moon's interior structure by looking at maps of the moon's gravity. Asteroid impacts from billions of years ago left dense pockets of material under the lunar surface, which can exert extra gravitational pull on spacecraft orbiting the moon. So by measuring that gravitationa