Science has long known about the curious gravitational variations of the moon's surface — called mascons — as evidenced by the Doppler shift of radio signals from passing spacecraft. However, researchers are now taking an unprecedented, high-fidelity look at the moon's gravitational field thanks to a pair of washing machine-sized orbiters.
The $US496 million Gravity Recovery and Interior Laboratory (GRAIL) is part of the NASA Discovery Program, and aims to determine the moon's interior structure via fluctuations in its gravitational field as well as to uncover potential clues about the formation of our solar system's rocky inner planets. See, mascons — MASs CONcentrations — are "positive gravitational anomalies" typically found in impact craters all over the moon, and are thought to be caused by basaltic lava flows collecting in these craters. These pockets of extra density can greatly affect the orbits of passing spacecraft over time, as well as efforts to "slingshot" craft into the outer solar system.
To see what's going on under the moon's surface, NASA launched a pair of orbiters, dubbed GRAIL A (Ebb) and GRAIL B (Flow), from Cape Canaveral Air Force Station, Florida, aboard a Delta II in September of 2011. The pair arrived at the moon 24 hours apart, on New Year's Eve 2011 and New Year's Day 2012. The spacecraft took their time reaching the moon (three weeks as opposed to the normal three days) in order to conserve their hydrazine fuel and reduce their lunar approach speed, which aided in achieving their super-low 50km orbit heights.
Each spacecraft is outfitted with a Lunar Gravity Ranging System, the same system used to precisely map the Earth's gravitational field during the GRACE mission, which uses Ka-band radar to collect measurements. As the spacecraft fly over gravitational fields of varying intensity, the LGRS measures the change in their relative positions and velocities with a precision of a few microns — about the diameter of a red blood cell. In addition to the LGRS, each craft is also equipped with a set of joystick-controlled cameras as part of the MoonKAM project (Moon Knowledge Acquired by Middle school students — MoonKAM), which allow school children to directly control the orbiters' digital eyes. [Wikipedia - NASA 1, 2 - LA Times]