NASA captures, quite literally, gravity's rainbow.

Scientists care about the moon in part because it is the moon -- our moon. But they care about it, also, because its body is in some ways a proxy for our Earth's: Its surface, like ours, bears the scars of a long existence in our little corner of the Milky Way. And new research suggests that that existence might have been much more violent -- and perhaps more hospitable to life -- than we humans initially believed. Yes.

The story begins, officially, around five years ago. In 2007, the Japanese lunar satellite Kaguya released two small probes into orbit around the moon. Those satellites, working in tandem, created the first gravity map of the far side of the moon -- a chart that showed, in color-coded detail, the variations in mass on our nearest planetary neighbor.

The same year, NASA announced plans for a similar mission: the launch of twin spacecraft that would spend several months doing their own moon-mapping work, measuring the lunar gravity field in unprecedented detail. In late 2011, NASA began that mission, sending a pair of spacecraft -- collectively known as GRAIL (Gravity Recovery and Interior Laboratory) -- to orbit the lunar surface. The pair of satellites (named, awesomely, Ebb and Flow) fly in formation around the moon, sending each other -- and Earth -- microwave measurements of our only natural satellite. The twin vehicles, each about the size of a washing machine, work by detecting tiny changes in the distance between them -- variations caused by lunar mountains, craters, and subsurface mass concentrations.

The GRAIL satellites and their operators use a technique pioneered by GRACE, the Gravity Recovery and Climate Experiment, launched in 2002 and run jointly by NASA and the German Aerospace Center. The Grace satellites have been taking detailed measurements of Earth's gravity field -- tracking, in particular, gravity changes related to the movement of mass within Earth (like, say, the melting of ice at its poles and changes in its ocean circulations). GRAIL has been applying the lessons of GRACE to our planetary neighbor -- helped along by the fact that the moon lacks an atmosphere, which allows the satellites to orbit wonderfully close to its surface: between 10 and 30 miles above the lunar crust. (The ESA's GOCE satellite, which does gravitational mapping of Earth, has to stay 10 times farther away from its target to avoid atmospheric drag.)