Though the moon is our closest neighbor in space, there's much we don't know about it. For instance: Did a second moon once crash into our moon, destroying the second moon and leaving our moon pockmarked with craters? NASA hopes to shed light on this and other questions as it sends a lunar probe named GRAIL (Gravity Recovery and Interior Laboratory) to orbit the moon. Here, a brief guide to this mission:

What will GRAIL do?
GRAIL, which launched Saturday, will split into two separate satellites, GRAIL-A and GRAIL-B. When the pair of solar-powered craft reach the moon in December, they'll orbit it 12 times a day for about three months. The main purpose of the mission is to create a digital map of the inside of the moon by analyzing the moon's gravitational field.

How will these lunar probes measure gravity?
Through "precision formation flying." As the two tightly coordinated GRAIL craft move above the moon, they'll slow down or speed up based on how strong or weak the moon's gravitational force is at different points across its surface. "By continually taking very precise measurements of the distance between the two spacecraft, the mission can measure changes in the gravitational field of the moon," says Brian McLaughlin at Wired.

So, studying gravity explains the interior of the moon?
Yes. It's the varying density of the crust, mantle, and other geological features of the moon's interior that determine gravitational changes, says Scott Gold in the Los Angeles Times. So mapping the moon's gravity "will allow for the first comprehensive assessment of the moon's crust, mantle, and core." This gravitational data will be used to produce a "map" of the moon's interior, so scientists can better understand how the moon was formed, why lava fields are found on one side of the moon but not the other, and perhaps even how rocky planets like Earth were created.

Sources: C-S Monitor, LA Times, Wired