The quest for GRAIL
The GRAIL (Gravity Recovery and Interior Laboratory) mission features two probes that will take a leisurely trip to the moon over the next three months.
On arrival, the twins will slip into orbit at different points around the moon and begin a detailed gravitational analysis. As they swing around the moon, they’ll detect slight changes in the gravitation pull that the moon exerts on them by accurately measuring the distance between themselves and a fixed point on or near the Earth, eventually building up a map of what’s inside the moon.
It’s similar to GRACE (Gravity Recovery and Climate Experiment), which put two satellites in orbit around the Earth in 2002 to measure the Earth’s gravitational field amongst other things.