A NASA mission launched in 2011 September to determine the internal structure of the Moon. It consisted of two spacecraft, Ebb and Flow, in low polar orbit. Tracking the changing distance between the two craft mapped the Moon’s gravitational field in detail. The two spacecraft each carried cameras to provide images of the lunar surface. The mission ended in 2012 December when the two craft impacted the Moon.

http://www.nasa.gov/mission_pages/grail/main/ Official mission website.