Two satellites are going to map the mass profile of the moon.
The first satellite, Grail-A, achieved orbit on Saturday, December 31: “Together, the satellites will make measurements that are expected to give scientists remarkable new insights into the internal structure of the Moon. This new data should clarify ideas about the Moon’s formation and resolve many mysteries, such as why its near and far sides look so different.”
The second satellite, Grail-B, entered orbit on Sunday, January 1.The two satellites will note tiny differences between the distance between them, caused by variation in gravity as they pass over the surface of the moon. This will enable scientists to calculate variations in the density of the moon’s crust. This is the first time two satellites have been placed around Luna.
You can read more at NASA.