smitty777 writes "The two washing machine sized satellites from the GRAIL program (Gravity Recovery and Interior Laboratory) launched in September are set to enter lunar orbit this weekend. As can be seen from this nifty infographic, the probes will monitor the gravitational field from orbit via the precise distance measurements of microwaves passing between the two satellites. From the article: 'The twin spacecrafts are named Grail-A and Grail-B. Grail-A will enter the moon's orbit on New Year's Eve, Grail-B will follow on New Year's Day. "The purpose of the GRAIL mission is to obtain gravity data on the Moon. And with that data, the scientists are able to determine the structure of the lunar interior, from crust to core," said David Lehman of NASA's Jet Propulsion Laboratory.' This is similar to the earlier GRACE project, which not only helped map out the gravity field of the Earth, but also helped map drought conditions in the U.S."