The Moon has received two new visitors from Earth. One entered orbit on December 31, with the other following on January 1. The twin spacecraft, which form the GRAIL mission, will make the most precise measurements to date of the Moon's gravitational field, which is quite "lumpy." Mapping the lumps will provide new information about the history and evolution of the lunar surface. The measurements also provide a look deep into the Moon's interior. This artist's concept shows the two spacecraft flying in formation, with differences in the Moon's gravity represented by different colors on the surface. [NASA/JPL]