This undated artist's rendering made available by NASA/JPL-Caltech shows the twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft which will map the moon's gravity field. Radio signals traveling between the two spacecraft provide scientists the exact measurements required as well as flow of information not interrupted when the spacecraft are at the lunar farside, not seen from Earth. The result should be the most accurate gravity map of the moon ever made. ??The mission also will answer longstanding questions about Earth's moon, including the size of a possible inner core, and it should provide scientists with a better understanding of how Earth and other rocky planets in the solar system formed. GRAIL is a part of NASA's Discovery Program. ??(AP Photo/NASA/JPL-Caltech)