More than 100 missions targeted at Earth's moon have been launched by space explorers since the late-1950s. NASA landed a total of 12 men on the lunar surface, collecting more than 800 pounds of moon rocks and lunar soil samples. But still, the moon remains a mystery, especially its formation. NASA's new mission aims to find out exactly how the moon came to be with the Gravity Recovery and Interior Laboratory, or GRAIL, part of NASA's Discovery Program.
