While the dream of another manned mission to the moon seems to be on hold for a while, there's still plenty that we don't know about Earth's satellite, particularly about its interior and its natural history, which might give us a better understanding of the solar system's formation. But scientists are hopeful that the twin GRAIL orbital probes, launched in September 2011 and scheduled to orbit the moon for several months starting in December 2011, will shed light on some of the moon's mysteries by creating a high-resolution map of the variations in its gravitational field.
The twin satellites, each about the size of a washing machine, will circle the moon together in a polar orbit, which means they'll fly over the moon's north and south poles, rather than around the moon's equator. The two satellites are designed to fly in tandem in an exact alignment. But as they pass over any feature on the moon's surface -- such as a mountain or crater, or an underground deposit of minerals -- that's big enough to cause a variation in the lunar gravitational field, that change in gravitational pull will slightly alter the position of the satellites in relation to each other. When that happens, super-sensitive instruments will measure the difference, down to a few microns -- the diameter of a red blood cell. That data will enable scientists to draw an extremely precise gravitational field map. Each spacecraft also will carry a set of cameras that will send images to NASA's MoonKAM educational Web site, where they can be viewed by teachers and students [source: NASA].