GRAIL -- The Twin Spacecrafts

The Gravity Recovery and Interior Laboratory, or GRAIL is a lunar mission by NASA and scheduled to be launched on September 8, 2011 from Cape Canaveral Air Force Station's Launch Complex 17B. Though there are launch opportunities till Oct 19, 2011.

The main objectives of GRAIL is to determine the structure of the lunar interior deep till core, and to advance understanding of the thermal evolution of the moon. GRAIL will fly twin spacecraft in near polar orbits of Moon, where one spacecraft will follow the other to measure its gravity field in extraordinary detail.

They will fly over the areas of greater and lesser gravity over the lunar surface for measuring Gravity range. They will move slightly toward and away from each other so that the instrument aboard each spacecraft will measure the changes in their relative velocity very precisely. This data will be translated into a high resolution map the Moon's gravitational field.

References:http://www.nasa.gov(Details from NASA website)