A pair of spacecraft, set for launch aboard the same rocket on Sept. 8, will head for the moon to study its gravity. NASA’s Gravity Recovery & Interior Laboratory (GRAIL) mission aims to assemble the most detailed map ever of the moon’s gravitational field. Such a map will allow scientists to glean information about the moon’s interior structure and composition and the evolution of its magnetism. Additionally, the data will provide insights into the moon’s heating and cooling history, as well as information about the moon’s origin. The two spacecraft will spend several months adjusting their orbits to fly in formation, one following the other. The differences in the moon’s gravity, produced by different densities and amounts of terrain, will cause the spacecraft to move toward or away from each other. The distance changes can then be translated into gravitational changes.