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Two spacecraft now circling the Moon will map its gravitational field with an unprecedented level of accuracy, providing scientists with a greater understanding of the Moon's interior-how large it is, whether it has a rocky core, and how it formed. NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission puts a pair of nearly identical spacecraft in orbit. By using radio waves to measure tiny variations in the Moon's gravitational field, they will create a high-resolution map that will give scientists a clearer picture of what lies beneath the surface and provide insight into the formation of terrestrial planets.