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CAPE CANAVERAL, Fla. -- At Space Launch Complex 17B on Cape Canaveral Air Force Station, the United Launch Alliance Delta II rocket that will launch NASA's Gravity Recovery and Interior Laboratory mission is ready for launch. Preparations are under way to roll the mobile service tower away from the rocket. GRAIL will fly twin spacecraft in tandem around the moon to precisely measure and map variations in the moon's gravitational field. The mission will provide the most accurate global gravity field to date for any planet, including Earth. This detailed information will reveal differences in the density of the moon's crust and mantle and will help answer fundamental questions about the moon's internal structure, thermal evolution, and history of collisions with asteroids. The aim is to map the moon's gravity field so completely that future lunar vehicles can safely navigate anywhere on the moon’s surface. Launch is scheduled for 8:37:06 a.m. EDT Sept. 8. For more information, visit http://www.nasa.gov/grail. Photo credit: NASA/Kim Shiflett