Media representatives prepare to photograph the launch of NASA's Gravity Recovery and Interior Laboratory mission at Press Site 1 on Cape Canaveral Air Force Station.

Media representatives prepare to photograph the launch of NASA's Gravity Recovery and Interior Laboratory mission at Press Site 1 near Space Launch Complex 17B on Cape Canaveral Air Force Station. Liftoff aboard a United Launch Alliance Delta II Heavy rocket is scheduled for 8:37:06 a.m. EDT.

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. For more information, visit http://www.nasa.gov/grail. Photo credit: NASA/Ken Thornsley