NASA’s GRAIL mission around moon

NASA's GRAIL mission to study the moon from crust to core successfully lifted off from Cape Canaveral Air Force Station's Pad SLC-17B on September 9, 2011.

The straight-line distance from Earth to the moon is about 402,336 kilometres. It took NASA’s Apollo moon crews about three days to cover that distance. Each of the GRAIL twin satellites will be taking about 30 times that , and covering more than 4 million kilometres to get there. This low-energy, high-cruise time trajectory is beneficial for mission planners and controllers, as it allows more time for spacecraft checkout. The path also provides a vital component of the spacecraft's single science instrument, the Ultra Stable Oscillator, to be continuously powered for several months, allowing it to reach a stable operating temperature long before beginning the collection of science measurements in lunar orbit.

GRAIL-A will enter lunar orbit on December 31, 2011, and GRAIL-B will follow the next day. When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. Regional gravitational differences on the moon are expected to expand and contract that distance. GRAIL scientists will use these accurate measurements to define the moon’s gravity field. The data will allow mission scientists to understand what goes on below the surface of our natural satellite.