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Flying in formation only 55km (34 miles) above the lunar surface, NASA's twin GRAIL (Gravity Recovery and Interior Laboratory) space probes have now officially begun mapping the lunar gravitational field. This science collection phase of the mission began late on Tuesday 6th March 2012 and will over the next 84 days produce a high-resolution map of the lunar gravitational field in unprecedented detail.

Far side of the Moon as photographed by Apollo 16 (NASA)

Maria Zuber, principal investigator for the GRAIL mission at the Massachusetts Institute of Technology in Cambridge said "The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do". She added, "but it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."

From data obtained by the mission, scientists will learn much more about the Moon's internal structure and composition and hope that GRAIL can solve mysteries such as why the far side of the Moon is more mountainous than the Earth facing near side.

Artist concept of the twin GRAIL space probes (NASA)

The GRAIL spacecraft were launched on 10th September 2011 from Cape Canaveral, Florida on a long (via Sun-Earth Lagrange point L1) but fuel-efficient journey to the Moon. Such a trajectory meant that the probes required nearly four months reaching their target, instead of three days needed by the Apollo astronauts on a direct flight. The spacecraft then slipped into lunar orbit around New Year and after a series of orbital adjustments are now at the required altitude and distance apart to begin measurements.

Scientists monitoring the probes vital signs said they're in excellent health. They have to wait to get a month's worth of data before they can start analyzing, but were pleased so far.

The spacecraft were originally named GRAIL A and B but are now called Ebb and Flow after a nationwide student contest to choose new names for them.

See also

GRAIL – NASA's twin Moon gravity probes successfully reach orbit
NASA launches GRAIL Moon mission twin probes