GRAIL twins arrive in lunar vicinity

The long 2.5 million mile tour to the Moon allowed the spin up of mission-sensitive equipment. GRAIL A and B will arrive in lunar orbit 25 hours apart following their more-than three month flight. The science mission will commence as after an 11 hour series of maneuvers bring the vehicles down to their 50 km high circular polar orbit [JPL].

Beginning with New Year celebrations this coming weekend the United States will start 2012 with a total of five unmanned spacecraft in orbit around the Moon simultaneously.

Following a low energy trajectory allowing more than three months flying time the twin lunar Gravity Recovery and Interior Laboratory (GRAIL A and GRAIL B) will be inserted into lunar orbit New Years Eve and New Years Day, respectively. There they will join the ARTEMIS P1 and P2 probes, two salvaged vehicles of the now-completed five vehicle THEMIS fleet subsequently redeployed to the Moon by way of L1 and L2, and the Lunar Reconnaissance Orbiter (LRO) that has been in lunar orbit since June 2009.

Maria Zuber of MIT, principal investigator for the GRAIL mission, joined David Lehman, project manager at NASA's Jet Propulsion Laboratory (JPL) for an audio news conference Thursday afternoon, December 28.

The Gravity Recovery And Interior Laboratory (GRAIL) spacecraft are intended to map the lunar gravitational field with unprecedented accuracy. 

GRAIL-A and GRAIL-B lifted off from Cape Canaveral on September 10. GRAIL-A will arrive in lunar orbit at 2100 UT) New Years Eve and GRAIL-B will arrive New Years Day at 2200 UT.

MoonKAM field of view. Piggy-backing on the small Lockheed Martin-built probes, four cameras in total will be operated by elementary school students signed up through the MoonKAM website [Zuber/MIT].

The science payload on-board each spacecraft is the Lunar Gravity Ranging System, which will measure changes in the distance between the two spacecraft down to a few microns - about the diameter of a red blood cell.

The GRAILs also carry cameras for the MoonKAM project, an educational outreach effort which will allow students and schoolchildren to download lunar imagery directly. The results of a contest to formally name both GRAIL A and B is scheduled to be announced following orbital insertion.

"The Moon is the nearest pristine airless body with a preserved surface intact from the time of the Solar System's formation," Zuber said. "We actually now know more about Mars than our closest neighbor." 

"We don't yet know fully why there is such a difference between the near and far side of the Moon, and the we think the answer lies within the Moon. GRAIL should complete a gravity map of the Moon 100 times as accurate as we have today."

As both spacecraft stay in line of site through June, approximately 50 kilometers above the surface in polar orbit differences in their distance will be measured to within a few microns, allowing the anisotropic nature of the Moon's mass to measured with unprecedented accuracy.

In addition the probes will carry four cameras between them, admittedly with no true scientific value, that will be operated by students signed up through the MoonKAM project under the direction of former astronaut Dr. Sally Ride.

Following the 84 day nominal mission there is a possibility of an extended mission through December 2012.

December 28 NASA/JPL press conference materials available HERE.