GRAIL Spacecraft on Final Approach for Lunar Rendezvous


The mirror-image GRAIL spacecraft are set to arrive in lunar orbit on Dec. 31. The two spacecraft are scheduled to compile the most comprehensive gravitational map of the Moon starting in March, 2012. Photo Credit: Alan Walters/

 The mirror-image GRAIL spacecraft are “number one” on the runway – at least in terms of approaching the Moon. On New Year’s Eve the spacecraft will enter lunar orbit to begin studying the Moon’s gravitational aspects – from its crust – all the way down to its core. GRAIL will conduct engine burns on New Year’s Eve and on New Year’s Day that will place the duo into orbit. The first will take place at 1:21 p.m. PST (4:21 p.m. EST) for GRAIL-A on Dec. 31, and 2:05 p.m. PST (5:05 p.m. EST) for GRAIL-B the following day. 

The space between the Earth and its celestial companion is approximately 250,000 miles (roughly 402,000 kilometers). GRAIL was launched on Sept. 10, 2011 and where it took the Apollo astronauts 3 days to reach the Moon’s sphere of influence (powered by the massive Saturn V rocket) – GRAIL took a far longer route. The two spacecraft traveled an estimated 2.5 million miles and nearly four months to reach the Moon. 


Video Courtesy of Jason Rhian

“Our team may not get to partake in a traditional New Year’s celebration, but I expect seeing our two spacecraft safely in lunar orbit should give us all the excitement and feeling of euphoria anyone in this line of work would ever need,” said David Lehman, project manager for GRAIL from NASA’s Jet Propulsion Laboratory (JPL) located in Pasadena, Calif.

JPL planned for the spacecraft to take its time in reaching its destination to allow mission planners time to verify the overall health of the spacecraft. According to a NASA press release, GRAIL only has a single science instrument on board – the Ultra Stable Oscillator. Taking the scenic route to the Moon allowed for this instrument to remain powered for several months and thus allowed it to maintain a stable operating temperature before ever reaching the Moon.

The GRAIL mission launched on Spet. 10, 2011 from Cape Canaveral Air Force Station's Space Launch Complex 17 (SLC-17). GRAIL was launched atop a United Launch Alliance (ULA) Delta II Heavy. Photo Credit: Jason Rhian

“This mission will rewrite the textbooks on the evolution of the moon,” said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology in Cambridge, Mass. “Our two spacecraft are operating so well during their journey that we have performed a full test of our science instrument and confirmed the performance required to meet our science objectives.”

When the two orbiters conduct their final approach they will fly in from the south – almost directly over the Moon’s South Pole. In the coming weeks flight controllers will place the spacecraft into elliptical polar orbits paring down these orbits from 11.5 orbits – to about two hours. JPL plans to begin the science phase of the mission in March of 2012. By this time the spacecraft will be in orbits that are almost circular and just 34 miles (55 kilometers) above the Moon’s surface.

It is hoped that by producing a gravitational map of the Moon that a better understanding of how the other terrestrial planets, Mercury, Venus, Earth and Mars were formed. GRAIL will transmit radio signals precisely defining the distance between them as they orbit the moon – thus providing an extremely accurate map of the Moon’s gravitational fields. The GRAIL spacecraft are linked together, as the Moon’s components (both seen and unseen) tug at the craft – their relative velocity will be monitored and recorded.

Once they have their orbits stabilized, the GRAIL spacecraft will begin conducting scientific research. The scientific phase of the mission is currently slated to begin in Marhc. Photo Credit: Alan Walters/

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