NASA GRAIL: Lunar twins Ebb and Flow have extended mission to map gravity

NASA’s twin GRAIL spacecraft, named Ebb and Flow, have successfully completed their primary mission of mapping lunar gravity. 

They have now entered an extended mission in order to obtain gravity maps of even higher resolution than before.

The two spacecraft entered this phase of the mission at 16:28 UT on 30 August when they moved to a lower orbit.

The orbital altitude of the extended mission will see the craft at an average of only 23 kilometres (14 miles) above the Moon’s surface, compared to the average orbital height of 55 kilometres (34 miles) during the primary mission.

At this lower altitude Ebb and Flow will be within 8 kilometres (5 miles) of some of the Moon's higher surface features.

The data collected during the primary mission of GRAIL - which stands for Gravity Recovery And Interior Laboratory - is being analysed and "holds the promise of producing a gravity field map of extraordinary quality and resolution" according to Maria Zuber, principal investigator for GRAIL.

"Mapping at a substantially lower altitude during the extended mission, and getting an even more intimate glimpse of our nearest celestial neighbour, provides the unique opportunity to globally map the shallow crust of a planetary body beyond Earth."

The extended mission will run from 30 August until 3 December, and it will map the gravitational fields of small lunar features, such as craters and mountains.

This unprecedented resolution will help scientists to comprehend the formation and evolution of the Moon.

"Ebb and Flow, and our mission operations team, are both doing great, which is certainly notable considering all the milestones and challenges they have experienced," said David Lehman, GRAIL project manager.

"The twins have endured the lunar eclipse of June 4, 2012, and 26 rocket burns since arriving in lunar orbit at the beginning of the year.

Down here in our control room, with all the planning and mission operations we have been doing, it feels as though we've been riding right along with them. Of course, they have the better view."

It was not guaranteed that the GRAIL twins would survive the lunar eclipse in June.

The spacecraft are mainly powered by solar panels, and there were fears that the batteries on board would not be able to power the craft for the duration of the eclipse.

Fortunately, the NASA spacecraft proved their resilience, enabling the extended phase of the mission to go ahead.

Leland Melvin, NASA Associate Administrator for Education, left, Maria Zuber, GRAIL Prinicipal Investigator at the Massachusetts Institute of Technology, and James Green, Director of the Planetary Science Division in the Science Mission Directorate at NASA Headquarters, right, applaud students from Emily Dickinson Elementary School in Bozeman, Mont. during a news conference, Tuesday, Jan. 17, 2012, at NASA Headquarters in Washington. 

Nine hundred classrooms and more than 11,000 students from 45 states, as well as Puerto Rico and the District of Columbia, participated in a contest that began in October 2011 to name the twin lunar probes.

Photo Credit: NASA/Paul E. Alers

 The GRAIL twins map the lunar gravity by measuring the varying distance between two craft using radio signals. The distance between Ebb and Flow will change depending on the amount of mass

below them, allowing the craft a glimpse into the lunar interior.

The GRAIL spacecraft were launched on 10 September 2011, arrived at the Moon over the New Year and began their primary science mission on 7 March 2012.