GRAIL Begins Extended Mission over the Moon

In this image, the mirror-image GRAIL spacecraft are readied for launch at the Astrotech facilities in Titusville, Florida. Photo Credit: Jason Rhian

The GRAIL spacecraft, Ebb and Flow, have begun their extended mission, and are now mapping the gravity field of the Moon and at half the altitude they were during the main mission.

NASA officials decided to extend the mission from August 30 to December 3 after the primary mission ended on May 29. During the primary mission, four of the mission’s six science goals were accomplished early due to the excellent performance of the mission spacecraft and the Deep Space Network.

Ebb and Flow had their only science instrument, the Lunar Gravity Ranging System powered off at 1:00 p.m. on May 29, while the probes orbited over the Sea of Nectar.

The Lunar Gravity Ranging System is a radio ranging system that precisely determines the distance between the two GRAIL spacecraft, named Ebb and Flow. By measuring the rate of change of the distance between the spacecraft, it is possible to map the intensity of the gravitational field of the Moon.

NASA’s Gravity And Interior Laboratory or GRAIL spacecraft have lowered their orbit to make a better map of the Moon’s gravitational field. Image Credit: NASA/JPL

This map will tell scientists about the concentration of mass below the lunar surface, helping them determine the history of the Moon’s geological processes.

It was necessary to deactivate the instrument to allow the spacecraft to endure a lunar eclipse on June 4.

The instrument was reactivated at 12:28 p.m. on August 30, while Ebb and Flow were over the Ocean of Storms. They will operate until December 3.

In order to make an even more accurate map of the lunar gravity field, the probes have nearly halved their orbital altitude from 55 km to 23 km. This brings the probes within 8 km of some of the Moon’s tallest geological features.

The mirror-image GRAIL spacecraft were launch from Cape Canaveral Air Force Station’s Space Launch Complex 17 in Florida in September of 2011. Photo Credit: Mike Killian – ARES Institute

Principal Investigator Dr. Maria Zuber puts it this way, “Flying two spacecraft in precise formation 20 km above the Moon is not for the faint of heart.”

Three orbital adjustments are needed every week just to keep the probes in proper formation. And the data the probes gathered during their primary mission is already being used to help the team navigate the spacecraft during their extended mission.

Dr. Zuber says that the lower altitude map will improve spatial resolution by a factor of 2. This will give scientists more insight into the effects of small features on the Moon and how they impact the gravity field.

Though both Ebb and Flow have exceeded their design life, all spacecraft systems are fully functional and mission operators are hopeful the spacecraft will last the month needed to collect enough data. Operators hope to continue running the spacecraft for three months, but one month of continuous data gathering should be just enough to create the gravity map.

The GRAIL spacecraft carried a special payload to lunar orbit, the MoonKAM. This simple camera allowed young students to pick targets on the Moon to be imaged by the MoonKAM. Unfortunately, the MoonKAM has been deactivated for the extended mission until operators can be sure that using the camera will not impact science collecting operations.

Ebb and Flow will continue on their extended mission until December 3.

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