After entering orbit around the Moon at the start of the year, NASA’s twin GRAIL (Gravity Recovery And Interior Laboratory) probes, Ebb and Flow, have completed their primary mission to study the Moon’s interior structure ahead of schedule. Operating around the clock since March 8, NASA says the spacecraft have provided unprecedented detail about the interior structure and evolution of the Moon and the data they have gathered will provide insights into how Earth and its rocky neighbors in the inner solar system developed.

With Ebb and Flow covering the entire surface of the Moon three times from an orbit that took them over the lunar poles, the Lunar Gravity Ranging System instrument onboard both spacecraft transmitted radio signals that allowed scientists to generate a high-resolution of the Moon’s gravitational field. The instruments were turned off on May 29 after transmitting their last data set of the prime mission when the spacecraft were 37 miles (60 km) above the Sea of Nectar.

But the spacecraft won’t be resting on their laurels, with both instruments only powered off until August 30, when extended science operations are set to begin that will continue through until December 3, 2012. With the goal of taking an even closer look at the Moon’s gravity field, the extended mission will see the operating altitude of the twin spacecraft halved so they are flying at the lowest altitude that can be safely maintained.

South pole of the far side of the moon as seen from the GRAIL mission’s Ebb spacecraft. (Image: NASA/JPL-Caltech)

"Orbiting at an average altitude of 14 miles (23 kilometers) during the extended mission, the GRAIL twins will be clearing some of the moon's higher surface features by about 5 miles (8 kilometers)," said Joe Beerer, GRAIL's mission manager.

NASA has also announced its MoonKAM (Moon Knowledge Acquired by Middle school students) program will be extended so students can add to the more than 70,000 student images of the Moon that have already been obtained by the mission.

But before the extended science operations get underway, the spacecraft will have to endure a lunar eclipse on June 4 that will expose them to sudden changes in temperature. However, NASA is confident the spacecraft can withstand the energy-sapping darkness of this event.

"Before launch, we planned for all of GRAIL's primary mission science to occur between lunar eclipses," said David Lehman, project manager of GRAIL from NASA's Jet Propulsion Laboratory (JPL). "But now that we have flown Ebb and Flow for a while, we understand them and are confident they can survive these eclipses in good shape."

Source: NASA