On August 30, the GRAIL (Gravity Recovery And Interior Laboratory) twin spacecraft, Ebb and Flow, started their extended mission to map the Moon’s gravity with even greater precision. About three months after finishing their primary mission and being placed in a state of hibernation to survive a lunar eclipse, the instruments of the two spacecraft were reactivated and they were moved to a lower orbit.

At the GRAIL mission control center at NASA’s JPL there was optimism for Ebb and Flow’s exit from hibernation because it was verified that the batteries had accumulated enough energy to resume their activities. Normally, the GRAIL spacecraft use the solar energy captured by their panels so the lunar eclipse was a critical moment because in the harsh conditions in which they work, problems can always emerge.

The GRAIL spacecraft work together by transmitting radio signals between them that determine with extreme precision the variations of their distance caused by the slight variations of the lunar gravity. Consequently, it would be enough that one of the two didn’t fully reactivated to cause the end of the mission.

Everything went well and at the control center there’s understandable satisfaction. The spacecraft Ebb and Flow started detecting small changes in the lunar gravity at an average height of just over 22 kilometers (about 14 miles), which will take them about 8 km (5 miles) above the highest mountains of the Moon. Their average height during the primary mission was about 55 kilometers (34 miles) so now the changes in the lunar gravity will affect the GRAIL spacecrafts more, therefore they’ll be able to detect them with even greater precision.

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This extended mission is scheduled to last until December 3 and in the coming months Ebb and Flow will take other pictures with their MoonKAM continuing their educational mission as well. Meanwhile at NASA they’re still analyzing the huge amount of data arrived during their primary mission. Next year a remarkably accurate map the Moon’s gravity should be available and it will greatly enhance our understanding of its composition and its evolution, which is closely linked to that of the Earth.