Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravity field, as depicted in this artist's rendering. Image by NASA/JPL-Caltech
On New Year's Eve, the first of two NASA spacecraft fired its engine and maneuvered into orbit around the moon after a three-month, 2.5-million-mile journey. Its twin followed suit 25 hours later.
Both GRAIL A and GRAIL B -- washing-machine sized satellites designed to map the surface and interior of the moon -- are now zipping around the moon in an elliptical orbit, with GRAIL B trailing 10 miles behind its twin. (GRAIL stands for Gravity Recovery and Interior Laboratory.) Over the next two months, they'll be moving into a low-altitude circular orbit at a distance of 35 miles from the moon's surface, preparing to start science observations on March 8.
The twin probes are designed to measure changes in the moon's gravitational field at extremely high accuracy. The mass of geological formations like mountains, valleys or deep basins, creates a slight gravitational pull or push on an orbiting object. So do materials and forces beneath the lunar surface that can't be seen on a topographical map.
"If you're orbiting the moon and you fly over a mountain, the spacecraft will literally sense additional pull," said Sami Asmar, deputy project scientist on the GRAIL mission. "A mountain has more mass than its surroundings. If you fly over a valley, you literally sense less gravity, you sense a gravity low. We'll try to correlate that with the topography."
Changes in gravity caused by these structures and forces will be transmitted as radio signals between the two spacecraft and beamed back to Earth.
"Bodies like the Earth and the moon are not homogeneous, they're not all with an equal distribution of mass and chemical elements," Asmar said. "They vary, and we want to know how they vary to understand the interior structure."
The moon's far side, for example, is rougher and more mountainous than its near side, which is smoother with more dips and valleys.
The hope is that these measurements will help answer some fundamental questions about the moon. For example, does it have an iron core and a surrounding liquid layer, like Earth? And if so, how big is the core and its mantle, and how massive?
The really interesting findings, he said, will occur when a gravitational pull is detected over a flat area, suggesting something underneath the surface, such as a buried asteroid or comet,or magma flow from the moon's interior.
One recent theory is that Earth once had two moons that combined during a collision, a possible explanation for why the far side of the moon is so much lumpier than the side that we see.
"It's an elegant theory, one that would be neat to shed light on," Asmar said.
Watch a video of the September launch and an interview with launch director Tim Dunn.
Image credit: Artist concept of GRAIL-B performing its lunar orbit insertion burn. Image by NASA/JPL-Caltech.
