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GRAIL spacecraft will use lunar gravity to peer inside moon

The GRAIL mission will study the moon’s gravitational field via minute differences in the distance between the two craft as they orbit above the moon’s surface.

As 2011 slid into 2012, NASA’s GRAIL spacecraft together slipped into orbits around the moon. GRAIL-A achieved orbit at 5 p.m. EST on Saturday, December 31, 2011. GRAIL-B followed at 5:43 p.m. EST on Sunday, January 1, 2012. The GRAIL (Gravity Recovery and Interior Laboratory) mission is designed to study the gravitational field of the moon via minute differences in the distance between the two spacecraft as they orbit above the moon’s surface.

Both craft launched from Cape Canaveral on September 10, 2011 from a Delta II rocket.  This cool video shows people on the beach, watching the GRAIL launch.

GRAIL-A separated from the rocket about nine minutes after launch, with GRAIL-B following eight minutes later, before the two craft embarked on their separate three-month trajectories to the moon. Both spacecraft successfully completed their final burns to achieve lunar orbit on schedule, in late December 2011.

Their journey is not quite done yet. The two craft are currently in an elliptical, 11-and-a-half-hour orbit. Over the next few weeks, the GRAIL team will use a series of burns to reduce the orbit to just over two hours and change its shape to a nearly circular. The craft will be in this orbit by the time the science phase begins in March. This next video explains more about that science.

We already know a lot about the moon. Since the late 1950s, the United States, Soviet Union, Japan, China and India have sent more than 100 missions there. Yet much about our companion world remains mysterious. Why is the lunar far side more hilly than the side that always faces Earth? What about those hidden masses – known as “mascons” for “mass concentrations” – below the moon’s surface? The difference in mass between the moon’s near and far sides are what cause one face of the moon to stay pointed in Earth’s direction. Scientists want to learn more about the interior of the moon, because, they say, this knowledge will help us understand how the moon evolved.

Once in orbit around the moon, the GRAIL spacecraft will be able to detect tiny variations in mass - mountains, craters, even unusual masses buried underground - on the moon. (NASA)

The highly precise craft will transmit radio signals to each other as they orbit the moon. Features both above and below the moon’s surface will exert different levels of gravitational pull on the craft, ever so slightly changing the distance between them. Scientists will be able to read these small changes in distances between the two craft – as small as the diameter of a red blood cell – to improve the existing map of the moon’s gravitational field. In this way, the data from GRAIL will help scientists “see” what’s below the moon’s surface, in turn increasing our understanding of how the moon, Earth, and other planets came to be.

Technicians work on the GRAIL craft after testing in a vacuum chamber in April 2011. Image Credit:

GRAIL is the first NASA mission to carry an instrument whose sole purpose is education and public outreach. The craft are carrying MoonKAM, a project spearheaded by Sally Ride, the first American woman in space, which will allow fifth- to eighth-graders whose teachers register the opportunity to ask for specific areas of the moon to be photographed. This month NASA will announce the winners of a student contest to give the craft a new name.

In a press release, NASA Administrator Charles Bolden said:

NASA greets the new year with a new mission of exploration. The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet. We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown.

Twin GRAIL spacecraft on final approach to moon

Moon-bound twin GRAIL spacecraft launch a success

Laura Dattaro