GRAIL's Twins Safely Reach the Moon

MIT scientist Maria Zuber has worked with numerous interplanetary spacecraft before, having helped dispatch instruments to the Moon, Mars, and Mercury. But I'm sure she'll sleep better tonight knowing that the twin spacecraft of the Gravity Recovery and Interior Laboratory (GRAIL) project, for which she's the principal investigator, have successfully reached their final destination.

GRAIL A arrived at the Moon late yesterday, firing its braking engine for about 40 minutes and settling into a looping polar orbit that ranges from 56 to 5,197 miles (90 to 8,363 km) from the lunar surface. GRAIL B wasn't far behind, arriving late today. Over the next three months, ground controllers will maneuver both craft into much lower final orbits, which will glide just 34 miles (55 km) above the Moon's cratered face.

(NASA won't be calling them A and B for much longer. Any day now, Zuber will announce the winners of a nationwide contest, which ended in November, to rename the craft. Hundreds of students wrote short essays to make the case for their suggested monikers.)

The GRAIL spacecraft left Earth way back on September 11th, so they weren't in any particular hurry to reach their destination. Instead, they followed convoluted but very energy-efficient, 3½-month-long trajectories that make use of a "balance point" between the gravitational fields of the Sun and Earth, the L₁ Lagrange point, to swing them toward the Moon.

As I noted after their launch, the GRAIL craft will buck the trend of recent lunar orbiters that have scrutinized the bleak surface with arrays of detectors. Instead, they'll exploit the Moon's own gravity to probe the deep lunar interior. From March to early June, radio antennas on Earth with keep very close tabs on the crafts' locations with respect to one another to within a few tenths of a micron — even though they'll be separated anywhere from 62 to 140 miles (100 to 225 km).

All that precision should reveal any minuscule changes in their velocities, deviations from perfectly Keplerian motion that betray "lumpiness" in the distribution of mass inside the globe. For the first time, Zuber and her team should determine whether the Moon has a core and, if so, its size and physical state, why the lunar crust is thinner on the hemisphere that continually faces Earth, and much more.

On June 4th much of the Moon — and the spacecraft — will be plunged into darkness during a partial lunar eclipse that will be challenging to survive: their electronic systems were designed to be in constant sunlight. But if the craft make it (and Zuber seems upbeat about their chances), then plans call for the orbital altitudes to be lowered to just 15 miles (25 km) — skirting the surface so closely that the craft should be able reveal gravitational irregularities under even modest-size craters.

This mission extension should run through December, and ground controllers will need to fine-tune to orbits constantly to keep the spacecraft from crashing into the Moon — which is their ultimate fate.

By the way, the project has teamed up with former astronaut Sally Ride to create a great opportunity for students. More than 2,000 classrooms have already signed up to use MoonKAM, a dedicated camera aboard each GRAIL spacecraft, and you'd better hurry if you want yours to be added to the list.