Over six decades of space exploration, scores of spacecraft have been hurled in the Moon's direction.
But despite all that scrutiny, planetary scientists still have basic questions about the structure and composition of the lunar interior. For example, is its core solid or partly liquid — or does one exist at all? Why is the crust so much thicker on the far side? Was its exterior once completely molten?
Today NASA took an important step in getting answers to these and other nagging geophysical questions, when a Delta II rocket roared into a partly cloudy Florida sky at 9:08:53 a.m. EDT. Aboard were the twin GRAIL spacecraft, destined to reach the Moon (via a swing near the L1 Lagrange point) one day apart at year's end. The launch, which had slipped two days from the planned liftoff on September 8th, was flawless.
GRAIL's twin spacecraft have one basic objective: to let ground controllers and each other know exactly where they are at all times. OK, there is a camera on board, called MoonKAM, but it's there to provide you-are-there opportunities for middle-school students. The real work of the Gravity Recovery and Interior Laboratory is to use the Moon's own gravitational field to reveal the structure of the lunar interior from core to crust.
A little background: The Moon is not uniform throughout but instead is layered and lumpy; its rocks vary in density from place to place and with depth. The denser the rock, the greater its gravitational force. So by carefully mapping the local gravity over the entire lunar globe, scientists can deduce what lies beneath its dusty surface.
Tracking data from the first lunar orbiters revealed that the huge pools of dense volcanic rock on and under the lunar maria created mass concentrations, or mascons. (The story of their discovery is fascinating in itself.)
But those early findings were crude because, as a rule of thumb, the gravity maps' resolution is roughly equal to the orbiter's altitude. Also, it's impossible to track a single spacecraft when it's out of view on the Moon's far side. Kaguya, a recent Japanese mission, sidestepped this problem by dispatching two smaller "subsatellites" to transmit and relay radio signals from the far side.
GRAIL aims to map the lunar gravity with 100 times (near side) to 1,000 times (far side) better sensitivity than ever before. By next March the two craft will be hugging the lunar surface in circular orbits with altitudes of just 34 miles (55 km). Each craft's radio transmissions will show slight Doppler shifts in frequency whenever it speeds up or slows down due to the changing gravity field attracting it from below. They'll keep tabs on each other closely, augmented by tracking from Earth whenever they're above the near side.
NASA scientists have used this same strategy to map Earth's gravity field using the twin GRACE (Gravity Recovery and Climate Experiment) spacecraft, which have been in orbit since 2002.
Plans call for the GRAIL spacecraft to be tracked for 82 days from March to May 2012, during which the Moon will slowly rotate beneath them three times and, crucially, the craft will remain in sunlight continuously. They'll never be out of each other's sight, with separations ranging from 62 to 140 miles (100 to 225 km). Scientists will focus on mapping near-surface gravity variations when GRAIL A and B are closest together, and they'll probe more deeply — all the way to the core — when the craft are farthest apart.
Flight controllers expect both to crash into the lunar surface next June, bringing the mission to a dramatic end. (No special targeting is planned.)
But by then GRAIL's scientists, led by geophysicist Maria Zuber (MIT), will be well on their way to answering longstanding questions about the origin and distribution of the dark maria, why the lunar crust is so much thicker on the far side, the size and state of the Moon's core, and how the lunar crust came to be magnetized — all of which will illuminate our understanding of how the Moon got there in the first place — not bad for a pair of spacecraft designed to do little more than go round and round!
By the way, Zuber has announced that there'll be an essay contest to name the two GRAIL spacecraft. It will be open to U.S. students and run from October 14th to November 11th.