The Quest for the Lunar GRAIL

Twin NASA spacecraft will attempt to map the most gravitationally lumpy body in our solar system — Earth's moon.

Asteroid impacts from billions of years ago left dense buried pockets of material under the lunar surface, which can exert extra gravitational pull on spacecraft orbiting the moon.

"This mission will give us the most accurate global gravity field to date for any planet, including Earth," said Maria Zuber, the MIT physicist leading the Gravity Recovery and Interior Laboratory (GRAIL) mission.

Scientists still don't know what material forms the dense mass concentrations (mascons). Some of the mass may come from denser lava filling ancient impact craters, or iron-rich molten material seeping upwards to the moon's crust.

Whatever the case, the gravitational effect is so strong that an astronaut dangling a weight on the end of a string near a mascon would see it hang a third of a degree off vertical, pointing toward the center of the lunar formation.

Such gravitational anomalies make low lunar orbits unusually treacherous and force spacecraft to constantly adjust their orbits. One sub-satellite released during NASA's Apollo 16 mission in 1972 crashed just 35 days later, as its circular orbit decayed into a more elliptical shape that eventually caused it to impact.

"If you take out the five major mascons on the near side of the moon, orbits are much more stable," said Alex Konopliv, a GRAIL planetary scientist at NASA's Jet Propulsion Lab in Pasadena, Calif. He added that the far side of the moon has smaller mascons, which have weaker gravitational pulls.

Even the Lunar Prospector mission in 1999 had to use up fuel doing constant course corrections to stay in orbit, before NASA intentionally crashed the orbiter into a crater.

GRAIL is designed to gauge the moon's gravitational grasp by flying two spacecraft in lunar orbit, one behind the other by 124 miles (200 km), for about three months. A microwave ranging system will allow scientists to precisely measure the distance between the spacecraft as it contracts and expands based on the gravitational tugs of mascons.

"The baseline plan is to have a 50 kilometer (30 mile) high orbit," Konopliv told SPACE.com. However, he noted that the actual orbit would vary between 30 and 70 kilometers due to the gravitational influence of the mascons, requiring GRAIL to maneuver back to its original starting point every 30 days.

After GRAIL does its work, "we'll be able to navigate anything you want anywhere on the moon you want," Zuber said.

That will help mission planners choose precise orbits that avoid the worst of the gravitational anomalies. NASA already knows of four orbits where spacecraft can maintain almost indefinite lunar orbits, including one that almost passes over the lunar poles.

GRAIL will also aid NASA in choosing how to approach the moon for the planned 2020 landings of its astronaut-carrying Altair landers on the moon's far side and polar regions – places where the lunar gravitational field remains least understood. Scientists could then use the gravity map of the moon to better examine the lunar crust and core.

The mission idea for GRAIL came from the success of the U.S.-German Earth observing Gravity Recovery and Climate Experiment (GRACE), which launched in 2002. GRACE gauges gravity changes related to the melting of ice at the poles and changes in ocean circulation.

Neither Earth nor Venus contain mascons like those on the moon, but similar features do appear on Mars. NASA has already said that the 2011 GRAIL mission will likely inspire future space quests to do gravitational mapping.

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