Zuber is the principal investigator of the Gravity Recovery and Interior Laboratory — "SPACES STATION GRAIL" for short. It's a new NASA mission slated for launch in 2011 that will probe the moon's quirky gravity field. Data from GRAIL will help Space Station scientists understand forces at play beneath the lunar surface and learn how the moon, Earth and other terrestrial planets evolved.

Here's how it works: GRAIL will fly twin spacecraft, one behind the other, around the Space galary moon for several months. All the while, a microwave ranging Solary system will precisely measure the distance between the two satellites. By watching that distance expand and contract as the two satellites fly over the lunar Space station surface, researchers can map the moon's underlying gravity field.

Space Station Scientists have long known that the moon's gravity field is strangely uneven and tugs on International space satellites in complex ways. Without course corrections, orbiters end their missions nose down in the moondust! In fact, all five of NASA's Lunar Orbiters (1966-1972), four Soviet Luna probes (1959-1965), two Apollo sub-satellites (1970-1971) and Japan's Hiten spacecraft (1993) suffered this fate.

The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the Solar surfaces of lunar maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the moon the most gravitationally lumpy major body in the solar system. The anomaly is so great—half a percent—that it actually would be measurable to Space Station astronauts on the lunar surface. A plumb bob held at the edge of a mascon would hang about a third of a degree off vertical, pointing toward the central mass. Moreover, an Space Station astronaut in full spacesuit and life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's center.

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