Magazine article Science News

Mapping with Grace: Twin Satellites Chart Changes in Earth's Gravitational Field

Magazine article Science News

Mapping with Grace: Twin Satellites Chart Changes in Earth's Gravitational Field

Article excerpt

Concerned about your weight? Don't go to the North Pole, where you're about 20 km closer to the center of Earth--and therefore a pound or so heavier--than at the equator. Head, instead, for India. There, you'd be standing over a less-dense landscape with a gentler gravitational pull. Yes, what you weigh depends on where you are. Your body doesn't change from place to place, but the gravitational field does. Topography, crust composition, and the planet's rotation-induced equatorial bulging are among the factors that influence Earth's gravitational pull at different locations. Furthermore, this uneven gravitational field changes slightly with the seasons, as precipitation carries moisture's mass from the oceans onto the continents.

For more than 30 years, scientists have been monitoring the planet's tug with several dozen satellites and sensitive instruments carried into the field. But the global gravitational model that they've compiled from that data has just been rendered obsolete by a pair of satellites that were launched last March.

Over their 5-year lifespan, the two spacecraft--dubbed the Gravity Recovery and Climate Experiment, Or GRACE--will produce gravity maps more than 1,000 times as accurate as those currently in use. With this enhanced accuracy, scientists will monitor subtle seasonal shifts in ocean currents, the changing mass of ice sheets, and the movement of water over and beneath Earth's surface.

ORBIT FOR TWO The twin GRACE craft--each about the size of a car and weighing half a ton--will orbit Earth at an altitude of nearly 500 kilometers, says Michael M. Watkins, a project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. They'll zip along a single trajectory, with one satellite leading the other by about 200 km. A microwave relay between the two craft will enable scientists to measure the distance between the satellites within a few millionths of a meter--about the width of a red blood cell or a particle of smoke. That accuracy is the key to constructing detailed maps of the planet's gravitational field, says Watkins.

Here's how the process works: As the first GRACE craft approaches a massive object on Earth's surface--a mountain, for example--it's pulled slightly forward in its orbit, away from its partner. …

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