A falling apple led the 17th century English scientist Isaac Newton to formulate his principle of gravity. But what Newton did not know is that apples don't drop at the same rate everywhere on Earth because gravity's pull varies slightly from place to place. The GRACE gravity mapping mission is to launch from Russia at 0923 UTC Saturday, March 16. A Russian rocket will launch two U.S.-German spacecraft from Plesetsk to map these variations. The findings will prove useful to scientists who study the oceans, geology and climate.

Gravity is everywhere and influences everything from the expansion of the universe to the structure of our bodies, which have developed muscles and strong bones to keep us from being pulled to the ground.

Gravity also affects Earth's structure, the motion of the seas and the movement of land water. To understand Earth's physical systems better, University of Texas scientist Byron Tapley says researchers want an accurate map of our planet's gravity. "We usually think of gravity as constant, but in fact, we know gravity varies over the surface of the Earth," he says. "It's lesser on the mountaintop than it is at the ocean surface, and this spatial variation is a very important property in a number of scientific studies and applications itself."

Gravity's variation which is only one percent or less - occurs because the distribution of Earth's mass is not the same everywhere. As Isaac Newton taught us, more mass means more gravitational pull.

But measuring gravity's worldwide fluctuations has been difficult because data must be combined from several dozen satellites. While they do a good job assessing large-scale features of Earth's gravitational field, they cannot pinpoint finer features or accurately show monthly variations.

That's where a new pair of U.S.-German satellites known together as GRACE comes in. U.S. space agency scientist Michael Watkins says they will race around Earth's poles 16 times a day about 220 kilometers apart sensing minute variations in the surface mass below, and hence differences in gravitational pull. "As they come upon a gravitational mass, the first spacecraft is pulled toward it slightly, and that changes the separation, so they go together and apart due to Earth's gravity field. Now the real key to the GRACE mission is that we map that separating distance extremely accurately, to about a millionth of a meter," he explains.

That's just one-tenth the width of a human hair, sensed by microwave rangefinders on each satellite. Byron Tapley says the result of this ultra-sensitive technology will be a series of extremely accurate gravity maps over the five-year lifetime of the GRACE mission. "The GRACE goal has as its objective to each 30 days measure the gravity field with an accuracy that's between 100-1,000 times better than current knowledge. In the five-year time frame, this means we'll get 60 measurements with this unparalleled accuracy, and it should provide a remarkable insight into the working of Earth's dynamic system," says Mr. Tapley

As an example, the GRACE satellites will be able to help determine if sea level rise is the result of melting glaciers or simply expansion due to heating. Glacier melt would add to water mass and therefore to gravitational pull.

GRACE will also help scientists track the movement and changes in ice sheets, study ocean currents, and follow alterations in Earth's solid structure, since all of these involve movement in the planet's mass.

Michael Watkins says that because ocean movements affect the atmosphere, the twin satellites will help feed data of importance to climate studies. "Those of us who have been working on ways to measure gravity see a very broad smorgasbord of things that we can apply this to polar ice mass, oceanography, hydrology and many others. For us I think that's really the exciting part," he says.

The GRACE mission is a joint effort between the U.S. and German space agencies.