By precisely calculating odd shifts in the orbit of Mercury, Einstein gained important support for his theory of general relativity, which posits that gravity arises because mass bends spacetime. By studying the orbits of two manmade satellites around Earth, scientists now say they have confirmed a much smaller effect predicted by the theory: namely, that mass drags spacetime with it as it rotates.

Astrophysicists think this "frame dragging" is a big deal for rapidly rotating neutron stars and black holes, but that near Earth it is extraordinarily small. In April, NASA launched the Gravity Probe B satellite, which carries high-precision gyroscopes designed to measure the effect with about 1 percent accuracy. Researchers writing today in Nature analyzed position measurements of two earlier satellites over an 11-year period, and determined that the 80-million-meter-long orbits of the satellites shifted by about two meters per year. Ignazio Ciufolini of the University of Lecce in Italy and Erricos Pavlis of the University of Maryland in Baltimore County conclude that the shift is within 1 percent of the rate predicted by relativity, although they only claim an accuracy of 10 percent. In an accompanying commentary, Neil Ashby of the University of Colorado calls the result "the first reasonably accurate measurement of frame-dragging."

The scientists used data from the LAGEOS and LAGEOS 2 satellites, which are intended to measure the movement of Earths land masses. Each beachball-size sphere is speckled with corner cubes to reflect light, much like the center dots on a highway. By timing the return of a short laser pulse, investigators can measure the distance to the satellite to within a few millimeters and compare it with predictions. The authors used recent, precise measurements of Earth's slightly lopsided mass and a comparison of the two satellites to isolate the tiny frame-dragging effect. Still, researchers will eagerly await the Gravity Probe B results in 2006, which should not require the large background corrections. "I fully expect that this claim will have a lot of scrutiny," Ashby says.

Although general relativity is widely accepted, other theories of gravity do exist, and precise tests sometimes yield surprises. For example, one term that Einstein had set to zero in his equations is now recognized as the "dark energy" that is accelerating the expansion of the universe.