Curve balls. LAGEOS's laser-ranging satellites revealed a twist in spacetime.

Satellites Spy Earth's Relativistic Wake

This world is a drag--and scientists have proved it. By studying the dance of two Earth-orbiting satellites, physicists have detected the subtle twisting of spacetime around a massive, spinning object. The measurement is the most convincing demonstration yet of a hard-to-spot consequence of Albert Einstein's general theory of relativity.

General relativity predicts that a spinning mass drags the fabric of space and time around with it, much as a restless sleeper drags the sheets around while twisting and turning in bed. This effect, known as the Lense-Thirring, or "frame dragging," effect, is difficult to detect. To spot it, one has to observe how a spinning body changes the orientations of nearby gyroscopes.

Physicists Ignazio Ciufolini of the University of Lecce, Italy, and Erricos Pavlis of Goddard Space Flight Center in Greenbelt, Maryland, and colleagues tried to measure the effect in 1998 by using two satellites, LAGEOS and LAGEOS II, as test gyroscopes. The satellites--half-meter-wide mirrored spheres--were launched in 1976 and 1992 as targets for laser range finders, which can track their position within a few centimeters. As the satellites spin around Earth, the Lense-Thirring effect twists the planes of their orbits slightly. The early measurements were "very rough," Ciufolini says, because the uneven distribution of Earth's mass distorts the orbits thousands of times more than does the Lense-Thirring effect. Because the mass distribution was poorly known, Ciufolini and colleagues had to make a few controversial estimates. As a result, their value of the Lense-Thirring effect had a large error—20%--and skeptics thought that there were subtle problems with estimates that the researchers made about the satellites' orbits. Now, thanks to better gravitational maps produced by twin satellites known as GRACE, as well as improved gravitational models and other refinements, the team now reports in the 21 October issue of Nature a much firmer detection with an error of about 10%. "This is the first reasonably accurate measurement," says Neil Ashby, a physicist at the University of Colorado, Boulder. Ciufolini predicts that a few more years of observation will reduce the measurement's error to just a few percent. By then, physicists hope, the gyroscope-laden satellite Gravity Probe B, which was designed to detect the Lense-Thirring effect, will have produced results with an error of about 1%--far lower than the two LAGEOS satellites can achieve. Related sites
More about the LAGEOS satellites from NASA
Science news article about Gravity Probe B
Probe B site at Stanford University

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