After 40-year prep, gravity test soars.

On April 20, a satellite conceived in the 1960s to test two aspects of Einstein's general theory of relativity finally roared into space from Vandenberg Air Force Base in California. Aside from two problems that have been solved--a backup thruster stuck open and a temporary computer malfunction due to radiation--all subsystems of NASA's Gravity Probe-B (GP-B) are functioning as planned, the satellites handlers say.

The mission had been canceled and reinstated seven times over a period of years. When the spacecraft reached its orbit and deployed all its solar panels, it "was a moment of considerable relief," says physicist C.W. Francis Everitt of Stanford University, longtime head of the project.

GP-B is basically a big tank of liquid helium with a tube running down its center. In the tube are four ultraprecise gyroscopes and a sharp-eyed telescope (SN: 11/1/03, p. 280).

By late May, mission controllers expect to spin up the gyroscopes and align their axes with a distant star as a reference point. Then, for 13 months, researchers will look for minuscule amounts of drift of the gyroscopes' axes. According to relativity theory, that drift should occur because Farth's gravity both bends space-time and drags it around with the rotating planet.

The mission's payoff? A powerful confirmation of Einstein's theory of gravity or a sign that the theory needs a makeover.--P.W.