Einstein's nearly 90-year-old General Theory of Relativity is getting one of its most rigorous tests high above the Earth. The Gravity Probe B (GP-B) has been in orbit since April 2004 conducting what could be a two-year test of the so-called "frame-dragging" effect-the "twisting" of the local space-time fabric. Critical to the tests: advances in design of gyroscopes.

The Probe has four gyros. The gyro rotors are made of fused quartz and ground to near-absolute sphericity. The near-perfection of the ping pong-size gyros is necessary because imperfection can distort their position: They will be pointing to a reference guide star. The frame dragging that distots the space-time fabric, theoretically would distort the gyroscopes too. But imperfections in the gyros themselves could also cause distortion, invalidating the experiment.

Engineers levitated the rotors with three saucer-shaped electrodes so they could suspend them in their cavity without disturbing the spin. To get and keep the rotors spinning, they directed a precise stream of helium gas at the rotors. And, they used superconductivity as the basis of a noninterfering pointer readout based on the superconducting quantum interference device (SQUID). It senses any angular shift in the rotor spin axis.

Though the GP-B has another 16 or so months to go, engineers have already achieved successes, including advanced gyro fabrication, near-perfect elimination of interfering magnetic fields, and telescope pointing and control.