The gyroscopes also have to avoid virtually all external acceleration, which is not as easy as it sounds. (When astronauts talk about zero-g, the g stands for gravity, but what they're really talking about is an environment free of gravitational acceleration. While gravitational forces exist everywhere in the universe, in a zero-g environment like the space shuttle, Earth's gravity is pulling the shuttle and everything inside it around Earth but not toward it, therefore gravity goes unnoticed and its force appears to be nearly zero.) Although Gravity Probe B will be in zero-g by astronaut standards once it reaches its orbit, it will still be buffeted not only by solar wind but by Earth's magnetic field and the upper part of the atmosphere.

"We want to fly as low as we can," says astronautical engineer Gaylord Green, who is in charge of the mission's spacecraft, "because we get our best [frame-dragging] signature there, but we have to fly high enough so that we're above the atmosphere. So we fly right at the upper fringes of the atmosphere, and where the atmosphere itself turns into space it's almost like ocean tops, little waves and sprinkles of molecules, that will keep hitting the satellite." The constant acceleration and deceleration that results would be unacceptably large for an experiment designed to measure a nearly infinitesimal effect.

The solution is to build Gravity Probe B as a drag-free satellite, a concept that physicist Ben Lange helped develop in the 1960s as his doctoral thesis at Stanford. As Lange explains it, the researchers enlist one of their four gyroscopes to act as a proof-mass, an object allowed to fall freely around Earth within its near-perfect environment. In effect it is allowed to respond to the local gravitational field without any outside disturbances. As it does so, devices called capacitance sensors check on its position with respect to the surrounding satellite. These sensors then tell the satellite's thrusters which direction to push the satellite to keep it centered around the mass. Although, as Green says, the word thruster is something of a misnomer. The satellite uses as its thrust the minuscule amount of helium gas that slowly boils out of the dewar to cool it, just the way evaporating sweat cools you down. "What we do is take the gas from the helium dewar," says Green, "sort of like a very gentle breath, and that's the gas we use to control the satellite. As a result, we're nearly seven orders of magnitude closer to zero-g than when astronauts float around in the space shuttle."

Once in orbit, the telescope on the probe, which is all of 13 inches long, will keep the satellite aligned on a distant star to within 20 milli-arc seconds. When Everitt gives a tour of the Gravity Probe B facility, he points out the telescope testing room, where the researchers have built what they call an artificial star to practice tracking. "One learns from this device," Everitt says, "an extremely healthy respect for a thousandth of an arc second." It seems that the solid ground is not so solid after all. It tends to vibrate naturally, shaking the telescope by 20 milli-arc seconds and making the testing considerably more difficult.

Nothing's simple on Gravity Probe B, not even selecting a guide star. "You want a reasonably bright star that you can track," explains Parkinson. "Bright and blue, preferably. You don't want a lot of infrared light, because it will heat the experiment up. You don't want it located too close to the plane of the solar system or the sun will cross the telescope's path and burn its eyes out. You would like it to be somewhere near the plane of the equator, because if not, you have difficulty picking out the frame-dragging effect. If it's pointing toward the North Pole, you won't see it because you're pointing along the axis of frame-dragging rather than perpendicular to it.''

They also want a star that isn't moving too much on its own. For instance, one with a large twin will wobble through space as the pair orbit around each other. If the star is also a radio source, the physicists could use its signal to measure its motion with respect to several strong radio sources known as quasars, which lie at the edges of the known universe. These quasars are too dim to track, but as radio sources they make ideal beacons with which to check the relative motion of potential guide stars. That way, the researchers can be assured that the star they choose is not going to botch the Gravity Probe experiment by wandering around the universe on its own.

In the early 1990s everitt and his colleagues engaged the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, to see if they could find a suitable candidate, which they figured would be easy considering how many stars there are to choose from. They were wrong, of course. It wasn't. It took the Harvard-Smithsonian researchers, led by Irwin Shapiro, several years to come up with all of two stars that fit the criteria--known by the astronomical designations hr-1099 and hr-8703, the latter also called Im Pegasi. ("That's in the constellation Pegasus," says Everitt, "and if you know the constellation Pegasus, you're a better man than I am, because I don't.")

Should the Gravity Probe B mission ever fly--"We think we've made most of our mistakes already," says Everitt, "or at least we hope we've made most of our mistakes"--it will perform the most precise test ever of any prediction of any aspect of general relativity. "That's clearly interesting," says Everitt. "The question is, is it more than interesting? And the answer to that is, we don't know." General relativity has become so well accepted and so well tested that there aren't any alternative theories to compete with it. On the other hand, because the force of gravity is so weak compared with the other forces of the universe--electromagnetism, for instance, which is 42 orders of magnitude stronger--physicists have been unable to test Einstein's theory with the same rigor that they've tested other favorite theories of the universe.

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