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Getting Gravity Probe B Ready Was Tough.

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Gravity Probe B.
Gravity Probe B.

After four months in orbit, NASA's Gravity Probe B is finally ready to begin its scientific mission - to find out if two predictions by Einstein about relativity are correct. It wasn't easy, though. Controllers expected to get the spacecraft orienting on its target star within a few days, but it took weeks because sunlight reflecting off of dust particles confused its tracking system. Engineers also had to tweak the spacecraft's software to compensate for cosmic rays that flashed into its telescope. Everything's working fine now, and in a year or so, we should have some answers.

It's "all systems go" for one of the most ambitious physics experiments ever attempted.

On August 27th, after four months in orbit, NASA's Gravity Probe B satellite began its year-long hunt for signs of a subtle space-time vortex around Earth predicted by Einstein's theory of relativity. The search isn't going to be easy, but for scientists involved, one of the hardest parts is already over: months of delicately starting up and checking out the satellite, when one wrong move could have ruined the experiment before it ever got started.

"It's a long and tortuous story," says Francis Everitt, principal investigator for Gravity Probe B (GP-B) and a professor at Stanford University.

One of the key parts of GP-B is an onboard telescope that locks on to the star IM Pegasus, which serves as a fixed point of reference in the sky. Everitt and his colleagues had figured that pointing the telescope at that star would be quick and painless, taking only three days after the launch.

Instead it took weeks.

First, sunlight reflecting off floating dust particles confused the satellite's star-tracking sensors. These sensors use the locations of constellations to orient the spacecraft, and the tiny shining specs looked like stars. The dust eventually cleared, but then another problem arose: Cosmic radiation in the form of high-speed protons peppered the telescope's light sensor, causing false signals. Mission scientists had to tweak the satellite's software to ignore these pulses. And on it went like this for weeks; scientists would solve one problem only to encounter another.

"Now it has become very routine, and we only take about a minute to acquire the star as we come up over the horizon," Everitt says. (The satellite loses sight of the guide star during each orbit because it passes behind the Earth, so it must reacquire the star as it comes back into sight.)

The purpose of the telescope and the guide star is to help scientists keep track of four spinning spheres, or gyros, onboard the satellite. These gyros, which will be listed in a forthcoming edition of the Guinness Book of World Records as the roundest objects ever manufactured, are the heart of the experiment. In the beginning, their spin axes are aligned with IM Pegasus. If space-time around Earth is really twisted, as Einstein says, the gyros will wobble, slowly drifting out of alignment with the distant star during GP-B's one-year mission.

"One of the things all of us were terribly worried about was getting some dirt in the gyro housings," Everitt says. The gyros float a near-perfect vacuum, and only a thousandth-of-an-inch gap separates the spheres from their casings.

"The gyros were cleaned before they went up, but we gave this thing a tremendous vibration during launch. Wouldn't you expect a piece of dirt to come in through one of the pump-out ports, land right on one of the gyros and jam it?" he says. "That would be the end of that gyro."

This time all the worrying was for nothing. "The gyros have all been as clean as a whistle," he says. They're suspended in their casings, aligned with the guide star, and spinning thousands of times per minute. "Amazing, delightful."

Now the gathering of science data begins. The satellite's onboard computers should be able to handle this phase of the mission automatically. Still, at least one person will be on duty monitoring GP-B at all times throughout the year, Everitt says. "It should run itself, but you can never relax."

After more than 40 years of methodical planning and four months of intense troubleshooting, GP-B's scientists feel "a real sense of gladness," he says. "What a difference it makes to be up there and operating. How thrilling that is. We all feel that."

"Some people," laughs Everitt, "are talking about taking a week or two of well-deserved vacation."

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