Relativity Revisited

NASA's Gravity Probe B spacecraft has begun its search for a bizarre prediction of Einstein's relativity.

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

On August 27th 2004, 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.)