NASA's Gravity Probe B spacecraft
has begun its search for a bizarre prediction of Einstein's relativity.
by Patrick Barry
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
Gravity Probe B in Earth orbit.
"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
"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."
A spinning spherical gyroscope in Earth orbit should wobble
in the whirlpool shaped space-time around our planet.
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
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."
Gravity Probe B mission control at Stanford University.