Gravity Probe B
In 1993, NASA dropped a space shuttle mission that might have helped
check out the equipment in space, but the gyros have proved themselves
so far in more than 90,000 hours of ground tests.
The perfection of the spheres is only a first step, however.
When they conceived this
project, they thought it was a five-year deal, not a 30-year deal. When you
are exploring the unknown, you cant predict how long it will
take. Bradford Parkinson
The quartz rotors pose a second, equally daunting test of ingenuity: How
can researchers measure the direction of spin of an unblemished sphere
without disturbing it or being able to see it? For that matter, how can
they keep the spheres from expanding as internal temperatures fluctuate,
or protect them from variations in the surrounding magnetic fields?
To do so, the researchers have taken advantage of how materials behave
at low temperatures.
Each sphere is coated with superconducting niobium and chilled to 2.5
degrees Kelvin, within a few degrees of absolute zero (minus 456 degrees
Fahrenheit). That permits the balls to be suspended electrically.
Any changes in the tilt of the gyroscopes' spin axes can be measured
indirectly by detecting changes in the direction of the magnetic field
surrounding the spheres with superconducting sensors that do not exert
any unwanted force on the rotors. The low temperatures also help to
stabilize the onboard telescope and shield the entire experiment from
outside magnetic interference.
To keep the gyroscopes at the proper temperature in space, the entire
package of instruments will ride inside what essentially is a thermos
bottle, called a dewar, brimming with 640 gallons of some of the coldest
liquid known to