nside the probe operated in a very quiet, low acceleration environment.
The Science Instrument Assembly was simple in its concept: A block of fused quartz 21-inches long, with a bonded quartz telescope on one end, held four gyroscopes. Very sensitive magnetometers provided the gyroscope readouts. The gyro-telescope instrument was held in the flight probe, which was inserted into the dewar, an extremely complicated Thermos-like bottle. The dewar held 613 gallons of superfluid, supercold helium that kept the instrument chilled to about 2 Kelvin (-455 degrees Fahrenheit) for the duration of the mission.
When Gravity Probe B was launched into a 400-mile-high polar orbit, its science instrument measured minute changes in the directions of spin of the four ultra-precise gyroscopes contained within. The gyros were so free of disturbances that they provided a nearly perfect space-time reference system. They were referenced to the science telescope that sighted on a far-field highly stable reference star.
"Gravity Probe B is one of the few space missions NASA has conducted with relevance to fundamental physics," stated a review of GP-B undertaken in 1995 by the Space Studies Board of the National Research Council. "If successful, it would assuredly join the ranks of the classical experiments of physics. By the same token, a confirmed result in disagreement with General Relativity would be revolutionary."
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