Gravity Probe-B

Gravity Probe B is an experiment being developed by NASA and Stanford University to test two unverified predictions of Albert Einstein's general theory of relativity. The experiment will check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in a satellite in a 650 km polar orbit.. The gyroscopes will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, have far-reaching implications for the nature of matter and the structure of the Universe.

Spacecraft
Spin stabilized (0.1 to 1 rpm). Attitude control thrusters use helium boiled off from the experiment's dewar. Experiment gyro used for attitude reference.

Payload
A block of fused quartz 21 inches long holds four gyroscopes and a proof mass, all bonded to a quartz telescope. The package is inserted into a 1500 L helium dewar to keep at 1.8 Kelvin. The gyroscopes are constructed from ultrasmooth quartz balls coated with niobium which becomes a superconductor as liquid helium temperatures, allowing the gyroscopes to be suspended electrically. Very sensitive magnetometers detect any changes in the gyroscope's spin axis. The gyros spin at 10,000 revolutions a minute.

Country of Origin	United States
Customer/User	NASA, National Academy of Sciences
Manufacturer(s)	Stanford University, Lockheed-Martin
Launch	Planned for 1999 aboard Delta 2 from WSMC
Orbit	650 km circular, polar
Design Life	16 months