Gravity Probe-B

American earth geodetic satellite. Gravity Probe B. Gravity Probe B was an experiment developed by NASA and Stanford University to test two unverified predictions of Albert Einstein's general theory of relativity.

The experiment would 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 would measure how space and time were 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, had far-reaching implications for the nature of matter and the structure of the Universe. The spacecraft was spin stabilized (0.1 to 1 rpm). Attitude control thrusters used helium boiled off from the experiment's Dewar. The experiment gyro used for attitude reference. The payload included a block of fused quartz 21 inches long holding four gyroscopes and a proof mass, all bonded to a quartz telescope. The package was inserted into a 1500 l helium Dewar in order to maintain it at a temperature of 1.8 Kelvin. The gyroscopes were constructed from ultra-smooth quartz balls coated with niobium which became 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 spun at 10,000 revolutions a minute.

Financial status as of 1997 was $ 236 million spent, $ 340 million to complete.