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 ultrasmooth 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.

Design Life: 16 months. Typical orbit: 650 km circular, polar.

• 2004 April 20 - Gravity Probe B - Launch Site: Vandenberg. Launch Vehicle: Delta. Mass: 3,145 kg (6,933 lb). Perigee: 640 km (390 mi). Apogee: 646 km (401 mi). Inclination: 90.00 deg. Period: 97.60 min.

  Gravity Probe B's mission was to confirm a prediction of Einstein's theory of relativity. The physics experiment, developed by Stanford University and Lockheed Martin, was to observe the magnitude 5 star IM Pegasi for over a year, attempting to measure the tiny shifts in the spacecraft gyroscopes' orientation caused by the Lense-Thirring gravitomagnetic (or `frame-dragging') effect. To accomplish this the spacecraft carried four gyroscopes kept at 1.8 deg Kelvin by a liquid helium dewar, laser retroreflectors and two GPS receivers for orbit determination, a drag compensation system, and a 14 cm aperture quartz telescope. The satellite was also to make an accurate measurement of the already-established gravitostatic warping of spacetime due to the Earth's mass.