Gravity Probe B (GP-B) was a satellite-based experiment whose objective was to test two previously-unverified predictions of general relativity: the geodetic effect and frame-dragging.

Gravity Probe B was expected to measure this effect to an accuracy of one part in 10000, the most stringent check on general relativistic predictions to date.

The Gravity Probe B experiment comprised four London moment gyroscopes and a reference telescope sighted on IM Pegasi, a binary star in the constellation Pegasus.

In polar orbit, with the gyro spin directions also pointing toward IM Pegasi, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.

Near-absolute zero temperatures were required to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive.

If one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only 2.4 m (8 ft) high.

Also important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals.

In preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over several years to understand its motion as precisely as needed.

Gravity Probe B marks the first time that Stanford University has been in control of the development and operations of a space satellite funded by NASA.

In April it was announced that the spin axes of the gyroscopes were affected by torque, in a manner that varied over time, requiring further analysis to allow the results to be corrected for this source of error.

In the data for the frame-dragging results presented at the April 2007 meeting of the American Physical Society, the random errors were much larger than the theoretical expected value and scattered on both the positive and negative sides of a null result, therefore causing skepticism as to whether any useful data could be extracted in the future to test this effect.

This created a classical dipole torque on each rotor, of a magnitude similar to the expected frame dragging effect.

As it was anticipated that "anything could go wrong", the final part of the flight mission was calibration, where amongst other activities, data was gathered with the spacecraft axis deliberately misaligned for 24 hours, to exacerbate any potential problems.

[needs update] A review by a panel of 15 experts commissioned by NASA recommended against extending the data analysis phase beyond 2008.

On 29 August 2008, the 18th meeting of the external GP-B Science Advisory Committee was held at Stanford to report progress.

The Stanford-based analysis group and NASA announced on 4 May 2011 that the data from GP-B indeed confirms the two predictions of Albert Einstein's general theory of relativity.