The Gravity Probe B experiment and early results

The NASA Gravity Probe B orbiting gyroscope test of General Relativity, launched from Vandenberg Air Force Base on 20 April, 2004 tests two consequences of Einstein's theory: 1) the predicted 6.6 arcs/yr geodetic effect due to the motion of the gyroscope through the curved space-time around the Earth; 2) the predicted 0.039 arcs/yr frame-dragging effect due to the rotating Earth. The mission required the development of many technologies that did not exist when experiment was conceived in 1960. Cryogenic gyroscopes with drift-rates 7 orders of magnitude better than the best inertial navigation gyroscopes, a < 1 marcs star tracking telescope, and other essential technologies were developed as a result of an intensive collaboration between Stanford physicists and engineers, NASA and industry. Gravity Probe B collected science data from August 27, 2004 through September 29, 2005. Analysis of the data began during the mission and is on-going. This paper describes the main features and challenges of the experiment and presents the preliminary results to date.