The Gravity Probe B Relativity Mission

The NASA/Stanford Relativity Mission Gravity Probe B (GP-B) experiment will provide two extremely precise tests of General Relativity based on observations of electrically suspended gyroscopes in a satellite in a 650 km circular polar orbit around the Earth. The project is now nearing completion. Final assembly of the instrument will take place later this year and launch is scheduled for October 2000. GP-B will provide a very accurate measurement of the frame-dragging effect, with its subtle connections to gravitomagnetism and Mach's principle. In addition to measuring frame dragging to 0.3%, it will measure the geodetic effect to approximately 1 part in 105. GP-B is a controlled physics experiment where error terms such as the Newtonian drifts of gyroscopes are reduced to negligible values, and where the apparatus is under the experimenters' control.