NASA - National Aeronautics and Space Administration

Artist concept of Gravity Probe B spacecraft in orbit around the Earth. Image Credit: Stanford

Gravity Probe B is the relativity gyroscope experiment developed by NASA and Stanford University to test two unverified predictions of Albert Einstein's general theory of relativity. Launched on April 20, 2004, from Vandenberg Air Force Base, Calif., Gravity Probe B spent 17 months collecting data while in orbit around the Earth. Fifty weeks worth of data has been downloaded from the spacecraft and relayed to computers in the Mission Operations Center at Stanford University, Stanford, Calif. Since October 2005, scientists have conducted painstaking data analysis and validation.

The experiment used four ultra-precise gyroscopes to generate the data required for this unprecedented test to precisely measure two effects predicted by Einstein's theory. One is the geodetic effect, the amount by which the Earth warps the local space time in which it resides. The other, called frame-dragging, is the amount by which the rotating Earth drags local space time around with it.

> View NASA fact sheet (PDF, 2.85 MB)
> Gravity Probe B Excecutive Summary (PDF, 1.9 MB)
> Gravity Probe B in a Nutshell (PDF, 500 KB)
> Gravity Probe B Overview Presentation (PDF, 911 KB)