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Gravity Probe B

Also named GP-B


Designation 28230 / 04014A
Launch date 20 Apr 2004
Country of origin United States
Mission Scientific:
Perigee/Apogee 637/659 km
Inclination 90°
Launch vehicle Delta 2 #304

Worth $750 million. Scientists first proposed what became Gravity Probe B in 1959. Over the decades, it weathered more than a half-dozen attempts at cancellation amid concerns over cost overruns and technical hurdles.

Attempted to verify two subtle physical effects predicted by Albert Einstein's General Theory of Relativity. It actually managed to prove that space and time bend!
The first result from the Gravity Probe B satellite confirms a prediction of Einstein's general theory of relativity to a precision of better than 1%.

Status: confirmed 2 predictions of Albert Einstein's general theory of relativity.

External resources


sat-index articles

Technical data


Prime contractor Stanford University
Mass at launch 3145 kg
Mass in orbit  
Solar array  
DC power  
Design lifetime 16 months

Integrated and tested by Lockheed Martin.

The payload is made of two parts: the dewar (structural component) and the flight probe (2.7 m long vacuum chamber) which contains the Science Instrument Assembly.

When the Gravity Probe B experiment begins, the instrument apparatus will measure minute changes in spin axis orientation of four ultra-precise gyroscopes contained within. The gyros will be so free of disturbances that they will provide a nearly perfect space-time reference system. They will measure two predicted effects of Einstein's theory: whether and how space and time are warped by the presence of Earth, and whether and how the rotating Earth drags space-time around with it. This will be by far the most accurate test of any of the predicted effects of Einstein's theory.

Carries 4 gyroscopes kept at 1.8 Kelvin by a liquid helium dewar, laser retroreflectors and 2 GPS receivers for orbit determination, a drag compensation system, and a 14 cm aperture quartz telescope. Will lock on its guide star IP Pegasi (precision of 1/100,000th of a degree).

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