Overview
Gravity Probe B was a relativity gyroscope experiment funded by NASA. Efforts were led by Stanford University physics department with Lockheed Martin as the primary subcontractor. Mission scientists view it as the second gravity experiment in space, following the successful launch of Gravity Probe A (GP-A) in 1976.
Some preliminary results were presented at a special session during the American Physical Society meeting, 14–17 April 2007. NASA initially requested a proposal for extending the GP-B data analysis phase through December 2007. The data analysis phase was further extended to September 2008, and possibly later, when definitive science results on the frame-dragging effect are expected. The mission plans were to test two unverified predictions of general relativity: frame-dragging and the geodetic effect.
The experiment planned to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 650 km (400 mi) altitude, crossing directly over the poles. The gyroscopes were so free from disturbance that they provided a near-perfect space-time reference system. They were intended to measure how space and time are "warped" by the presence of the Earth, and by how much the Earth's rotation "drags" space-time around with it. This is the so-called frame-dragging effect, an example of gravitomagnetism. It is an analog of magnetism in classical electrodynamics, but caused by rotating masses rather than rotating electric charges.
Previously, only two analyses of the laser-ranging data obtained by the two LAGEOS satellites, published in 1997 and 2004, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively, whereas Gravity Probe B aims to measure the effect to a precision of 1%.
However, Lorenzo Iorio claimed that the level of total uncertainty of the tests conducted with the two LAGEOS satellites has likely been greatly underestimated. A recent analysis of Mars Global Surveyor data has claimed to have confirmed the effect to a precision of 0.5%, although the accuracy of this claim is disputed. Also the Lense–Thirring effect of the Sun has been recently investigated in view of a possible detection with the inner planets in the near future.
The probe has also detected the so-called geodetic effect, a much larger effect caused by space-time being 'curved' by the mass of the Earth. A gyroscope's axis when parallel transported around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. The angle 'missing' may be thought of as the amount the gyroscope 'leans over' into the slope of the space-time curvature. A more precise explanation for the space curvature part of the geodetic precession is obtained by using a nearly flat cone to model the space curvature of the Earth's gravitational field. Such a cone is made by cutting out a thin 'pie-slice' from a circle and gluing the cut edges together. The spatial geodetic precession is a measure of the missing 'pie-slice' angle. Gravity Probe B should measure this effect to an accuracy of one part in 10,000, the most stringent check on general relativistic predictions to date.
The launch was planned for 19 April 2004 at Vandenberg Air Force Base but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On 20 April, at 9:57:23 AM PDT (16:57:23 UTC) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 AM (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months.
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