What is Gravity Probe B?

Michael Anissimov

Gravity Probe B was a satellite-based physics experiment, launched on 20 April 2004, which used four gyroscopes and one telescope to make delicate measurements. The purpose of the experiment was to detect the curvature created by the earth’s gravitational field, along with associated ‘frame-dragging’ effects. The gyroscopes, suspended in vessels of superfluid helium at temperatures close to absolute zero, are the four most spherical objects ever created by human hands. Gravity Probe B continues to orbit the earth, but its instrumentation became useless in late 2005, having served its course.

Gravity Probe B was funded by NASA and led by Stanford University’s physics department.
Gravity Probe B was funded by NASA and led by Stanford University’s physics department.

The Gravity Probe B craft used some of the most sophisticated scientific instrumentation available at the time. Funded by NASA and led by Stanford University’s physics department, Gravity Probe B marks the first time that a university has been in charge of a NASA-funded project. Lockheed Martin was the main subcontractor for the project, building most of the launch hardware. The science package was designed by engineers and physicists at Stanford University.

In the absence of gravitational interference, a spherical gyroscope keeps spinning in the same direction it gets an initial push in. Gravity Probe B’s gyroscopes, which are uniform spheres to within 40 atomic diameters, should especially move in uniform rotations. When there is gravitational interference, a gyroscope does not move in a uniform way. As Gravity Probe B makes a complete orbit around the earth, the gyroscopes are perturbed slightly by the curvature in spacetime caused by the earth’s gravity field. The exact angle of perturbation can be used to calculate the degree of spacetime curvature and give us other important information about the relativistic qualities of the region around the earth.

The changes in rotation angle among the gyroscopes were measured by super-accurate Superconducting Quantum Interference Devices (SQUIDs) which measure changes in magnetic fields as small as a few nanoteslas. The SQUIDs in Gravity Probe B were made of niobium and consisted of a tiny layer on the surface of the gyroscopic spheres. The bulk of the spheres was made of fused quartz, which can be made extremely homogeneous in density.

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