Gravity Probe B was a satellite-based physics experiment that was launched on April 20, 2004, and made delicate measurements using four gyroscopes and one telescope. The goal of the experiment was to detect the curvature of the earth’s gravitational field, as well as the ‘frame-dragging’ effects that come with it. The four most spherical objects ever created by human hands are gyroscopes suspended in superfluid helium vessels at temperatures close to absolute zero. Gravity Probe B is still orbiting the earth, but its instrumentation failed in late 2005, after serving its purpose.
The Gravity Probe B craft was equipped with some of the most advanced scientific equipment available at the time. Gravity Probe B, a NASA-funded project led by Stanford University’s physics department, is the first time a university has been in charge of a NASA-funded project. The project’s main subcontractor, Lockheed Martin, built the majority of the launch hardware. Stanford University engineers and physicists created the science package.
A spherical gyroscope keeps spinning in the same direction it gets an initial push in the absence of gravitational interference. The gyroscopes on Gravity Probe B, which are uniform spheres to within 40 atomic diameters, should move in uniform rotations in particular. A gyroscope does not move in a uniform manner when gravitational interference is present. Gravity Probe B’s gyroscopes are slightly perturbed as it completes a full orbit around the earth due to 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 provide other useful information about the region around the earth’s relativistic properties.
Super-accurate Superconducting Quantum Interference Devices (SQUIDs), which measure changes in magnetic fields as small as a few nanoteslas, were used to measure the changes in rotation angle among the gyroscopes. Gravity Probe B’s SQUIDs were made of niobium and consisted of a thin layer on the gyroscopic spheres’ surface. The majority of the spheres were made of fused quartz, which has a very uniform density.