Gravity Probe B is a satellite-based, relativity gyroscope experiment that launched in 2004 for the purpose of measuring the space-time curvature near Earth in order to test Einstein’s theory of general relativity. The experiment was to check, very precisely, tiny changes in the spin and angle of four gyroscopes contained in a highly sophisticated housing inside the Earth satellite. According to Nature News, the gyroscopes are “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.”
To conduct the experiment, the Gravity Probe B (GP-B) gyroscopes must spin with near-zero constraints. When the experiment was first conceived, the technology required was non-existent. In fact, Stanford, Lockheed Martin, and NASA had to invent over a dozen new technologies in order to meet the constraints required to get the gyros spinning in a manner required for the spacetime experiments.
Firstly, the gyroscope had to be spherically near perfect. It could have no imbalance in mass or density distribution nor any uneven surface deviations. And the friction between the bearings and axle of the gyroscope must be near zero. To meet these requirements, a 1.5 inch sphere was constructed from pure fused quartz, polished perfectly round to within 40 atoms. GP-B rotors are now listed in the Guinness Database of World Records as the roundest object ever manufactured. The surface of the spheres is less than three ten-millionths of an inch from perfection. If the spheres were enlarged to the size of Earth, its highest peak would be only 8 feet high.
To solve the problem of friction between the bearings and the axle of the gyroscope, bearings were removed entirely and the gyroscope levitated using six evenly-spaced electrodes. As a result, the rotors spin at 4,000 revolutions per minute, a mere 32 microns from their housing walls and are free from any mechanical or fluid supports.
Finally, a stream of pure helium was used to spin up the gyroscopes to 4,000 rpm after which, the housing evacuated so as little as a few molecules of helium remained inside. Given these requirements, the gyroscopes would then operate in a near perfect environment. The system is so effective, the gyroscopes have an average spin-down time constant of 15,000.
Sources: Nature News, Stanford, Guinness Book of World Records