Gravity Probe B - Experimental Setup

Experimental Setup

The Gravity Probe B experiment comprises four London moment gyroscopes and a reference telescope sighted on HR8703 (also known as IM Pegasi), a binary star in the constellation Pegasus. In polar orbit, with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.

The gyroscopes are housed in a dewar of superfluid helium, maintaining a temperature of under 2 kelvins (−271 °C, −456 °F). Near-absolute zero temperatures are required to minimize molecular interference, and enable the lead and niobium components of the gyroscope mechanisms to become superconductive.

At the time, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms (less than 10 nm). If one of these spheres were scaled to the size of the earth, the tallest mountains and deepest ocean trench would measure only 2.4 m (8 ft) high. They are composed of fused quartz and coated with an extremely thin layer of niobium. A primary concern is minimizing any influence on their spin, so the gyroscopes must never touch their containing compartment. They are held suspended with electric fields, spun up using a flow of helium gas, and their spin axes are sensed by monitoring the magnetic field of the superconductive niobium layer with SQUIDs. (A spinning superconductor generates a magnetic field precisely aligned with the rotation axis – see London moment.)

IM Pegasi was chosen as the guide star for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions near at the celestial equator of the sky coordinates. Also important was its well understood motion in the sky, which was helped by the fact that this star emits relatively strong radio signals. In preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over the last few years to understand its motion as precisely as needed.