GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Figure 2-11. Exploded diagram of the GP-B spacecraft & payloadThe Science Instrument Assembly (SIA) containing the quartz block and telescope forms the pristine spacebornelaboratory for making the measurements of the GP-B experiment. The SIA is located at the center of massof the spacecraft/dewar, along its main axis. It is mounted inside a cigar-shaped vacuum canister called theProbe (described below).The quartz block houses the four spherical gyroscopes and SQUID readout instruments (Super ConductingQuantum Interference Devices—the magnetometers that read the gyroscopes’ spin axis orientation). Eachspherical gyroscope rotor is enclosed in an elegant, two-piece cylindrical quartz housing, mounted in the quartzblock and surrounded by antimagnetic shielding. The gyro rotors are electrically suspended by six roundelectrode pads—three in the top half of the housings and three in the bottom half—with only 32 microns(~0.001 inch) clearance from these pads, which are embedded in the housing walls. The average spin rate of thegyro rotors during the science phase of the mission was approximately 4,300 rpm.Bonded to the top of the quartz block using a hydroxide catalyzed optical bonding process, is the quartzreflecting Cassegrain astronomical telescope, which focuses on the guide star, IM Pegasi. Hydroxide catalyzedbonding is a patented method of fusing together quartz parts, without the use of any glue or fasteners to ensurethat the SIA did not distort or break when cooled to cryogenic temperatures. The line of sight of the telescope isrigidly aligned to the SQUID readout loop of each gyroscope. As such, the quartz telescope provided the frameof reference for measuring drift in the spin axes of the gyroscopes.The SIA is mounted in a two-meter long cigar-shaped canister, called the “Probe,” which was inserted into thedewar prior to launch. The Probe is an amazing feat of cryogenic engineering, designed by Lockheed Martin inPalo Alto, California. It provided both mechanical and structural stability for the SIA. The Probe was designedto provide a free optical path for the telescope to view distant space through a series of four preciselymanufactured windows, mounted in its upper section. Three of these windows served to reduce thermalconductivity into the dewar, and the fourth, which is made of sapphire, also forms the vacuum seal at the topend of the Probe.Gravity Probe B — Post Flight Analysis • Final Report March 2007 37