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

Figure 2-10. Schematic diagram of the GP-B experimentAccording to Einstein’s theory, over the course of a year, the geodetic warping of Earth’s local spacetime shouldcause the spin axis of the gyroscope to drift away from its initial guide star alignment by a tiny angle of 6,606milliarcseconds (0.0018 degrees). Likewise, the twisting of Earth’s local spacetime should cause the spin axis todrift in a perpendicular direction by an even more minuscule angle of 39 milliarcseconds (1.1x10- 5 degrees),about the width of a human hair viewed from 1/4 mile away.Gravity Probe-B’s measurement of the geodetic effect has an expected accuracy of better than 0.01%—far moreaccurate than any previous measurements. The frame-dragging effect has never directly been measured, butGravity Probe-B is expected to determine its accuracy to better than 1%.2.1.7 The GP-B SpacecraftAll of the Gravity Probe B technologies are integrated into one of the most elegant and sophisticated satellitesever to be launched into space. The GP-B spacecraft is a marvel of engineering and truly a beautiful sight tobehold. From it largest to smallest parts, it is filled with the cutting edge technologies and materials describedthroughout this report, many of which were invented specifically for use in the Gravity Probe B mission.2.1.7.1 Inside the DewarThe spacecraft is built around a 650-gallon dewar, one of the largest and most sophisticated ever flown in space.The dewar is nine-feet tall and forms the main structure of the spacecraft. The vacuum area just inside thedewar’s shell contains multiple reflective surfaces that cut down on heat radiation. The dewar also containsvapor-cooled metal shields that help maintain its internal cryogenic temperature, and slosh baffles helpsuppress tidal motions in the superfluid helium inside. When cooled to nearly absolute zero temperature, liquidhelium transforms into a state called “superfluid,” in which it becomes a completely uniform thermalconductor. Only helium exhibits this, and other special properties of superfluidity.Figure 2-11 shows the dewar’s location in the center of the spacecraft, along with an exploded view of the mainpayload components inside the dewar.36 March 2007 Chapter 2 — Overview of the GP-B Experiment & Mission