Flight performance of gravity probe B cryogenic system

Gravity Probe B (GP-B) is a cryogenic and space-based test of Einstein's General Theory of Relativity by means of precision gyroscopes, The GP-B spacecraft was launched into a polar orbit from Vandenberg AFB on April 20, 2004. The launch and operation of GP-B represented the culmination of forty years of planning, technology development, hardware fabrication, and testing. The superfluid liquid helium became depleted on September 29, 2005, giving a lifetime of 17.3 months compared to the requirement of 16.5 months and a thermal model prediction of 16.6 months. The flight dewar contained 2320 liters of similar to 1.8 K superfluid helium at launch and housed the science instrument consisting of four precision gyroscopes and a telescope. A porous plug phase separator effected the venting of the helium boiloff gas. This venting helium was used to operate 16 thrusters, which are the actuators that effect precision pointing on a fixed star and adjust the orbit to be drag free or close to true zero-g.