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 ∼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.