Measuring 5 meters long and 1.6 meters in diameter, the fully integrated Gravity Probe B spacecraft was a remarkable example of complete synchronicity of vehicle and experiment. The spacecraft consisted of the structural core of the experiment within the dewar, bolted onto a welded aluminum truss, and including batteries, four solar arrays, sun sensors, sunshield, and payload electronics. The entire spacecraft centered upon the drag-free concept. This meant including a “proof mass” identical in size and shape to the four rotors and housed within an evacuated cavity. Electronic sensors communicated the sphere’s location to proportional helium thrusters which responded by “chasing” the mass thereby following an ideal gravity in the 400-mile-high orbit.
Image credit: Stanford University/Barron Storey