Optical measurements and attitude motion of HERMES after loss of stabilization

The three-axis stabilized Hermes communications satellite relied on the gyroscopic stiffness of a momentum wheel for roll and yaw stability. The basis for stability is essentially the dual-spin principle with a minimum moment of inertia configuration, the central body and solar sails being the 'platform' and the momentum wheel the 'rotor'. If the wheel despins, a loss of attitude stability results. On day 329 of 1979, an earth sensor malfunctioned and, in conjunction with degraded telemetry and batteries the wheel despun, resulting in loss of attitude stability and ultimately shutdown of the satellite. From day 331 of 1979 to mid-1980, the sun's reflections from Hermes were periodically recorded at the Ground-Based Electro Optical Deep Space Surveillance (GEODSS) experimental test site (ETS) in New Mexico. During this time, Hermes made a transition from the unstable state to a flat spin about the yaw axis, and its flat spin decayed to zero. Attention is given to the ETS observations, and the deduced attitude motion.