Investigation of a " Phantom" Disturbance Torque Sensed on Space Station during Russian EVAs

This paper describes the investigation, analyses, and tests carried out to determine the source of an unknown " phantom" disturbance torque that was saturating the Control Moment Gyroscopes, causing loss of attitude control of the International Space Station during Russian Extravehicular Activities. Exhaustive flight data analyses, analytic simulations for flight data reconstruction, and an on-orbit " Motionless" test collectively and conclusively show that venting from a heat exchange/cooling sublimator mounted on the backpack of the Russian Orlan spacesuit is the sole source of the disturbance torque. A loss of attitude control is highly undesirable, as it adversely affects most space station subsystems, such as solar power collection, communication and tracking, and thermal control. In the short term, identification of the source of the disturbance torque has allowed space station EVA planners prepare for loss of attitude control on subsequent EVAs. As a long-term solution, EVA planners may take measures to minimize the net resultant and duration of the thrust from the sublimators in order to mitigate loss of attitude control on subsequent EVAs, or the Orlan spacesuit may be modified such that the sublimator vents non-propulsively.