Current collection at the shuttle orbiter during TSS-1R high voltage charging

We compare measurements of collected currents and Space Shuttle Orbiter potentials, taken during the Tethered Satellite System (TSS) missions, with predictions of a numerical model. The model assumes thin potential sheaths about the Orbiter and contributions from both ram and thermal currents. It was originally developed to explain the smaller than expected Orbiter charging detected throughout the first TSS mission (TSS-1). During periods of the TSS reflight (TSS-1R) when the Orbiter potential was > -100 V the model effectively described current collection. Deviations from model predictions appeared at more negative potentials. These indicate bounds of applicability for the model and the growing importance of unaccounted physical processes. Data acquired near the time of the tether break suggest that additional current during the break was carried by secondary ions created in collisions between surface-generated electrons and ambient neutrals within the Orbiter's high-voltage sheath.