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

Abstract. We compare measurements of collected cur-rents and Space Shuttle Orbiter potentials, taken dur-ing the Tethered Satellite System (TSS) missions, with predictions of a numerical model. The model assumes thin potential sheaths about the Orbiter and contribu-tions from both ram and thermal currents. It was orig-inally developed to explain the smaller than expected Orbiter charging detected throughout the first TSS mis-sion (TSS-1). During periods of the TSS refiight (TSS-1R) when the Orbiter potential was>-100 V the model effectively described current collection. Devia-tions 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 hat additional current during the break was carried by secondary ions created in colli-sions between surface-generated electrons and ambient neutrals within the Orbiter's high-voltage sheath