CURRENT COLLECTION AT THE SHUTTLE ORBITER DURING TSS-1R HIGH-VOLTAGE CHARGING

We compare measurements of collected currents and Space Shuttle Orbite r potentials, taken during the Tethered Satellite System (TSS) mission s, with predictions of a numerical model. The model assumes thin poten tial sheaths about the Orbiter and contributions from both ram and the rmal 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 p otential was > -100 V the model effectively described current collecti on. Deviations from model predictions appeared at more negative potent ials. These indicate bounds of applicability for the model and the gro wing importance of unaccounted physical processes. Data acquired near the time of the tether break suggest that additional current during th e break was carried by secondary ions created in collisions between su rface-generated electrons and ambient neutrals within the Orbiter's hi gh-voltage sheath.