Thruster effects on the shuttle potential during TSS 1

We have surveyed measurements taken by the Shuttle Potential and Return Electron Experiment (SPREE) in the shuttle payload bay during the satellite deployment phase of the Tethered Satellite System (TSS 1) mission. The shuttle was flying in an inverse airplane mode with its engine bells facing the ram direction to facilitate ion current collection. SPREE ion spectral measurements indicate that with no thrusters firing, the shuttle charged to, at most, a few tens of volts negative. We identified 13 intervals during which the tether was electrically connected to the shuttle by a low-impedance shunt and thruster firings occurred. In previously reported examples of neutral gas releases from charged spacecraft the magnitude of the vehicle potential decreased. Thruster firings during our events either left the TSS circuit unchanged or caused the current measured in the tether to diminish and the shuttle to become more negatively charged. In the latter case it appears that thruster gas impeded access of ambient ions to the current collecting surfaces of the shuttle. The changes in the distribution of the motionally induced potential between the tether and sheath calculated using Ohm's law are in agreement with SPREE measurements.