POSITIVE SPACECRAFT CHARGING AS MEASURED BY THE SHUTTLE POTENTIAL ANDRETURN ELECTRON EXPERIMENT

During the deployed phase of the Tethered Satellite System 1 Mission ( TSS-1), the Orbiter was observed to charge positively on a number of o ccasions during operation of the 100 mA, 1 keV Fast Pulsed Electron Gu n (FPEG) of the Shuttle Electrodynamic Tether System (SETS). The occur rence of positive charging was determined both through the measurement of the voltages in the system and through observation of the charging peaks in the electron spectra measured by the Shuttle Potential and R eturn Electron Experiment (SPREE). Here we present data from the two c ases of highest positive charging during the deployed phase of the TSS -1 mission. These cases occurred in darkness during periods of depress ed ambient plasma density. Positive Orbiter charging was observed from ten to a hundred volts. During the operation of the FPEG prior to Orb iter charging, the SPREE electrostatic analyzers measured intense flux es of electrons at energies up to the energy of the emitted beam. Duri ng the charging periods, the SPREE electron spectra displayed a peak w hose position in energy was consistent with the positive potential of the Orbiter as determined from SETS and the Italian Space Agency (ASI) Deployer and Satellite Core Equipment (DCORE and SCORE) measurements. At energies above the charging peak, the shape of the electron distri bution function during charging was consistent with a simple accelerat ion of the pre-charging spectrum by the electric field produced by the positively charged Orbiter. At energies below the charging peak, inte nse, isotropic fluxes of electrons were measured either with a power l aw spectrum or with a spectrum peaked at 10 to 20 eV.