ENHANCED ELECTRODYNAMIC TETHER CURRENTS DUE TO ELECTRON-EMISSION FROMA NEUTRAL GAS-DISCHARGE - RESULTS FROM THE TSS-1R MISSION

During the reflight of the first electrodynamic Tethered Satellite Sys tem (TSS-1R) mission, the unplanned separation of the tether at the Or biter end resulted in the highest tether current during the mission. I n the moments just prior to the tether separation with 19.7 km of teth er deployed and a generated electromotive force (EMF) of 3482 V, curre nts reaching approximately 0.97 A were shunted through the tether to t he Orbiter electrical ground, which was in contact with the ionosphere primarily through its main engine surfaces. This current level was ne arly twice as large as observed during any nominal operating period. A s the failure point of the tether entered into the ambient plasma, the current increased to 1.1 A and maintained this level even after the b reak for approximately 75 s. The principal surprise in these results w as that the broken end of the tether, with only a few short strands of copper wire, could support higher currents than the much larger Orbit er conducting surface areas. Analysis of possible current enhancement mechanisms revealed that only a gas-enhanced electrical discharge, pro viding an electron emission source, was plausible. Ground plasma chamb er tests confirmed this analysis. The TSS-1R results thus represent th e highest electron current emission from a neutral plasma source yet d emonstrated in a space plasma. This is of interest for current collect ion processes in general and plasma contactor development in particula r.