Deep Space 1 Measurements of Ion Propulsion Contamination

The Ž rst in situ measurements are reported of ion propulsion-induced contamination from an interplanetary spacecraft, NASA’s New Millennium Deep Space 1. Deep Space 1 carries two quartz crystal microbalance and calorimeter pairs to characterize contaminant deposition due to ion thruster operation. One pair of sensors has a direct line-of-sight view of the ion engine whereas the other pair is shadowed from direct view of the engine. After about 2750 h of ion thruster operation in the Ž rst year of the mission, the line-of-sight sensor has collected about 250 E Aofmolybdenum,and thenon-line-of-sightsensor hasonly collected theequivalentmassof a 25E A-thick deposit of molybdenum.The line-of-sight deposition rate appears proportional to the thrust level, whereas the non-line-ofsightdepositionrate increasesmuchmore rapidlywith the thrust level.Measurement results suggest that signiŽ cant back ow of ionized molybdenumparticles would occur primarily at high thrusting levels when both the molybdenum ionization rate and the plumepotential relative to the spacecraft are higher than that at lower thrusting levels.