Deep Space 1 measurements of ion propulsion contamination

The first in situ measurements are reported of ion propulsion-induced conta mination from an interplanetary spacecraft, NASA's New Millennium Deep Spac e 1. Deep Space 1 carries two quartz crystal microbalance and calorimeter p airs to characterize contaminant deposition due to ion thruster operation. One pair of sensors has a direct line-of-sight view of the ion engine where as the other pair is shadowed from direct view of the engine. After about 2 750 h of ion thruster operation in the first year of the mission, the line- of-sight sensor has collected about 250 (Angstrom) of molybdenum, and the n on-line of-sight sensor has only collected the equivalent mass of a 25-Angs trom -thick deposit of molybdenum. The line-of-sight deposition rate appear s proportional to the thrust level, whereas the non-line-of-sight depositio n rate increases much more rapidly with the thrust level. Measurement resul ts suggest that significant backflow of ionized molybdenum particles would occur primarily at high thrusting levels when both the molybdenum ionizatio n rate and the plume potential relative to the spacecraft are higher than t hat at lower thrusting levels.