Deep Space 1 measurements of ion propulsion contamination

The first 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 first year of the mission, the line-of-sight sensor has collected about 250 Angstrom of molybdenum, and the non-line of-sight sensor has only collected the equivalent mass of a 25-Angstrom -thick deposit of molybdenum. The line-of-sight deposition rate appears proportional to the thrust level, whereas the non-line-of-sight deposition rate increases much more rapidly with the thrust level. Measurement results suggest that significant backflow of ionized molybdenum particles would occur primarily at high thrusting levels when both the molybdenum ionization rate and the plume potential relative to the spacecraft are higher than that at lower thrusting levels.