On-orbit optical observations of Electric Propulsion Space Experiment 26-kilowatt arcjet

During the course of eight flight firings of the ESEX 26-kW arcjet in March and April 1999, optical observations from onboard and ground-based sensors were obtained. Images of the thruster plume at 656 nm confirm expectations that the plume luminescence in,the space environment is more compact than that from a thruster operated in the laboratory at higher background pressure. Observations using a ground-based telescope reveal blackbody and line emission spectrum over the range 325-675 nm. The spectral features are consistent with ground tests. Line ratios observed in flight show a moderately higher degree of excitation than ground tests, which is consistent both with the higher specific power and the less collisional plume expansion of the flight test compared to ground tests.