1. A wide band RF antenna on board SERT II viewed the Earth through the ion engine thrust beam. Noise levels measured in the 1680-1720 MHz and 2090-2130 MHz bands are about those expected from thermal noise from the Earth. (14)
2. Under normal conditions, optical interference will not be a problem on spacecraft, The exhaust plume from ion thrusters is very tenuous, and the densities are so small that there is little absorption of radiation. With cesium, for examplei Lyon(9) reported line widths of the order of 10- 2 cm or less under thruster exhaust conditions. The width of all of the cesium lines, even if total absorption should occur within the lines, is negligible when compared to the width of the visible spectrum, and the quantity of energy absorbed also is negligible in comparison to the total incoming energy. Similar results are to be expected with mercury thruster exhaust plumes. Telescopes, star trackers, and similar instrumentation will encounter few problems due to absorption of incoming visible radiation,
3. Decay of un-ionized but excited neutral atoms in the exhaust plume of mercury thrusters has been experimentally investigated. <25) The reported brightness of the resonant lines may exceed 10-8 w/cm 2 ster, but the author concluded that this was not enough energy to perturb the operation of star trackers provided the tracker field of view of the plume was sufficiently small. Further, this brightness would be expected to decrease rapidly with increasing distance from the thruster exhaust plane because of the combined effects of excited atom decay and decreasing atom density.