Space vehicle optical contamination by ram glow

Abstract

The properties of the vehicle ram glow have been measured on a number of shuttle flights (STS-3, STS-4, STS-5, STS-8, STS-9, 41-D, and 41-G). The intensity of the glow is not proportional to the cosine of the ram angle and for relatively large ram angles there is still an appreciable glow, provided the surface is not shadowed by some other spacecraft structure. Material samples were also exposed in the ram direction during night side of orbits and the glow surrounding the samples was photographed. The glow intensity appeared to depend on the nature of the sample surface. Re-analysis of the data shows that the glow intensity differences are consistent with a model in which the ram glow intensity is a function of the surface temperature. Absolute intensity measurements were obtained on STS-7, STS-9 and 41-G. The column integrated ram glow intensity on STS-7 and STS-9 were of between 10 and 20 Rayleighs per angstrom. The glow layer in front of spacecraft can reach of the order of several hundred Rayleighs per angstrom with slant viewing enhancement. On STS 41-G the glow intensity was substantially less. The intensity was derived from absolute calibration as well as from comparison to known airglow emission intensities within the imaging instrument's field of view. The various factors which may contribute to the glow intensities will be discussed. Thruster induced glow was also investigated. Thrusters were initiated individually and the resulting luminosities were recorded by photographic and television techniques. On the STS-8 mission when the altitude was 220 km the thruster induced spacecraft glow was found to decay with a time constant which is 1 5th of the time constant obtained on STS-3. These and other results were incorporated into a model which predicts the dependence of the glow intensity on spacecraft altitude and spacecraft surface temperature.