IR EXCITATION OF CONTAMINANT WATER BY OXYGEN FOR THE SPACE-SHUTTLE ATLOW-EARTH-ORBIT ALTITUDE

As the water outgas cloud of a space shuttle passes through the rarefi ed atmosphere at orbital altitude, collisions occur between the gases with sufficient energy to excite infrared-active water molecules to va rious vibrational and rotational states. An infrared contaminant model (IR model) has been developed to study the shuttle-induced excitation and emission of water molecules outgassed from the space shuttle. The focus of the first application of the model is translation-to-vibrati on (T-V) energy transfer since estimates suggest that this process sho uld dominate the production of vibrationally excited H2O under typical low Earth orbit conditions. Using the velocity and position distribut ion functions of interacting neutral gases obtained from a neutral gas es interaction model, the spatial distributions of excitation and IR r adiation from contaminant water are computed, and typical results are presented. Infrared spectral data (450-2500 cm-1), measured by the Cry ogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS-1A) senso r on STS-39 (April 28 to May 6, 1991) at an altitude near 265 km, are used to test model predictions. The dependence of the radiant emission structure and brightness on outgassing rates and altitudes is discuss ed. The time history of the contaminant water outgassing rates is infe rred for STS-39, and it is compared with the mass-spectrometer-based r esults for STS-4 (June 26 to July 4, 1982). Also, estimates of H2O col umn density at mission elapsed time (MET) 50 hours are compared for mi ssions STS-2, STS-3, STS-4, and STS-39.