Molecular Beams in Space:  Sources of OH(A→X) Emission in the Space Shuttle Environment

Abstract

OH(AfX) emission bands have been observed in the molecular beam jets produced by Space Shuttle engine exhaust using the GLO imager spectrograph located in the payload bay. Spectra were collected at a resolution of 4 Å for both daytime and nighttime solar illumination conditions, all at an altitude of ∼390 km. A spectral analysis is presented that identifies and quantifies four separate OH(A) excitation processes. These include (i) solar-induced fluorescence of the OH(X) in the exhaust flow, (ii) solar-induced photodissociation of H 2 O in the exhaust at the strong Lyman-R solar emission line (1216 Å), (iii) solar-induced photodissociation of H 2 O in the far UV, at shorter wavelengths than Lyman-R, and (iv) luminescent collisions between atmospheric species and exhaust constituents, most probably the reaction O + H 2 O f OH(A) + OH(X). Process (i) produces a very rotationally cold and spectrally narrow component due to the rapid cooling of the OH(X) in the supersonic expansion of the exhaust flow. Processes (ii) and (iii) produce extremely excited OH(A), not well characterized by thermal vibrational or rotational distributions. The O + H 2 O chemiluminescent reaction has a substantial activation energy, 4.79 eV, and is only slightly above threshold for the ram geometry, where the engine exhaust is directed into the atmospheric wind. Evidence for process (iv) is observed in the night ram but not the night perpendicular exhaust atmospheric interaction, consistent with the threshold energy. Through the use of a nonequilibrium spectral emission model for OH, the integrated intensity, spectral distribution, and OH(A) internal state characterization for each of the above processes was deduced. Additional confirmation of the analysis is provided through the use of a model simulation of the space experiment to predict the total integrated intensities for processes (i) and (ii), for which the underlying spectroscopy, absorption cross sections, and solar excitation intensities are well established. Analysis of process (iii) has established, for the first time, a value for the far-UV conversion efficiency of absorbed photons to OH(A) photons of 0.26, which is twice the established value for Lyman-R. Under the assumption that O + H 2 O collisions are the source of process (iv), the analysis has established a chemiluminescence cross section at ram conditions of 1.7 × 10 -2 Å 2 . Evidence of OH(A) emission bands from predissociated vibrational levels suggests that the total reaction cross section for process (iv) may be significantly higher. While this cross section assumes a single-step reaction of O with H 2 O, the possibility of a two-step process of O with other plume species has yet to be explored.

Potential Surfaces and Dynamics of the O(3P) + H2O(X1A1) yields OH(X2Pi) + OH(X2Pi) Reaction

“…which has been observed in space-based 11 and laboratory molecular beam measurements. 10 The electronically excited OH is apparently formed through high-lying conical intersections in the potential energy surfaces of the four-atom complex.…”

“…Collisions of O( 3 P) with H 2 O(X, 1 A 1 ) give rise to spectral radiation observed from the long wave infrared (~20-10 µ) [1][2][3] , through the infrared (10-1 µ) [1][2][3][4][5][6][7][8][9] and up to the vacuum ultraviolet (~0.25 µ), [10][11][12] depending on the relative collision energy. These emissions are important to characterize because they can be a significant background noise source for telescopes mounted on spacecraft in low-Earth-orbit or LEO.…”

Analysis of Space Shuttle Primary Reaction-Control Engine-Exhaust Transients

“…The GLO-2 mission on STS-63 that flew in February 1995 obtained spectral data on PRCS engine firings during daytime conditions at similarly low thermospheric densities at an orbital altitude of nearly 400 km [5]. Figure 7 shows spectra recorded during a 3 s PRCS burn and approximately 3 s after the burn.…”

“…Figure 7 shows spectra recorded during a 3 s PRCS burn and approximately 3 s after the burn. The spectrograph line of sight was nearly parallel to the thrust centerline, and the spectral acquisition times were 2 s. The spectral features in the bottom spectrum, which is on an expanded vertical scale in comparison with the top spectrum, are due to OH, NO, and NH exhaust species (see [5] for an assignment of the bands). The top spectrum exhibits a broad background with increasing intensity with wavelength.…”

“…The GLO spectral measurements are reported here for the first time. The spaceborne GLO instrument has been described in detail before [3,4], including GLO measurements of engine-exhaust interactions with the upper atmosphere reported by Bernstein et al [5,6]. GLO was mounted in the rear section of the shuttle bay.…”