Spacecraft glows from surface-catalyzed reactions

Existing data on the optical glows that extend from low earth-orbiting space vehicles are shown to be consistent with recombination of ambient atmospheric species on ram-exposed surfaces. This radiation is most intense from the velocity direction because of the greatly enhanced fluxes of air atoms and molecules; if these particles' up-to-10 eV relative kinetic energy plays any part, it would be creating and maintaining sites from which internally-excited fragments can be desorbed—“conditioning” the surface—rather than overcoming potential barriers of reactions with substrate or putative contaminant species. Surface-catalyzed exothermic recombination qualitatively explains the reported differences as well as similarities among spacecraft in spectral intensities and spatial distributions of glows, and predicts further emissions at ultraviolet (among them O 2 A → X, NO B → X, and perhaps also N 2 a→ X bands), infrared (NO 2 and NO vibrational cascade), and to a lesser extent visible, wavelengths. A systematic survey of the literature shows that similarly unequilibrated desorbates are commonly observed in the surface-science and kinetics laboratory, a finding that provides a framework for future experiments on chemiluminescence excited by spacecraft. Quantitatively, vehicle glow presents a severe technical challenge because of the complexity and virtual uncharacterizability of the materials exposed to the airflow and the potential for interference among surface reactions of the several incident aeronomic species.