Optical radiations from interaction of effluent gases with the low-orbital atmosphere

The radiance distributions of the short-wavelength infrared and visible glows associated with interaction of exhaust from Shuttle orbiter's primary reaction control system thrusters with background air are interpreted as resulting from independent kinetic processes involving major and fuel-fragment components of the hypervelocity combustion gas. These luminous volumes become essentially stable in the spacecraft's moving reference frame by 1 s after the engine's liquid bipropellant ignites, with dimensions on the order of kilometers. Viewed perpendicular to the vehicle trajectory, they have Gaussian brightness profiles in ram and, at visible wavelengths, an edge-enhanced ogival shape in wake injections. Strong vibrational emission arises from collisions of water molecules, which also emit over a broad infrared range when outgassed from low-orbiting spacecraft. Other phenomenological features of these optical-contaminant glows quantified from ground-based radiometric images include 1) total exoatmospheric photon yields, 2) volume emission rates and downstream movement of the surface brightness maxima as the ambient reactant species becomes depleted, and 3) cross sections for quenching the luminosity. The absolute visible-band intensities from three exhaust directions indicate a potential barrier for the excitative reactions, and with the patterns of projected radiance apply in remote sensing of angles between the thrust axis and the spacecraft velocity.