Radiative Lifetime Analysis of the Shuttle Optical Glow

Photographs taken during the third flight of the Space Shuttle revealed the presence of a diffuse optical glow above the surfaces of the vehicle in the ram direction. The origin of the glow is unknown but is clearly a manifestation of the interaction of the spacecraft with the ambient atmosphere. The line-of-sight intensity is found to be approximately three times more intense than the airglow seen at the limb of the Earth. We assume that the glow is produced by excited molecules created in a chemical reaction at the surface, which leave the surface isotropically. By analyzing the spatial distribution of the glow, we obtain a characteristic decay length of 20 cm. The corresponding radiative lifetime depends on the velocity with which the species leaves the surface. For a velocity within a factor of 2 of the thermal velocity, the derived radiative lifetime lies between 0.3 and 1.3 ms. The production efficiency for the visible Shuttle glow is 2.5xlO~6 photons per impacting oxygen atom. The species responsible for the Shuttle glow and the Atmosphere Explorer satellite glow appear to be different. I. Introduction N IGHTTIME photographs taken by the crew of the STS-3 mission revealed an optical emission of unknown origin appearing above parts of the Orbiter surface in the ram direction of the vehicle. Figure 1 is a nighttime photograph of the STS-3 payload bay, showing the vehicle glow above the surfaces of the vertical stabilizer and the engine pod. The airglow emissions from altitudes of about 100 km are visible in the background. The emission has an intensity comparable to that of the airglow emission seen at the limb of the Earth1 and may limit the sensitivity of optical and infrared astronomical and aeronomical experiments planned for future Space Shuttle missions. A similar emission has beem observed by photometric measurements onboard other spacecraft.2'4 Torr et al. 2 noticed some unexplained enhancements in the airglow intensities measured by the Visible Airglow Experiment (VAE) onboard the Atmosphere Explorer-C satellite,5 which they attributed to some form of interaction between the satellite and the atmosphere. After correcting the VAE measurements for the galactic background and atmospheric emissions, Yee and Abreu3'4 substantiated the finding of Torr et al. 2 and demonstrated that the intensity of the interaction-induced glow is proportional to the density of the ambient atomic oxygen atoms. They attributed the emission to collisions of oxygen atoms with the satellite surface in which metastable molecules are formed. The metastable molecules leave the surface and radiate, producing the spatially extended glow. The identities of the metastable molecules are uncertain, and the processes leading to their production are not fully understood. Yee and Abreu6 showed that the AE optical glow has a diffuse band spectrum that ranges from the near ultraviolet to the near infrared and probably peaks in the red or in the infrared. Slanger7 pointed out that the OH airglow emissions in the spectral regions around 7320 and 6563 A have the same relative intensity as the AE glow and argued that OH is a plausible candidate for the emitter. A more extensive comparison by Langhoff et al., 8 which used spectral data over a wider range of wavelengths and which is based on a different model of the excitation mechanism, provided support for the OH identification. An alternative theory attributes the glow to impact excitation of some unidentified species by fast electrons created by plasma wave interactions.9