Photometric-Photogrammetric Analysis of Video Images of a Venting of Water from Space Shuttle Discovery

摘要

The patterns of scattering of visible sunlight by ice particles that formed from fuel cell-product water vented just before dawn into the wake of Shuttle Orbiter are photometrically reduced and analyzed. This initially coherent heated liquid stream breaks up within approx. 1 m from the nozzle into irregular polydisperse drops of mean size comparable with its initial 1-1/2 microns diameter (much as was observed in laboratory simulations), which are accompanied by a much less-dense cloud of submicron ice droplets produced when the evaporated/sublimed water gas overexpands and partially recondenses. Discrete large drops dominate the images of the backlit trail taken by intensified-video cameras onboard the spacecraft, while the radiance from the efficiently-scattering small particles is responsible for virtually all of the signal at a long focus (1/2 deg-field) camera precisely tracked on the spacecraft from the Air Force Maui (mountaintop) Optical Station. We calculated the mean radius of these smaller ice particles along the approx. 2 1/2 km of detectable trail from the video photocurrent distribution and their radiative-evaporative energy balance in the low-earth orbital environment, finding that application of a small correction for surface roughening (such as was seen in laboratory experiments) would fit the sublimation rates. The small ice particles transition from geometric (Mie) to Rayleigh scattering over this trajectory (relative to the spacecraft); in contrast the large particles lose only a very small fraction of their mass.