Ultraviolet Image Analysis of Spacecraft Exhaust Plumes

Data obtained during the April 26, 2000 MirEx experiment is used to infer the chemical mechanisms responsible for ultraviolet radiation observed to emanate from Russian spacecraft exhaust plumes in low Earth orbit. The principle objective of this experiment was to use the Mir Space Station instruments to study the plume collisional processes occurring in the rarified atmosphere of the far-field; at distances greater than one kilometer away from the spacecraft. This was accomplished by observing the automated Progress cargo ship as the auxiliary engines were being retrofired in a ram-burn configuration at 330 kilometers in altitude above Earth. The primary emitters, observed within the 240 nm to 360 nm operating region of the instruments, were the molecular transitions of OH (A ²Σ+ -7 X ²Π), occurring at 306 nm, and NH (A ³Π -7 X ³Σ⁻), occurring at 336 nm. Proposed reactions for forming the excited-state OH and NH molecules

begin with the ambient gases present in the upper thermosphere. The collision pair for the atmospheric species is postulated to be gaseous water, which is expelled as a product of rocket engine combustion. It is concluded that the most likely atmospheric species are atomic oxygen and nitrogen. The activation energy for both reactions is the same; that energy required to break an O-H bond in the water. The inclusion of the atmospheric wind velocity contribution to the collision velocity is proven to provide sufficient energy for each of the proposed reactions to proceed.