Infrared background of spacecraft in low‐Earth orbit attributable to ion chemistry

Abstract

A diurnal variation in the infrared background radiation observed on Spacelab 2 suggests that ion chemistry plays a part in the generation of those infrared emissions. Because water is the major contaminant in the environment of spacecraft in low earth orbit, it is thought to be the source of vibrationally excited and infrared active H2O+ formed in O+(4S) + H2O charge‐transfer reactions. H2O+ infrared luminescence spectra (0.01 µm FWHM) are calculated for the predicted nascent vibrational distribution as well as for steady‐state conditions assuming a charge‐transfer relative velocity of 7.8 km s−1, the low‐Earth orbital velocity. The calculated luminescence spectra exhibit an intense band between 3 and 3.5 µm, attributable to symmetric stretch deexcitation, and a minor band between 6 and 8.5 µm associated with bending vibrational transitions. The effect of charge‐transfer product rotational temperature, H2O partial pressure, and motional electric field on the infrared background spectra are investigated.