 line-of-sight relationship to the material. If the outgassed material were to deposit on thermal control surfaces or sensitive optics, the performance of these items would become degraded. Because the deposition occurs basically one molecule at a time, this type of contamination is referred to as molecular contamination. Non-line-of-sight transport may also occur when collisions with other ambient atmosphere molecules changes their trajectory. Secondary desorption from an intermediate surface often results in particles which are ionized and hence may be attracted to charged surfaces by electric fields. UV exposure may interact with the molecular contamination process to enable photochemical deposition. Mitigation provisions focus on careful selection of stable materials and coatings, pre- treating materials with vacuum bake-out, configuring the design such that outgassed material is not allowed easy access to sensitive devices, and including additional design margin in the application of sensitive devices to allow for expected degradation. Also related to the vacuum environment is the level of exposure to ultraviolet (UV) light; most UV is absorbed by the ozone in the earth's atmosphere. Solar UV degradation of the absorptance of radiators used in thermal control systems is one potential area of concern. Materials must be evaluated and chosen based upon susceptibility to degradation when exposed to UV. Neutral Environment The impact between a spacecraft and individual atoms results in aerodynamic drag and may result in sputtering and chemical reactions with surface materials. Concern over the effects of the neutral environment is most profound at very low altitudes, decreasing with increased orbit altitude, and is not a significant issue above 1000 km. Atomic oxygen (AO), produced by the decomposition of ozone into molecular oxygen which is then broken down into atomic oxygen, is most prevalent between 175 km and 650 km; helium is the dominant contributor above 650 km. Physical sputtering occurs when neutral mol