The influence of commonly used materials and compounds on spacecraft contamination

Abstract

The United States and other countries have had earth orbiting vehicles for over 30 years. During that period of time, aluminum has been used extensively as the structural material of choice. As weight tolerances have grown tighter, advanced materials have begun replacing aluminum in many applications. The outgassing complexity of these new materials, along with the inability to model thin layers and material interactions, reduces the confidence level in comparison to aluminum. Another issue which makes contamination control more difficult, is the reduced operating temperature of current optics and their heightened sensitivity to contamination. In many attempts at contamination control of sensitive optical instruments, there is a tendency to let the selection of materials by handbooks eclipse the impact of mass transport, outgassing rates and the subsequent effects of any condensate. Several indepth contamination assessments for optical instruments and sensors based on low-earth orbiting (LEO) spacecraft and platforms have recently been conducted. This paper will discuss the effects of molecular contamination on several of these instruments and their sensitivity levels to contamination. There are a number of manuals and documents which screen materials on the basis of outgassing, primarily with respect to the criteria of ASTM method E595. While useful for screening materials, this approach does not provide the collected ratio of volatile condensible material (VCM) to the total VCM (total weight loss) as a function of receptor temperature. In general, there are three (3) levels of effort that, when used in series, can attain acceptable spacecraft contamination control. These levels are: (1) materials selection, (2) contamination modeling of the existing design, and (3) a thermal vacuum test of the hardware with contamination monitors. This paper will also discuss improvements to the ASTM method when a material will coexist on a spacecraft or platform with optical instruments.