Thermal and UV degradation of four common space-grade polymer films have been studied in situ using electron spin resonance (ESR) spectroscopy. By recording subsequent spectra at a sufficient rate, the time dependence of the radical concentration could be followed, allowing more detailed study of the kinetics of the degradation process. The thermal degradation was found to consist of two main processes, one being a stabilization process and the other the actual degradation, whereas the UV experiments showed biexponential degradation kinetics. Additionally, to compare in situ and ex situ experiments, we monitored the stability of the generated radicals after the exposure. The radical concentration decreased only slightly when samples were stored in a vacuum, whereas storage in air led to a significant loss of radicals.

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velocity. Technology improvements allow higher resolution measurements; computing efficiency provides higher precision modeling; and our understanding of the space environment constituency improves. This overview will provide a general description of the space environment and point to some key features that spacecraft designers need to consider to reduce risk and improve engineering performance. The best metric that spacecraft designers can use to assess the engineering performance of the spacecraft is to understand environmental effects on the spacecraft materials, particularly through flight experiments exposed to the space environment in which the spacecraft will operate. This understanding begins with characterizing the natural space environment, then progresses to ground-based materials testing, and culminates with flight experiments. The following are process steps that spacecraft engineers should ideally follow to assess the performance and guide selection of candidate materials for spacecraft application:Define mission-specific space environment.Consider induced environment factors that could influence material engineering performance.Determine failure criteria for candidate materials.Perform literature search for previous space environment testing or flight experiments on candidate materials.Determine acceptable acceleration factors for ground testing.Perform ground tests, including appropriate/available synergistic effects.Expose material on a flight experiment in the space environment. Space EnvironmentThe natural space environment is a dynamic synergy of its constituents, comprised of many components that vary in intensity and interact with each other through both constructive and destructive mechanisms. The first step in understanding the engineering performance of a material in the space environment is to understand the operational space environment and the interactions with the spacecraft materials. Today, the space environment engineer has a very good understanding of the environmental constituents and has models to predict the flux, fluence, and spatial distributions of each component. The part of the predictive tool that is missing is a thorough understanding of the synergy of how these individual components of the environment interact and produce effect in materials. Often these synergistic effects are not accurately simulated in ground test facilities.

A review of the contamination physics and of the most widespread engineering approaches to contamination assessment was carried out. The two main approaches are the physical and the empirical one. The main questions still open to validate the physical approach to outgassing and deposit physics were then studied. Among others, special attention was paid to the important point of a realistic separation of chemical species, probably a prerequisite for a physical modeling. Several original results were obtained. Some lead to a quite clear conclusion, like the preeminence of the limitation by desorption over the limitation by diffusion for outgassing. This observed trend needs yet to be validated on other materials. Other major results are progress on the validation of the physical approach and on the ambitious species separation program. Nomenclature D = Fick's law diffusion coefficient, cm 2 :s 1 E A = activation energy, J:K:mol 1 f = contaminant flux density, g:cm 2 :s 1 k = outgassing kinetic constant, s 1 L = effective diffusion length of the sample, m M = contaminant molar mass, g=mol m, m evap = deposited, evaporated or outgassed surfacic mass, g:cm 2 m 0 = contaminant monolayer mass, g:cm 2 n vol = contaminant volume density, g:cm 3 n surf = contaminant surface density, g:cm 2 P S = saturation vapor pressure, mbar R = gas constant, 8:314 J:K:mol 1 T = temperature, K t = time, s W = relative contaminant mass, % = outgassing or deposit evaporation characteristic time, s

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