## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Atomic Oxygen  
AO is the main atmospheric component in LEO at altitudes of up 700 km. It is a species with large harmful potential over many materials. As a free radical of a very electronegative element, it has an intrinsic reactive capacity, which, added up to the relative velocity between the orbiting spacecraft and the thermal distribution of orbital AO, strengthens oxygen's capacity to react with and sputter off the target material. It is also an indirect source of contamination, as its interaction with organic materials, such as polymers, may originate secondary volatile compounds, which in turn might condensate on critical elements of the telescope, such as on optical surfaces. Optical surfaces are degraded in a level directly proportional to AO fluence. This, in turn, is determined by several factors, including [39] spacecraft altitude, as AO decreases with altitude; optical surfaces orientation, as surfaces in the ram or windward direction will be exposed the most; orbital inclination, as high-inclination orbits expose optics to cosmic radiation, which in turn may increase the AO generation and hence exposure; solar activity, as the Sun emits radiation and charged particles that can promote the generation of AO; and mission duration. The degradation issues caused by the impact of AO in the space environment have been investigated by several authors [40]. AO is particularly harmful in LEO, where it is formed through molecular oxygen dissociation promoted by solar UV radiation at altitudes greater than 100 km. When combined with the typical spacecraft orbital velocities of several km/sec, it has the effect of exposing the optical system to a stream of AO at an energy of approximately 5 eV. Hence, the optical components intended to operate in LEOs need to be designed to resist atomic oxygen. Nowadays, most of the flight optics undergo a critical 5 eV energy AO test for their space qualification, where the AO total fluence and exposure time on the coatings is typically calculated from numerical models and intended to mimic the extent of the entire mission [41]. While most of the oxide-based substrates are resistant to AO, bare metal surfaces and coatings may be vulnerable. The Evaluation of Oxygen Interactions with Materials III (EOIM-III) experiment tested  
Figure 1: Overview of different degradation effects on a silver mirror. (**a**) Reflectivity of protected Ag mirrors from several bibliographic sources, unexposed. The differences in the mirror spectral performance are explained by the differences in the composition and thickness of the protective coatings on the Ag layer. (**b**) Effect of several degradation mechanisms on the reflectivity of protected Ag mirrors. Red curve: degradation after 37 h of exposure to a 5 eV AO beam (\(4\times 10^{20}\) atom/cm\({}^{2}\)) [34]. Blue curve: degradation after 279 h of simultaneous exposure to 10 keV electrons (\(5.3\times 10^{15}\) e\({}^{-}\)/cm\({}^{2}\)), 2 keV protons (\(3.5\times 10^{14}\) p\({}^{+}\)/cm\({}^{2}\)), and 5 keV protons (\(3.4\times 10^{14}\) p\({}^{+}\)/cm\({}^{2}\)) [35]. Green curve: degradation after 1436 h of simultaneous exposure to solar-equivalent UV, 10 keV electrons (\(1.4\times 10^{18}\) e\({}^{-}\)/cm\({}^{2}\)), and 5 keV protons (\(1.6\times 10^{17}\) p\({}^{+}\)/cm\({}^{2}\)). These dosage levels are equivalent to the radiation exposure at the L2 orbit location over a 5-year mission lifetime [36]. Brown curve: degradation after 240 h of simultaneous exposure to purified air mixed with Cl\({}_{2}\) (10 ppb), H\({}_{2}\)S (10 ppb), and NO\({}_{2}\) (20 ppb), at 30 \({}^{\circ}\)C and a 70% relative humidity. These conditions are fairly similar to pre-launch environments [37]. Magenta curve: degradation after 30 thermal cycles from \(-\)80 to +35 \({}^{\circ}\)C [38]. the resistance of several optical materials to AO during space shuttle mission 46 [42].