## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Ultraviolet Radiation  
UV radiation comprises the spectral range of wavelengths between 10 nanometers up to 400 nm. The effects of high-energy photons on mirrors are not strictly related to their reflectivity or morphological properties. The effects from these photons are not the determining factor contributing to radiation damage. However, chemical changes such as reduction and oxidation reactions can induce optical absorption in thin-film layers, and UV photons can promote such reactions, changing the composition of the materials. For these reasons, space UV and ionizing radiation, the durability of materials must be considered. Importantly, the radiation effects are synergistic with other effects and must be considered together [80]. One of the principal effects of UV radiation is the polymerization and darkening of silicones and hydrocarbons, which are ubiquitous contaminants in space telescopes. This darkening effect is often enhanced by electron irradiation [81]. Hence, the UV resistance of mirrors is often tested during space qualification tests. It is common to use a distribution of Xe lamps (or similar sources) to obtain a spectral intensity profile similar to the solar irradiance, and the mirrors are exposed for a time equivalent to the intended operation hours under solar ultraviolet exposure [82]. UV exposure may have effects on polymers and other materials used in lightweight mirror material in spacecraft applications. In this latter case, the effects of UV exposure need to be accounted for due to their potential impacts on the thermal management of a spacecraft during application in composite mirror structures [83; 84; 85]. The earlier space optical thin films used for band-pass filters were based on thermally evaporated soft materials such as ZnS and MgF\({}_{2}\). Exposure to the space environment containing ionizing radiation, solar UV, atomic oxygen, and high vacuum revealed the unstable operation of those coatings. ZnS deposited by evaporation was used as a coating material decades ago for its moderate FUV reflectance and its transparency above 400 nm. [PERSON] et al. [86] evaluated the resistance of a ZnS film to intense UV irradiation, as it would be expected in a space instrument. ZnS experienced a dramatic reflectance decrease in UV after a long UV irradiation in air, whereas the reflectance decrease was relatively small at wavelengths around 400 nm. The outermost 15-20 nm thickness was seen to have changed from polycrystalline ZnS to amorphous ZnO. The authors also studied a multilayer with single Al and Ge films under the outermost ZnS film to enhance the FUV reflectance and to decrease the near UV and visible reflectance. The Ge/Al/ZnS multilayer was UV irradiated in vacuum, which resulted in a severe FUV reflectance decrease and increasingly more severe towards shorter wavelengths, whereas no change was observed at wavelengths around 260 nm. Again, there was a predominant presence of oxygen over sulfur in the outermost 10-20 nm. The paper reported that, even in the total absence of oxygen upon UV irradiation, sulfur is expected to sublime, leaving a metallic film of Zn. All these behaviors recommend caution in employing ZnS as the outer coating of optics in space. [PERSON] at al. [87] reported the on-orbit degradation of Ag mirrors on the Suomi-NPP spacecraft. They identified an important degradation in near-IR bands of the Visible Infrared Imaging Radiometer Suite instrument, but little degradation in the green and blue channels. They first considered the possibility that the mirrors had become contaminated either before launching or on-orbit, and that the contaminant was darkening with UV exposure. However, the spectral signature of the degradation was uncharacteristic of UV darkened molecular contamination, which typically results in greater losses in the short wavelengths rather than the NIR.