## 2 Degradation of Materials in Space--Stability Issues on Mirrors  
Common to all orbits is the degradation of materials by the hazardous space environment, whose importance in space technology is undeniable [30; 31]. Degradation may be caused by atomic oxygen, thermal stress, electromagnetic radiation, telescope outgassing or self-contamination, charged particles, space debris, and micro-meteorites. In Low Earth Orbits (LEOs), atomic oxygen (AO) is the main source of degradation, while in the interplanetary medium the solar wind and solar electromagnetic radiation dominate the degradation effects. Most of the materials used for space optics need to be evaluated for their behavior under several of the aforementioned degradation mechanisms. It is known that these degradation mechanisms can significantly degrade materials and lead to changes in their mechanical behavior or thermo-optical properties [30]. These changes can cause early failures of satellite components or even failures of complete space missions.  
The main challenge in the assessment of the degradation of materials in space is the development and choice of the most representative ground testing and extrapolation to end of-life conditions for the thermal cycle and for the charged particles, AO, UV irradiation, and high-velocity impacts of microparticles. These tests have to account for the different environments in which the mirrors will operate, ranging from Low Earth Orbit (LEO) to interplanetary orbits and deep space.  
Investigations of the behavior of optical materials and coatings in the space environment started being reported in the 1970s. Pre-launch acceptance testing and the evaluation of mirrors coated for use in space are almost never performed on the actual flight mirror. Smaller witness mirrors, coated at the same time as the flight component, are used as test proxies for the spaceflight component. The intent of the acceptance testing is generally to identify any mirror surface quality problems before performing the qualification testing of the final and larger mirror. The use of test samples to verify the performances of the whole mirror is even more important for complex optical coatings such as reflective multilayers [32; 33]. Environmental tests are performed to check the resistance of a mirror coating that is exposed to ambient conditions simulating the space environment for the instrument lifetime. As an example, Figure 1 shows reflectance degradation as a function of wavelength in the UV-Vis spectral range of protected Ag mirrors under various degradation sources. This combination of environmental resistance tests helps to predict, model, and account for the in-orbit degradation of the optical system.  
For each of the key degradation sources (i.e., AO, UV radiation, thermal cycling, charged particles, telescope outgassing, and space debris and dust), several mitigation techniques and strategies have been proposed, most of them based on the use of protective coatings. Coatings performing critical optical functions have been used in space instrument applications for NASA, ESA and other international and national space agencies for more than 50 years. The performance of earlier coatings launched into space has been observed to change with time. Starting from that, pre-flight testing in simulated space environments has been developed to verify the spectral and efficiency performance, which are desirably able to predict the changes observed in space.  
The effect of real or simulated space conditions on mirrors has been investigated during the last few decades, and in the following sections we will discuss the main results and developments reported in the literature. The next subsections address the main degradation sources in the space environment. A large emphasis is given to far UV (FUV, \(\lambda\) in the 100-200 nm) and extreme UV (EUV, \(\lambda\) in the 10-100 nm) due to the enhanced degradation that arises due to the strong absorption of most contaminants in these ranges compared to longer wavelengths.