## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Charged Particles
In the case of low-energy particles, the damage amount depends on the energy, flux, and fluence. Low-energy proton irradiations (\(<\)500 keV) with fluences lower than 10\({}^{16}\) p/cm\({}^{2}\) have been shown to determine negligible changes in the near infrared reflectance of SiO\({}_{2}\)-protected Al mirrors [109; 110]. A degradation dependent on the proton fluence has been instead observed in the visible and near ultraviolet ranges. For example, [PERSON] et al. and [PERSON] et al. [111; 112; 113] investigated the effect of 60 keV protons on Al protected with SiO\({}_{2}\). These mirrors were measured from the near UV to the IR. Reflectance was monotonously degraded with the proton dose (see Figure 3a); with a fluence of 10\({}^{16}\) p/cm\({}^{2}\), a reflectance drop of 5% at 700 nm and 10% at 500 nm was observed, whereas in the UV range this drop goes to over 20%. Moreover, fluences in the order of 10\({}^{17}\) p/cm\({}^{2}\) heavily compromise the Al-protected mirror reflectance up to over 1000 nm. Similarly, Al/MgF\({}_{2}\) mirrors of unknown design but optimized for the near-UV or visible range, and hence with a thicker MgF\({}_{2}\) protective coating than FUV mirrors, were exposed to a geostationary orbit simulator consisting in simultaneous irradiation with UV, electrons and protons [80]. The UV reflectance decay was found to depend on the specific mirror, which had been prepared by a specific vendor, so that some degradation could not be discarded for the mirrors in space environment. Such degradation may not have been due to the presence of contaminants but to the shallow penetration of electrons and even more to less penetrating protons. Such a reflectance decrease was attributed to the change of Al optical constants or to the appearance of ripples and hillocks on the surface of the Al mirror. Calculations on the effect of protons over generic metallic surfaces predict the recombination of protons to form H\({}_{2}\) bubbles, in turn resulting in a significant roughness increase. Importantly, the proton energy used in the irradiation experiments greatly influences the degradation results. For example, based on the results reported in [110], it can be observed that a fluence of 10\({}^{17}\) p/cm\({}^{2}\) induces a higher reflectance degradation when the proton energy is low. This fluence at 60 keV induces a reflectance drop of 99% at 400 nm, while at 160 keV this drop is about 20% (see Figure 3b). This degradation is due to the different ion implantation profile inside the coating: the lower is the energy, the shallower is the ion implantation peak. In case of metallic mirrors, if the implantation peak falls in the topmost part of the metallic layer or inside an eventual protective layer, the bubble formation inside the coating will provide a greater degradation. [PERSON] and [PERSON] [114] exposed Al/MgF\({}_{2}\) and Al/LiF FUV mirrors to 10 keV protons to simulate several-year exposure in a synchronous earth orbit. Such irradiation resulted in a broad-band reflectance decrease centered at \(\sim\)210 nm (Al/MgF\({}_{2}\)) and \(\sim\)190 nm (Al/LiF), with the reflectance decrease growing with the proton dose. Furthermore, for the analyzed mirrors the reflectance decrease was negligible at the short-end of the high FUV reflectance range, which was explained with the presence of contaminants but not coating degradation. The contamination thickness was calculated to be 4-5 nm. Even though an undulating pattern on the coating surface was induced by irradiation, its small width did not result in observable scattering. Most of the reflectance degradation could be reverted, hence approaching the original reflectance after exposing both Al/MgF\({}_{2}\) and Al/LiF mirrors to AO, a result attributed to the oxidation of the contaminants, which took a volatile form. [PERSON] and [PERSON] [114] also exposed Pt mirrors to 10 keV protons to simulate a long exposure in a synchronous earth orbit.