## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Charged Particles  
An additional concern in space optics regards the mirrors degradation occurring when they are exposed to charged particles and ions. During an inter-planetary journey, galactic cosmic rays background and Sun are the main sources of such particles and ions. Galactic Cosmic Rays (GCRs) are a continuous and isotropic flow of charged particles reaching the solar system from outside the heliosphere. They are approximately composed of 85% protons, 14% helium, and the residual 1% heavy ions. The energy spectrum ranges from few MeV up to GeV with particles fluxes that decrease with increasing energy. Inside the heliosphere, GCRs decrease by a few %/Astronomic Unit (AU) with heliocentric distance (R), while the solar activity modifies the GCRs flux. As the solar activity undergoes an 11-year cycle, the GCR flux varies with the maximum during solar minimum periods [99]. The Sun emits particles such as protons, electrons, alpha particles, and less abundant heavy ions such as O\({}^{+6}\) and Fe\({}^{+10}\) continuously (solar wind), either as part of eruptions (unpredictable occurrences) or as coronal mass ejections. The solar wind is an outflow of completely ionized gas originating from the solar corona and expanding outwards to the interplanetary regions. Different components are contained in the solar wind, which differ for particle speed, the spectral flux (particles/eV cm\({}^{2}\) s) of the constituents, and the solar region of provenience. For instance, the \"quiet\" solar wind in the ecliptic plane is constituted by protons of \(\sim\)1 keV and alpha particles of \(\sim\)4 keV, whereas out of the ecliptic such energies can increase up to four times [100]. More severe but transient disturbances can be caused by energetic particles events occurring during coronal mass ejections or solar flares. These events can potentially lead to high fluxes of protons in the energy range from tens to hundreds of MeV, whose effects can be occasionally detected even on Earth surface. This proton emission occurs randomly and usually during periods of solar maxima, and it is accompanied by heavy ions. In general, the fluence of solar energetic particles scales with distance from the Sun as R\({}^{-3}\) at a few MeV and R\({}^{-2}\) at tens of MeV and above [101]. Around planets, the space environment is also affected by their magnetosphere, which interacts with charged particles present in the heliosphere. Moreover, the albedo neutrons generated by GCRs' interaction with the planet atmosphere decay into protons, giving an additional source of ions around planets. These particles are confined via magnetic mirroring and trapped preferably in some regions around the planets, forming radiation belts [102]. For example, Earth has two main electron belts at about 3000 and 25,000 km altitudes, with energies varying from few keV up to 10 MeV; protons are instead confined in a belt at around 3000 km altitudes, in which the energies span between 100 keV and several hundred MeV. Outside these radiation belts the distribution and flux of particles depends on the characteristics of the magnetosphere, the planetary atmosphere, the Sun distance and the phase of the solar cycle. Earth geostationary orbits (GEO; circular orbits at 35,786 km altitude) has an electron flux ranging between 10\({}^{9}\) e/(cm\({}^{2}\)s) and 10\({}^{8}\) e/(cm\({}^{2}\)s) in the energy interval 1-10 keV and 10\({}^{5}\) e/(cm\({}^{2}\)s) at 1 MeV. The proton fluence in the same orbit is 10\({}^{10}\) p/(cm\({}^{2}\)s) at 1 MeV and decreases by two orders of magnitude at 10 MeV and four at 100 MeV. The magnetosphere of giant planets, such as Jupiter, becomes an important source of high-energy electrons (\(>\)10 MeV) in interplanetary space [103]. The spacecraft components need to be protected from highly penetrating radiation and particles encountered in the operational environment.