## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Dust and Space Debris  
Dust (or meteoroids) and space debris are important sources of mirror degradation. In extreme conditions, meteoroids may cause a full spacecraft failure. One example was the Olympus communications spacecraft (ESA), in which the general failure of the satellite was attributed, with a high probability, to a Perseid meteoroid impact [125]. Although dust and debris are small in weight and size (e.g., millimeter- and micron-size particles are the most abundant in LEO), their high velocities, ranging from few m/s up to dozens of km/s, represent a threat for space optics. In the near-Earth area, space debris is generated by launch activity and subsequent operational practices, with size range between 10 \(\upmu\)m and 1 mm [126]. However, meteoroids are generally more harmful than space debris, as the average velocity of the former is higher. Beyond LEO, space dust is dominant, where short-period  
Figure 4: Effect of charged particles on multilayer coating (**a**) Evolution of the EUV reflectance at 30.4 nm of an uncapped and Ir-capped Si/Mo multilayer coating versus 1 keV proton and 4 keV He ion fluence (data from ref. [119; 120]). (**b**) Delamination occurring on a Si/Mo multilayer under a 1 keV proton irradiation with a fluence of 3.6-10\({}^{16}\) p/cm\({}^{2}\) (image from Ref. [119]—2011 OSA). comets with aphelia less than 7 AU have been identified as a major source of interplanetary dust released through the sublimation of cometary ices [127]. Aside from the mass and velocity, the effect of these particles can be further exacerbated by the directionality of the optical surfaces relative to the ram direction. The exposure time is also critical; it has been reported that even after a short exposure to the space environment, exposed surfaces can be covered with impacts from small-size debris and meteoroids [128]. The chemical composition of dust is diverse, but most compounds are silicates, ice, iron, and organic compounds. Depending on several factors, dust can accumulate on the surface of the optics and increase the scattered radiation [129], or flake the protective coating, leaving reactive materials exposed (which could be subsequently degraded by AO, for instance), or can even blast away the coating and produce craters on the substrate. In extreme conditions, high-velocity collisions may produce plasma, generating side-effects that may be more damaging than purely mechanical effects [125]. Additionally, the collateral effects of impacts may induce the damage or contamination of the optics; an example will be provided below in the description of one of the NASA Long Duration Exposure Facility (LDEF) experiments. Experimental verification is often necessary to better understand the effect of high-velocity particles. This can be performed in dedicated testing facilities, such as the Heidelberg Dust Accelerator, in which particles of various materials can be accelerated to velocities up to 40 km/s [130]. [PERSON] et al. [131] utilized the aforementioned facility to simulate the effect of the impacts of iron (1.2 \(\upmu\)m diameter) and latex (0.75 \(\upmu\)m diameter), o mimic inorganic- and organic-based meteoroids at velocities of 2-20 km/s on an oxide-protected, Au-coated Be witness mirrors for the James Webb Space Telescope (JWST). It was found that both latex and iron particles had the ability to blast away the protective coating, creating craters where be substrate was exposed. The authors reported ratios between the crater diameter and the incident particle kinetic energy of 0.09 \(\upmu\)m/n) for latex and 0.07 \(\upmu\)m/n) for iron. Yet, most of the knowledge of the mass and velocity distribution of the dust, composition, flux, and effect of impacts in space instrumentation has been gathered in the last decades from dedicated experiments in space, such as dust detectors on board of GALILEO and ULYSSES, or the cosmic dust analyzer (CDA) on board of CASSINI [130].