## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Dust and Space Debris
The CDA instrument had two sensors; the first one was a high-rate detector to count the number of particles, and the second one analyzed the dust's charge, speed, size, and direction. Further knowledge related to the dust and debris characteristics has been obtained from satellites or parts thereof returned from space (Shuttle, Solar Max, Palapa, Westar, MIR, EURECA, HST). As an example, the chemical analysis of the craters on solar cells recovered from the HST showed that caters with diameters of 100-3500 \(\upmu\)m were produced by meteoroids, whereas craters with diameters of 1-100 \(\upmu\)m were produced mostly by space debris composed of aluminum and aluminum oxide, indicative of solid rocket motor fuel debris [132]. The Long Duration Exposure Facility (LDEF) experiment deserves special mention; in this experiment, a trap with several optics was mounted in the exterior of the spacecraft (see Figure 5a,b), and exposed for 5 years and 8 months (32,422 orbits in LEO, from 842 km to 340 km) to micrometeoroids and space debris. This extended duration presented a unique opportunity to study the long-term effects of space exposure on the coatings and substrate materials flown. Among the most spectacular results, a 1 mm-diameter impact in a bare CaF\({}_{2}\) substrate produced a two-direction full cleavage, breaking the sample into three pieces, as shown in Figure 5c. Another impact on a PbTe/ZnS multilayer-coated Ge substrate caused a coating delamination in the surroundings of the spallation area of 4.5 mm diameter. Posterior analysis of the multilayer coating showed that the impact did not add stress or induce any further coating damage beyond the spallation area. The contamination of the SiO-coated Si substrate by aluminum provided the best example in the LDEF of the secondary ejecta and collateral effects of impacts. An impact occurred in the edge of the aluminum sample holder (see Figure 5d), leaving secondary ejecta spray patterns of molten aluminum on the surface of the sample [133; 134]. To summarize, space dust and debris can affect the performance of mirrors and coatings, or even determine a full mission failure. However, the body of spacecrafts should be protected from micrometeoroids and space debris impacts. [PERSON] proposed in 1947 [135] that a steel \"skin\" of one millimeter thickness spaced one inch from the main spacecraft body would disintegrate along with the high-velocity meteoroid upon impact, thus preventing the latter causing damage to the spacecraft. Even though this protective system has been verified, implemented, and improved since then, this cannot be used to protect the primary mirror of an optical telescope, for instance. Hence, the protection of exposed optical elements mostly relies on prevention and prediction. In this respect, all space agencies pursue the common goal of reducing the generation of space debris from in orbit explosions and collisions and from rockets' upper stages, and discouraging anti-satellite missile tests. In terms of prediction, models which can precisely account for (1) meteoroid velocity and mass distributions as a function of orbit altitude, (2) the flux of meteoroids of larger sizes (>100 microns), (3) the effects of plasma during impacts, and (4) variations in the meteoroid bulk density with impact velocity have been identified as a powerful tool to foresee the effect of dust and debris in future space observatories [126].