## 3 Conclusions  
Optical coated elements for space instrumentation are mainly optimized in terms of their efficiency and required working spectral band. After fabrication, witness samples undergo a series of laboratory tests required by space agencies in order to qualify the components. However, laboratory tests rarely reproduce the conditions in space; because the quality of the vacuum is not the same as in space, some contaminants coming from the satellite itself are neglected. The space environment is not always known precisely, and the flux of particles and contaminants is lower in space than in accelerated tests. Mirror substrates and coatings are the key components of space optics. Space mirrors must withstand a harsh environment, where servicing campaigns to clean or replace degraded optics are very limited or, most often, impossible. Mirrors must be able to keep acceptable performance through  
Figure 5: Effect of space debris (**a**) Tray B08 with several mounted optics. There were bare substrates and coated substrates, among other samples. (**b**) The LDEF in orbit. The location of tray B08 in LDEF can barely be seen, but it is indicated with a red arrow. (**c**) Impact and cleavage of the BaF\({}_{2}\) substrate. (**d**) Impact on the edge of the Al holder for the SiO-coated Si sample. Molten Al spray patterns can be seen on the sample surface (**a,c,d**) and are available at www.reading.ac.uk and [[https://www2.physics.ox.ac.uk](https://www2.physics.ox.ac.uk)]([https://www2.physics.ox.ac.uk](https://www2.physics.ox.ac.uk)), Infrared Laboratory, LDEF. (**b**) Available at the NASA Image and Video Library. missions that may have a lifetime as long as decades. In fact, the optical performance of the components strongly affects the scientific data outcomes, and their degradation can lead to data misinterpretation due to an unknown change in the instrument radiometric response. In a more dramatic scenario, unpredicted mirror degradation may kill an expensive mission along with the high expectations of the community for decades. Hence, space opticians need to predict the behavior of coatings and substrates at the specific orbit and space conditions and for the full mission lifetime. In order to accomplish this objective, more experimental data need to be collected and shared. This is particularly important, as presently very few experimental data are made available by the re-testing of components in those few cases in which the optics have been collected after a flight. The results of qualification tests are rarely published and made available to the scientific community, as they are perceived as small technical details and because there is not a reference scientific journal which offers a solid background in this field, as is the opposite of the case of electronics components. The clear definition of testing procedures to assess the robustness of optical components against the operational environment is of pivotal importance for preventing in-flight failures, to fabricate robust coatings, or simply to model their degradation. In situ testing experiments in which simple optical systems are coupled to mirror optics for efficiency measurements over time during a flight could be an advantage. This paper is intended to contribute to the formation of a background knowledge in the field. Attention has been devoted to the main sources of mirror degradation: atomic oxygen, thermal processes, ultraviolet radiation, outgassing and cross-contamination, charged particles, and space debris and dust. An effort has been made to cite and comment the main literature on the degradation effects of all these sources on mirrors all over the electromagnetic spectrum, with an emphasis at short wavelengths. Available information combines space simulation in the lab and also the heritage of six decades of space optics. Despite the large amount of literature on space mirrors and degradation/stability issues, significant improvements are still desired for future space observatories.