## 4 Conclusions  
Long-duration space flights imply the integration of architectural concepts into human space missions, with the introduction of common elements of the living environment, such as windows, within the spacecraft design. The proposed solutions, of course, must be subjected to close certification, in order to establish their compliance with the severe space environment conditions and requirements. Light, transparent thermoplastic polymers such as poly-sulfones are believed to be ideal candidates fur such applications, thanks to their significant properties of toughness, impact strength and flame-retardant capability. The commercial poly-sulfone Radel R was characterized by flame exposure test, as well as by ultraviolet and atomic oxygen erosion rate measurements, in order to validate such material as thermoplastic polymer for space employment. Flammability testing showed that Radel R is self-extinguishing, with no afterglow phenomena, thus establishing the optimal behavior of poly-sulfones for anti-flame applications. Mainly, except for small charred areas, the melted zones appeared to keep visible transparency after the flame exposure, thus allowing us to assess the material suitability for the required applications. The results of UV and AtOx erosion test demonstrated a suitable chemical stability of Radel R, even if further optimization treatments on the base poly-sulfone surface should be performed to achieve optimal results. In fact, the hitting mechanism by UV radiations gives rise to a significant interaction with the impinged polymer surface, resulting in a lowering of the polymer transparency. Even if not relevant when considered alone, the AtOx effects result in being more damaging due to the synergistic effect with UV radiation, which may cause more severe erosion of spacecraft materials, thus compromising their optical properties. Moreover, AtOx corrosion effects should be closely considered in terms of flammability index lowering, which would make simultaneous UV exposure able to light the spark of fire ignition events. The proposed coating treatment performed by using the commercial hybrid ceramic polymer Ceramer appeared to worsen the surface properties of the base poly-sulfone Radel R. In particular, a significant amount of material spreads during the flammability test and there is a sharp increase of UV/AtOx aging, thus establishing the unsuitability of such kind of poly-sulfone coating for space applications. Further surface optimization treatment should be developed in order to gain a fully compliance with the operative requirements. By this way, the next activities of the research project will be focused on the development of a coating process able to smooth the optical properties degradation due to LEO environment effects, while keeping unchanged (or eventually improving) the anti-flame effectiveness of the base poly-sulfone material. Future research will be addressed also to take into account thermal properties over as wide a temperature range as possible, as well as the evaluation of the depth of transparency  
Figure 6: UV/AtOx aging characterization: chamber inside during the test, and Radel R samples before and after exposure, for both AtOx alone and UV+AtOx irradiation. degradation as function of UV exposure time. Finally, it is worth noting that, since this is a ground-based study, the comparisons are inevitably valid only for normal gravity: A definitive assessment of the applicability of any of these test methods to a micro-gravity environment awaits a quantitative interpretation of the role of gravity, especially as far as the flammability is concerned. Conceptualization, M.M. ; Methodology, M.M. and F.S. ; Software, R.P. ; Validation, M.A. and A.D.; Formal Analysis, R.P., M.A. and A.D.; Investigation, M.A. and A.D.; Data Curation, R.P. ; Writing--Original Draft Preparation, R.P., M.A. and A.D.; Writing--Review & Editing, R.P. ; Supervision, M.M. and F.S. All authors have read and agreed to the published version of the manuscript. This research received no external funding.