Contamination modeling in Europe has long been based on physical mechanisms, such as desorption. However other physical mechanisms, such as diffusion, evaporation or mixing effects exist. These alternative mechanisms were experimentally evaluated and modelled. It was yet observed that, without an experimental capability to reliably separate the (re)emitted chemical species, it is very difficult to determine whether the modeling and its underlying physical mechanisms are representative of reality, or simply a mathematical fit of reality. This is the reason why in the last years emphasis was put on the experimental separation of species, mostly through TGA/MS coupling. This paper presents a review of these efforts and promising results on species separation to reach a really physical modeling of outgassing, deposition/reemission and UV synergy.
Growing evidence was accumulated on the deleterious effects of the photofixation of contaminants on solar arrays power and on the optical properties of coatings. UV irradiation indeed promotes contamination accretion, even on surfaces on which condensation would not occur and strongly degrades the optical properties of contamination layers. Recent research conducted at ONERA enabled to implement a photofixation model in the numerical tool COMOVA. Present work aims at assessing the ability of this model to reproduce in-orbit cases and at estimating the sensitivity of the results to input parameters. Simulation results are reasonably close to the in-orbit degradations.
The in-orbit aging of thermo-optical properties of thermal coatings critically impacts both spacecraft thermal balance and heating power consumption. Nevertheless, in-flight thermal coating aging is generally larger than the one measured on ground and the current knowledge does not allow making reliable predictions<sup>1</sup>. As a result, a large oversizing of thermal control systems is required. To address this issue, the Centre National d’Etudes Spatiales has developed a low-cost experiment, called THERME, which enables to monitor the in-flight time-evolution of the solar absorptivity of a large variety of coatings, including commonly used coatings and new materials by measuring their temperature. This experiment has been carried out on sunsynchronous spacecrafts for more than 27 years, allowing thus the generation of a very large set of telemetry measurements. The aim of this work was to develop a model able to semi-quantitatively reproduce these data with a restraint number of parameters. The underlying objectives were to better understand the contribution of the different involved phenomena and, later on, to predict the thermal coating aging at end of life. The physical processes modeled include contamination deposition, UV aging of both contamination layers and intrinsic material and atomic oxygen erosion. Efforts were particularly focused on the satellite leading wall as this face is exposed to the highest variations in environmental conditions during the solar cycle. The non-monotonous time-evolution of the solar absorptivity of thermal coatings is shown to be due to a succession of contamination and contaminant erosion by atomic oxygen phased with the solar cycle.