In space, satellites are immersed in a complex radiative environment consisting of highly energetic charged particles (protons, electrons, ions, gamma rays...). These particles interact with spacecrafts and degrade the physico-chemical structure of materials, leading to changes in their optical and thermal properties also known as spectral signature. Cold satellite coatings such as white paint, for example, see their solar absorptivity and IR emissivity change, compromising the satellite's thermal equilibrium. Moreover, these changes are an emerging issue for space surveillance.

To anticipate these phenomena, we need to understand and predict the evolution of the thermo-optical properties of the materials used in these coatings, as a function of the level of radiation received.

Our work is focused on the change of optical response of space paints in the range [250nm, 2500 nm] after irradiation. It is divided in two parts. One part is devoted to experimental irradiations and optical properties measurements. For this purpose, white paints from CNES formed by PDMS, a silicone resin, and zinc oxide pigments (ZnO) have been irradiated with 240 keV incident protons. The second part of our work aims at developing a radiative transfer code capable to reproduce the optical signature of the paints as a function of the amount of received radiation. The transfer model is based on the Monte Carlo method: photons are transported step by step through scattering, refraction and absorption interactions. We use the optical module of the GEANT4 library [1] to develop the radiative transfer code and configure these transport phenomena. An empirical model is implemented in our simulation to account for the evolution, as a function of the deposited dose, of the refractive index responsible for the absorption of paint constituents. Calculated reflectances and transmittances of white paint samples are compared with our experimental measurement results.

[1] https://geant4.web.cern.ch/