**2. Environment effect on dielectric properties**

As already mentioned, when spacecrafts are evolving into space, they are submitted to a large number of charged particles and encounter several operational anomalies. It has been demonstrated that the harmful anomalies are related to spacecraft charging and the risk of electrostatic discharges (ESDs). For instance, Koons et al. [10] established a list of spacecraft accidents between 1973 and 1997, where he shows that 54.2% of the 326 cases analyzed were directly due to ESDs. Other studies confirm the link between ESDs and anomalies recorded on satellites and try to establish links with surface and/or internal charging [11, 12]. The final idea is always to recommend solutions so as to improve satellite's design that mitigate the occurrence of ESD on future spacecraft even if they are submitted to extreme environmental conditions. Many other studies tend to give access to a broad panel of information concerning the origin of those anomalies through the space weather analyses [11, 12].

Usually spacecraft charging is attributed to three main categories called absolute charging, surface charging, and deep-dielectric charging. The absolute charging is associated to the charging of the whole structure versus its environment; the surface charging refers to the external charging and can be associated to the differential charging between the various part of satellites; and at last the deep-dielectric charging concerns the charging in bulk (**Figure 1**). Deep-dielectric charging can occur in materials located on the satellite surface or inside the main structure. It is due to higher energy particles that can cross thick materials on the way without being stopped. In most of the cases, it is surface materials that are studied as they are directly exposed to various sources of ageing and at the origin of most of the discharges.

**Figure 1.** *Schematic representation of satellite charging.*
