**2.1 Charging and discharging phenomena and hazards**

In the field of spacecraft charging and discharging, the potential of the spacecraft is relative to the zero potential of the space plasma. Although the density of space plasma is fluctuating, it is much faster than the change of the spacecraft potential on the time scale. The spacecraft potential is floating. The spacecraft operates in a harsh space environment, such as plasma, high-energy electrons, atomic oxygen, etc., and charging and discharging phenomena will occur in the surface or deep layer of the spacecraft [2]. **Figure 1** depicts a schematic diagram of the spacecraft floating potential.

When the energy of the incident particles is low, the charge exchange process will appear between spacecraft and the surrounding environment. Environmental electrons or ions interacting with target atoms on the surface of the dielectric material will generate the secondary electrons and backscattered electrons. In addition, when the spacecraft is operating on the sunny side, photoelectrons are generated on the surface of the dielectric material. The combined effects of the above processes will cause charging and discharging phenomena on the surface of the dielectric materials. For different dielectric materials, due to their different secondary electron emission coefficients, backscattering coefficients and photoelectron coefficients, the surface charge exchange processes are different. Consequently, different surface potentials appear on the dielectric materials, which will cause unequal charge between the dielectric materials [2].

**Figure 1.** *The schematic diagram of spacecraft floating potential in space plasma environment [2].*

The deep dielectric charging refers to the process that high-energy electrons (MeV) penetrate through the dielectric surface and deposit within the insulating materials [22]. Incident electrons penetrate into insulating materials, and their energy will gradually transfer into target atoms, owing to the physical mechanism of elastic scattering or inelastic scattering. For high-resistivity polymer, the intrinsic conductivity is very low. High-energy electrons penetrate the surface and deposit inside the material. These charges are called deposited electrons. Under the radiation of the space electron spectrum, electrons of different energies have different penetration distances inside the material, resulting in the formation of deposited charge layers of different depths. The charge accumulation will cause distortion of the electric field, which is likely to cause internal electrical breakdown of the dielectric materials [23].

From 1980 to 2005, the statistics of 156 anomalies of orbiting spacecraft showed that 45% of spacecraft anomalies were caused by the failure of the power system of the spacecraft [24], among which the insulating materials and structure of solar array and its drive assembly are most likely to discharge. The spacecraft power system fails once the solar array or its drive assembly fails. Even worse, the spacecraft will be out of control. A Nigerian satellite launched by China in November 2008 completely failed due to the failure of solar array drive assembly [25]. Especially with the increase of spacecraft operating voltage and power requirements, the coupling effect of high operating voltage and space radiation environment will pose a greater threat to the insulation system of spacecraft.

### **2.2 Research process of dielectric charging and discharging**

As early as the 1920s, Mott-Smith and Langmuir began the initial theoretical exploration of the electrostatic charging of isolated bodies in space [2]. With the launch of the first artificial satellite in 1957, humankind entered the era of space, and the related issues of space dielectric charging have gradually attracted researchers' attention. Before 1980, it was believed that the charging and discharging of the dielectric surface was the main cause of spacecraft anomalies, and related research focused on the surface charging phenomenon [26]. With the occurrence of abnormal spacecraft failures and the launch of CRRES satellite (Combined Release and Radiation Effects Satellite) in the 1990s, deep dielectric charging of the spacecraft came into focus and research on spacecraft charging entered a new era [27]. H.B. Garrett published two review papers in 1981 and 2000 [26, 27], which summarized the research progress of spacecraft surface charging before 1980 and research development of surface charging and deep charging between 1980 and 2000. Lai published a review paper in 2003 [28], which summarized the suppression methods of dielectric charging.

Since the twenty-first century, great achievements have been made in space environment exploration, basic theoretical research and ground simulation experiments. However, the charging and discharging of dielectric materials is still the main factor threatening the safe operation of spacecraft. Especially with the development of high-voltage and high-power spacecraft, the field of dielectric charging and discharging is facing new challenges.
