Spacecraft charging
Space is mostly a perfect vacuum but there is a certain amount of charged particles, both electrons and positive ions, mainly coming from the solar wind. These particles form what is called a ‘plasma’ and has an electrical potential associated, producing spacecraft charging that can be detrimental or harmful to its elements.
Any object in space will be exposed to the space plasma and capture some electrical charge to eventually set to same electrical potential. Spacecraft charging may affect the electronic measurements onboard. It may damage electronic instruments. It may also affect the telemetry and even the navigation system. In severe cases, it may terminate the spacecraft mission. Both surface charging and deep dielectric charging
are important in causing spacecraft anomalies.
In general, the lower the orbit altitude the lower the particle energy and the lower potential a satellite will have. Higher orbits like GEO can expose satellites to kV what can be dangerous for the electronics.
Satellite charging is not simple. On one side, metals and dielectrics behave different. Metal has surface charge and charge can be distributed across the metal bodies. Dielectrics capture charge that can even have opposite sign and won’t produce currents. The energy of the incoming particles also has an effect, very energetic particles may escape being captured. Electrons, because of their smaller mass, are more likely to be captured than positive ions, therefore spacecrafts tend to acquire a negative potential respect to its surrounding plasma. Secondary and backscattered electrons can also be produced in space. Impending photons from sunlight, even if they carry no charge, can produce electrons by photoelectric effect.
As any satellite goes round the Earth it passes through areas of different potentials because of the assymmetry in the Earth magnetic field created by the solar wind.
Most charging effects happen when the satellite transits from the eclipse area where hot electrons abound to the sunlit area. In that period, charge changes rapidly and currents can be important. Also, charge accumulated in dielectrics can arc towards areas with a different potential.
Solar panels if present, have also an important effect. They create a difference of potential within the spacecraft that alters the overall potential. Typically, a spacecraft body is grounded, i.e. connected to the batteries negative pole. Part of the solar arrays will have a positive potential what will cause the spacecraft body being more negative for the whole to have the same equilibrium potential as without this effect.
Space charging is something potentially harmful and proper measures are needed. NASA has this article and a valuable guideline for designers.
An additional risk associated with space charging is with docking manoeuvers, for example when space shuttles dock into the ISS or when satellites docking other satellites either for servicing or debris removal. Because docking is a critical operation where failures can be disastrous, discharge currents must be managed in a way they won’t affect the on-board electronics.
