Electrostatic Charging of Spacecraft in Geosynchronous Orbit

Any satellite whose orbit passes through the terrestrial magnetosphere will encounter several plasma regimes with widely differing temperatures, densities and compositions. The spacecraft itself will act as a plasma probe; each surface element changes its potential until no net current flows between it and the plasma. Differential charge build-up between adjacent surfaces (for example of different material or geometrical characteristics) can induce electrostatic discharges. The resulting current pulse, or RF interference gives rise to 'operational anomalies' (spurious switchings, telemetry drop-outs etc), or in extreme cases leads to permanent damage (eg short circuits in strings of solar cells). This thesis is a study of the spacecraft charging phenomenon applicable to satellites in geosynchronous orbit. Results are derived from three sources. Firstly, Meteosat F2 (a meteorological satellite launched by ESA in July 1981) which carried an electron spectrometer to make direct measurements of the geosynchronous orbit electron environment, and in addition, suffered a series of operational anomalies and surface charging events. Secondly, an experimental test programme was undertaken using a monoenergetic electron beam to irradiate some common spacecraft surface materials, and thereby examine their electrostatic charging properties. Thirdly, numerical simulation codes were written and employed to model several different situations, ranging from the electron beam experiment to a full three-dimensional simulation of Meteosat F2. Analysis of the electron beam irradiation data shows that the surface conductivity of some common insulating materials (Kapton and Teflon) plays a much greater role in the current balance equation than was thought previously. Furthermore, the process whereby bulk and surface conductivity is enhanced by large electric fields (deviations from pure Ohmic behaviour) is also shown to be a significant factor in the current balance equation. These results are of importance both to satellite surface design, and to high voltage insulating systems. Spacecraft charging simulation codes must accurately model the detailed interaction between plasma energy spectra and the energy dependent material surface properties (such as secondary electron emission). The thesis demonstrates that it is vital to use real, measured spectra (for example from the Meteosat spectrometer) rather than simple Maxwellian plasma definitions, if spacecraft charging is to be modelled successfully. Also, the need for an improved database of reliable secondary electron yield measurements is demonstrated.