Importance of Surface Conditions for Spacecraft Charging

Abstract

Abstract : Spacecraft charging is determined by the ambient plasma environment and the surface properties. Whereas the ambient plasma environment is measured in-situ and in real time, the surface property data rely on measurements conducted previously in the laboratory and empirical formulae derived from the laboratory measurements. This note addresses the importance of surface conditions in determining the spacecraft surface potentials. For example, the surface smoothness, thickness, surface composition, and surface contamination are important factors that govern the current balance at equilibrium and the onset of spacecraft charging. These parameters of surface conditions can also greatly influence the accuracy of model calculations of the spacecraft potential. We will provide supporting data to illustrate the main points and to make the case. For spacecraft design, it is inadequate to look up published tables of the coefficients for a given type of surface material. It is necessary to measure the secondary electron coefficient, for example, of an actual piece of the material, because the thickness, smoothness, and surface composition, etc. do matter. In the harsh space environment, surface conditions can also change gradually as a result of the unceasing bombardment by the incoming electrons and ions. All these factors pose uncertainty to spacecraft charging.