Predicting spacecraft self-contamination in space and in a test chamber

Abstract

The self-contamination of spacecraft (defined as the return and deposition of outgassed molecules on its critical surfaces, either in orbit or while undergoing vacuum) is considered. Theoretical relations for the flux, density, and pressure of the emitted gas as a function of altitude, radius, and distance from the spacecraft surface are developed. The flux of the outgassed molecules that return to the emitting surface is also obtained and shown to be dependent on altitude, spacecraft dimensions, and the magnitude of outgassing. The rate of condensation and the time for the formation of a monolayer of the returning molecules can be calculated. The self-contamination of spacecraft undergoing vacuum chamber test is also theoretically examined and compared with the equivalent parameters for orbit conditions. It is concluded that, depending on the dimensions of the spacecraft relative to those of the chamber and the wall capture coefficient, ground tests conducted in the more usual space simulation chambers can provide returning fluxes and self-contamination comparable to those occurring in space up to an altitude of about 400 km. For higher altitudes and return fluxes less than 0.001 of those emitted, the chamber test can produce a greater contamination. In this case, the ground results can be related to those obtained in space, provided that the wall capture coefficient is known or if the ratio of returned to emitted flux at the spacecraft surface is measured