Modeling Molecular Outgassing Transport and Deposition for Spacecraft Operating in Vacuum

Abstract. The science objectives of increasingly ambitious space missions require meeting stringent contamination requirements. The process of outgassing under vacuum exposure is often a primary source of spacecraft molecular contamination. Materials, such as cabling insulation and structural adhesives, evolve molecules that can be transported to and deposit on contamination-sensitive surfaces. Such deposition can negatively impact spacecraft performance: contaminant films may compromise scientific observations made by optical instruments, and may degrade the efficiency of radiators or solar arrays. The design of spacecraft and their mission operations is informed by contamination modeling, an engineering discipline combining empirical data and established science principles. Empirical data obtained from short-duration ASTM E1559 testing is used to determine outgassing rates from source materials. Scaling laws, based on Fick’s law of diffusion and Arrhenius factors to account for temperature variations, are used to extrapolate outgassing test data to mission conditions and durations. Outgassing contamination transport processes in vacuum are modeled as free molecular transport using a view factor formulation. Data from a thermo-gravimetric assessment performed at the conclusion of the ASTM E1559 test is used to estimate contamination condensation on spacecraft surfaces. An example describing the contamination model—incorporating source outgassing, free molecular transport, and condensation onto spacecraft surfaces—is presented for a representative mission.