Abstract: Test particle Monte Carlo (TPMC) method is presented. Return flux on four geometric surfaces due to ambient scatter of outgassing molecules is simulated. Return flux ratio (RFR) obtained for flow past a sphere is in good agreement with DSMC results. RFR on outgassing and freestream conditions for flows past three geometric bodies, including a circle flat plate, a convex and concave hemisphere, is investigated. RFR for flows past a circle flat plate and a concave hemisphere is much greater than that for flows past a convex hemisphere. Outgassing molecules collide on outgassing surfaces directly forming direct flux contamination for flows past concave surfaces, which is much greater than RFR. Thus, using convex outgassing surfaces in spacecraft design can decrease return flux contamination effectively.

Key words: outgassing molecules, ambient scatter, return flux, test particle Monte Carlo, hemisphere