Abstract: The test particle Monte Carlo (TPMC) method is used to determine the return flux on spherical and cylindrical simplified spacecraft surfaces due to ambient scatter of outgassing molecules. The return flux ratio (RFR) obtained for the flows past a sphere is in a good agreement with that acquired by the direct simulation Monte Carlo (DSMC) technique. The computation and the analysis of the return flux on cylindrical simplified spacecraft surfaces with different length-to-radius ratios due to ambient scatter show that the most returned molecules are distributed in the up-wind regions on the side surface of the cylinder. In addition, the RFR is increased with the decrease of the ratio of the freestream molecular mass to the outgassing molecular mass, with the increase of the ratio of the freestream temperature to the temperature of the outgassing surfaces, and with the increase of the freestream molecular speed ratio and the number density. The variations of the RFR with the four aforementioned parameters at different length-to-radius ratios are similar, and the RFR for the slenderest cylinder is the maximum, the RFR for the dumpiest geometry the minimum, and the RFR for the moderate-size geometry takes values in between. Depending on the effective up-wind area, the variations of the RFR with the freestream angle of attack at different length-to-radius ratios are no longer similar.