To quantify the potential contamination induced by exhaust plume from a 10 N bipropellant thruster firing, plume impingement analyses on the sensitive surfaces were performed. The heat and mass fluxes of chemical species on the spacecraft surfaces and on the specific area of sensor units (optical port and radiators) are evaluated. A fully unstructured three-dimensional direct simulation Monte Carlo (DSMC) code was developed and validated with measurements. A geostationary satellite with an optical port and radiator was selected and the actual configuration of the satellite is simplified by considering two communication antennas, satellite panels and a bipropellant thruster, which is a major plume source to the sensitive surfaces such as the optical port and radiator. The DSMC computations show that the mass distribution or plume gas composition is unevenly distributed due to the mass separation effect which occurs from the rapid expansion into vacuum especially around the nozzle lip at the backflow region. $H_2O$ is known as the most critical contaminant species, however, its plume impingement effect on the sensitive surfaces is negligible as compared to its evaporation rate. Not only have the results contributed to the geostationary satellite development, but also these findings are expected to be significant in designing spacecraft configurations and useful in the trade-off studies. Grouping species method was proposed to reduce computing time and efforts. Gas species are grouped by their molecular mass, diameter, and the number of degrees of freedom based on the Dalton’s law. The proposed grouping species method was applied to the present exhaust plume of bipropellant thruster with reasonable accuracy, while reducing computing time by 50%. Therefore, the proposed grouping species method could enhance the analysis efficiency of parametric studies with many design parameters, e.g. thruster tilt angle, position, and the distance between thruster position and the sensitive surfaces.