In this study, the performance of an open source, two-temperature thermochemical nonequilibrium solver, hy2Foam, is evaluated under rarefied hypersonic flow conditions representative of atmospheric reentry. The focus is placed on the solver’s ability to capture the physical effects of rarefaction and transport modeling at high Knudsen number regimes where continuum breakdown occurs. A series of validation cases are conducted for the Stardust reentry capsule using well-established results from Martin and Boyd. Key flow properties such as stagnation line pressure, heat flux, and translational and vibrational temperatures are compared. The study investigates the influence of mass diffusion models and mixture rules on solution accuracy. Furthermore, the impact of rarefaction is explored by varying the altitude, corresponding to different Knudsen numbers. The results show persistent discrepancies in vibrational temperature predictions despite improved transport models, suggesting that the primary source of error originates from reaction and relaxation modeling in rarefied regimes. This work highlights both the potential and limitations of continuum-based open-source solvers in modeling high-enthalpy, rarefied, chemically reacting flows.