As illustrated in February 2021 by NASA’s landing of Perseverance on Mars, one of the most dangerous and challenging aspects of any planetary exploration mission is a spacecraft's hypersonic entry into a planet's atmosphere. Achieving this goal has involved many succesful missions over the past years. The physical processes occurring around the spacecraft are quite complex and involve the synthesis of chemical kinetics, quantum mechanics, radiation physics, and ablation effects with fluid dynamics. To further complicate matters, the atmosphere is often rarefied and conventional fluid dynamic analysis is no longer applicable. Such high energy, high speed, rarefied conditions are very expensive and often impossible to reproduce in wind tunnels here on Earth. Actual flight tests are even more expensive and measured data is limited to that collected during the 1960's Mercury, Gemini, and Apollo programs. If numerical simulation can reproduce the experimental data that is available, it can then be used with confidence as a fast and inexpensive design tool for new spacecraft flying new missions.

At low altitudes (below ~80km for Earth) the atmosphere is sufficiently dense such that molecules undergo a vast number of collisions as they move over the spacecraft. Under these conditions the gas can accurately be assumed to behave as a continuum and the Navier-Stokes equations can be solved using methods from Computational Fluid Dynamics (CFD). CFD methods are very mature and are capable of incorporating advanced physical models such as chemical and thermal non-equilibrium, radiation, and even ablation. For very high altitudes (above ~100km for Earth) the atmosphere is rarefied to the point where molecules undergo far fewer collisions invalidating the continuum assumptions inherent in the Navier-Stokes equations. In this regime the most mature numerical method is the direct simulation Monte Carlo (DSMC) method which is also capable of incorporating advanced physical models. Since the DSMC method simulates the gas on the molecular scale it provides accurate results in all regimes, however under continuum conditions, large numbers of particles and collisions demand impractical computational resources. Thus, in general, the DSMC method is used to simulate atmospheric entry at high altitudes and CFD is used at lower altitudes. Of course there is a large overlap regime in which the flow around the spacecraft exhibits regions of both continuum flow and non-equilibrium or rarefied flow. For this reason current research is not only focused on using CFD and DSMC to simulate the aerothermodynamics of atmospheric entry, but also focuses on incorporating these methods into a hybrid particle-continuum code.