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Sample return from asteroids and comets beyond the main belt is considered to be one of the most worthwhile future deep space missions [1]. Japan Aerospace Exploration Agency (JAXA) is currently entertaining a Jupiter Trojan sample return mission using a solar power sail [2], following the heritage of the Hayabusa sample return mission [3]. Because of high orbital energy corresponding to outer bodies, a sample return capsule (SRC) for such a future mission is expected to reenter the Earth’s atmosphere at velocities higher than 14 km/s and, hence, to encounter much severer aerodynamic heating environments than those of any past SRCs. Especially, the radiative heat transfer is considered to play a significant role in heat transfer processes in the shock layer, since the radiative heat transfer exponentially increases with the atmospheric flight velocity. To optimize the aerothermal design of the SRC and to minimize the mass of the thermal protection system (TPS) equipped with the SRC aeroshell, it is essentially required to accurately predict the aerothermal environments around the SRC along the reentry trajectory.

The prediction of the nonequilibrium radiative heat flux experienced by a sample return capsule relies on a combined approach of numerical simulation and ground test experiments. This paper presents an analysis of the VUV radiation measurements from heated air mixtures carried out in the High Velocity Shock-Tube (HVST) facility at JAXA Chofu Aerospace Centre. The spectral signatures of atomic lines were analyzed under equilibrium and nonequilibrium conditions. The electron number densities were inferred from hydrogen lines and used as a benchmark to assess the ionization model currently implemented in the in-house solvers.