Huygens Probe Radiative Environment

Abstract

The Cassini mission is a NASA/ESA program planned to explore the Saturnian system, and consits of a Mariner Mark II Orbiter sending the on-board Huygens probe to Titan during its fly-by of Saturn. The Huygens probe uses aerobraking and decelerates through Titan atmosphere from 6190 m/s at 1250 km altitude down to 400 m/s near 170 km. This extremely high velocity results in dissociation and ionization in the forebody shock layer. Moreover, the relaxation times of chemical and thermal processes involved are of the same magnitude than the characteristic time of the flowfield, the shock layer being so in full nonequilibrium. As the Titan atmosphere major constituents are N2 (more than 77% in molar fraction), CH4 (from 0 up to 3.5% ) and Argon which may exist up to 20%, non-equilibrium reactions result in CN creation which is known to be a strong radiator. Dedicated preliminary calculations show that, for certain flight conditions, the radiative heat flux may exceed the convective one. In the thermal protection design process, accurate radiative heat flux calculations are needed in order to insure a conservative design and to avoid mass penalties of the thermal protection. It is also desirable to validate our radiative calculations by comparing our results with experiments. Previous shock tube tests have been performed by C.S. Park (1991) in a Titan representative atmosphere gas medium.