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Analysis of the Aerothermodynamic Environment during the Entry of the Huygens Probe in the Titan Atmosphere

F. Mazoué ⁽¹⁾, J. Graciano, F. Dengra, L. Marraffa, T. Blancquaert, J.P. Lebreton ⁽²⁾

(1) Atos Origin, Leiden, The Netherlands
(2) ESA/ESTEC, Noordwijk, The Netherlands

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The Cassini/Huygens mission is a NASA/ESA/ASI program, planned to explore the Saturnian system. The spacecraft consists in a Saturn orbiter and the Huygens Titan entry Probe. Cassini/Huygens was launched in October 1997. The Huygens separation from Cassini is planned for December, 25th 2004 for an an entry in Titan on 14th January 2005. This paper addresses analysis of the entry conditions that are performed at ESA/ESTEC in support of the industrial activities for the re-validation of the Huygens entry conditions, taking into account the new entry geometry and the latest atmospheric models (Lebreton & Matson, SSR 104, 59-100, 2002; Clausen et al., SSR, 104, 155-189, 2002).

The first objective of this work was to analyse the radiative, convective and total heat fluxes during the entry phase as function of the new entry conditions and Titan atmosphere composition as described by the Yelle model (ESA SP-1177, 1997). It consists in three different profiles with different compositions: i) recommended (nominal) profile (95%N2, 3%CH4, 2% Ar); ii) minimal profile (95%N2, 5%CH4) and iii) maximal profile (87%N2, 3%CH4, 10% Ar). Previous studies have shown more significant heat fluxes were obtained for minimal and nominal atmospheres. Thus, analyses have been limited to these profiles. The effects of the gravity waves (Strobel & Sicardy, ESA SP 1177, 1997) have also been studied.

In addition to engineering studies related to re-validation of the entry conditions, this work also provides useful information for the observation possibility from Earth of the Huygens entry plume in Titan atmosphere (R. Lorenz, An Artificial Meteor on Titan ?, Journal of the Royal Astronomical Society, October 2002). Indeed, the study is used as a validation of the ESA/ESTEC tools and codes for computations of flow-field and emission around the Huygens probe.

Simulations have been performed with different codes; Traj3d code for trajectory parameters and free-stream conditions; PMSSR and TINA (Thermodynamic Implicit Non-equilibrium Algorithm) codes for temperatures and molar fractions variations inside the shock layeras well as convective heat flux. The NEqAir (Non-Equilibrium Air Radiation) code is used to compute the radiation emission of the flow, from which the radiation heat flux on Huygens probe is derived by integration over the volume of the shock layer.

The validation of PMSSR/NEqAir and TINA/NEqAir code has been performed by comparing with Nelson's results (1991) for a reference Titan atmosphere composition of 98% N₂ and 2% CH₄. Good agreements have been obtained with both chains in terms of temperature and mole fractions evolutions and radiative heat flux on the stagnation point. Preliminary entry plume luminosity values are derived.