Rebuilding of ICOTOM radiometer data during the Schiaparelli Martian entry

International audienceIn spite of the crash of Schiaparelli, the Exomars Descent Module in October 2016, following a nevertheless successful aerothermodynamic entry, data from the infrared radiometers ICOTOM embedded of the COMARS modules was sent to the orbiter before and after the blackout phase. Three pairs of radiometers were located in line on the back shield of the probe in order to monitor the infrared radiation received by the thermal protection system from CO and CO2 molecules. Within a pair, one ICOTOM (B1) was dedicated to the range 4,17-5 µm (2000-2400 cm-1) and the other one (B2) was dedicated to the range 2,6-3,36 µm (2950-3850 cm-1) in order to collect cold and CO2 bands as well as CO rovibrational radiation. Flight data provided radiative heat flux densities in both ranges for three locations in the shield, especially at the end of the hot phase. Lower housing temperatures than expected led to recalibration of the ICOTOM depending on the incoming flux and on the sensor own temperatures.In order to derive of maximum of information about the thermodynamic status of the gas behind the spacecraft, CNES gathered laboratories and companies to rebuild the aerodynamic field, the chemical composition, the non-equilibrium status and the radiative transfer within the back body plasma from numerical simulation and on-ground experiments.The main purpose is to obtain a comprehensive numerical tool able to recover the flight measurements and to deal with similar entry situations in the future. Only few hypotheses were made especially about the equilibrium status of the plasma and about the angle of attack of the probe. Then the implicit-in-time code has been developed for 3D geometry and includes partial state-to-state chemistry. Kinetic chemical models developed for 0D and 1D geometry were reduced in order to make them compatible with reasonable computation times. However, a full (vibrational and electronic) model is used along lines in the flow with a Lagrangian approach. A specific scheme has been established to describe the vibrational structure of CO2. That model has been made compatible with chemical and radiative calculations. Solving the radiative transfer equation within the back body plasma implies to take into account possible non-equilibrium effect in CO2 vibrational populations. Following a line-by-line model working out of equilibrium, a statistical narrow-band model was developed in order to reduce the time cost of calculations involving aerodynamics, chemistry and radiation. However, radiation transfer calculations were not coupled to the aerothermochemistry calculations and are carried out as a post-treatment on the fields of densities and temperatures (when applicable).Another important part of the project consists of ground tests of ICOTOM radiometers on plasmas close to those encountered by Schiaparelli. That experimental approach is to confront the signals given by the ICOTOM radiometers and the heat flux densities derived from them with laboratory measurements obtained by optical and laser spectroscopy and interferometry. Ground plasma flows are then used as test cases both for ICOTOM measurements and for numerical simulation. Various facilities such as ICP and arc-jet plasmas were used within that study