Contribution of CO2 IR Radiation to Martian Entries Radiative Wall FluxesSupport to the ESA EXOMARS Mission for Exploration of Planet Mars

M. Lino da Silva

Instituto de Plasmas e Fusao Nuclear, Instituto Superior Tecnico

Framework of the ESA EXOMARS Mission

I ExoMars (Exobiology on Mars) is aEuropean-led robotic mission to Marscurrently under development by theEuropean Space Agency (ESA) andNASA.

I ExoMars is the first mission in ESA’sAurora Exploration Programme, aimedat extending Europe’s capabilities inplanetary exploration.

I Mission originally conceived as a roverwith a static ground station, to belaunched in 2011 aboard a Soyuz Fregatrocket.

I Delays in the definition of the missionled to a modified framework: The newESA/NASA Mars Joint ExplorationInitiative signed in July 2009.

I Other technological challenges, also as aconsequence of the contribution byIPFN, showed that the original missionprofile was inpractical.

I Two missions defined: The launch of anESA Entry, Descent, and LandingDemonstrator in 2016, and the launch ofthe robotic ESA-rover onboard a NASASpacecraft in 2018.

I Prime contractor (Fluid Gravity Eng.(UK)) tasked with producing the CFDflowfields.

I IPFN has been tasked with thesimulation of the radiative properties ofthe plasma surrounding the EXOMARSspacecraft during its atmospheric entry,using its in-house line-by-line codeSPARTAN.

Figure: Artist concept of the EXOMARS SpacecraftEntry

Figure: Simulation and experimental validation ofthe hypersonic flowfield around EXOMARS;Gottingen HEG Plasma Windtunnel c©DLR

Figure: Spacecraft scale model tested in ashock-tube facility; TCM2 Shock-Tube c©Universitede Provence

I Objective: Simulation of the local radiative properties of the plasma (emission andabsorption coefficients) for a large spectral range (VUV to IR) and radiative transferfrom the plasma towards the wall points using a Ray-Tracing routine.

Hardware Used for Simulations

I Calculations carried in a Linux DebianIntel x86 8-core machine with 32GB ofRAM and 2+2TB storage space.

I Radiative field with a size of 25–50GB.Large I/O overheads need specialcomputational techniques.

I Creation of a 28GB ramdrive with anassociated stack systm for preallocatingradiative data.

Computational Fluid Radiative DynamicsModelling ofthe EXOMARS Entry

I Simulations on initial mission profile(larger vehicle) and new profile (smallervehicle), for 6 entry trajectory points.

I 5 chemical species in the flowfield: CO2,CO, O2, C and O.

I Population of radiative states obtainedfrom a Boltzmann distribution,considering two temperatures (T,Tvib).

I Need to define a coarser radiative griddue to storage constraints (line-by-linecalculations yield spectra with millionsof points). Figure: Sample temperature field and wake for a

5km/s Martian entry

Radiative Systems Accounted by the Simulation

species system upper state – bands species database model electroniclower state (v′max, v′′max) levels

Martian-like molecular systems atomic photoionization

CO X-X X1Σ+−X1Σ+ (20; 20) C Topbase level Qa 361

Fourth-Positive A1Π−X1Σ+ (20; 20) O Topbase level Qa 245Third-Positive b3Σ+

−a3Π (2; 20)Angstrom B1Σ+

−A1Π (2; 20) molecular photoionizationTriplet d3Π−a3Π (20; 20)Asundi a′3Π−a3Π (20; 20) O2 – total Qa –

CO – total Qa –CO2 – – 613 bands CO2 – total Qa –

Earth-like molecular systems molecular photodissociation

O2 Schumann-Runge B3Σ−u−X3Σ−g (10; 10) O2 – T-dependent Qa –

Atomic lines atomic photodetachment

species database model electronic C− – total Qa –levels O− – total Qa –

C NIST – 272O NIST – 377

Ray-Tracing and Radiative Transfer Procedures

Fundamental equations for radiative transfertowards the spacecraft wall:

dIdl

= εν − α(ν)l

Iw(ν) = 2∫ π

0

∫ π/2

0Iw(θ, φ) cos (θ) sin (θ) sin (dθ) dφ

Iw(W/m2) =

∫∞

0Iw(ν)(W/m2cm−1)dν

Figure: Sample rays over a spacecraft wall point

I Sampling of 22,500 rays for 50 wall points (450 per wall point, covering ahalf-hemisphere of ' 5◦ solid angle).

I The radiative fluxes from the different hemispherical angles are then summed,accounting for the inclination relative to the wall.

Results and Discussion

I Simulations highlighted the predominance of CO2 IR radiation, which accounts for over 95% of the overallwall fluxes.

I Previous works neglected IR contributions, therefore severely underestimating radiative fluxes.I The radiative peak occurs at lower velocities than the convective pek, in the case where CO2 is heated

without dissociating.I Radiative fluxes in the spacecraft backcover exceeded 1W/cm2, mandating the application of additional

thermal protections, and invalidating the first mission profile.I Approach can be straightforwardly extended to other studies, such as radiative transfer inside a Tokamak. Figure: Integated radiative wall fluxes Iw (left), spectral-dependent wall fluxes

Iw(ν) (right), for the 50 sampled wall points

Instituto de Plasmas e Fusao Nuclear Workshop, 12 November 2010