The effect of the spin-forbidden CO (1Σ⁺) + O (³P) → CO₂(1Σ_g⁺) recombination reaction on afterbody heating of mars entry vehicles

Vibrationally excited CO₂, formed by two-body recombination from CO(¹Σ⁺) and O(³P) in the wake behind spacecraft entering the Martian atmosphere, is believed to be responsible for the higher than anticipated radiative heating of the backshell, compared to pre-flight predictions. This process involves a spin-forbidden transition of the transient triplet CO₂ molecule to the longer-lived singlet. To accurately predict the singlet-triplet transition probability and estimate the thermal rate coefficient of the recombination reaction, ab initio methods were used to compute the first singlet and three lowest triplet CO₂ potential energy surfaces and the spin-orbit coupling matrix elements between these states. Analytical fits to these four potential energy surfaces were generated for surface hopping trajectory calculations, using Tully’s fewest switches surface hopping algorithm. Preliminary results for the trajectory calculations are presented. The calculated probability of a CO + O(³P) collision leading to singlet CO₂ formation is on the order of 10^-4. The predicted flowfield conditions for various Mars entry scenarios predict temperatures in the range of 1000K-4000K and pressures in the range of 300-2500 Pa at the shoulder and in the wake, which is consistent with a heavy-particle collision frequency of 106 to 107 s^-1. Owing to this low collision frequency, it is likely that ¹Σ_g⁺CO₂ molecules formed by this mechanism will mostly be frozen in a highly nonequilibrium ro-vibrational energy state until they relax by photoemission.