Uncertainty estimation of incoming comet trajectories with a thermophysical non-gravitational force model for Comet Interceptor encounter planning
The European Space Agency Comet Interceptor mission is designed to fly-by an as-yet undetermined incoming long-period or dynamically-new comet, which will require an accurate prediction of its trajectory. Cometary orbits are perturbed from purely gravitational trajectories by non-gravitational accelerations (NGAs) stemming from their outgassing, however, and these must be taken into account in the navigation. We here test the ability of ground-based astrometry and standard NGA-fitting techniques to predict the position and velocity of incoming comets perturbed by a Monte Carlo sampled thermophysical acceleration model. We find that, due to the difficulties of predicting NGA evolution as well as the limitations of reasonably simulated astrometry, predicted accuracies are unlikely to meet the mission goals of 1000 km and 1 m s$^{-1}$ in position and velocity, sixty days before intercept. We instead recommend expanded uncertainties of $10,000$ km and several m s$^{-1}$. Our NGA fits struggle to reproduce the reference accelerations and trajectories when outgassing is rising steeply at a few AU-incoming, but with more observations up until perihelion, do eventually converge, leading to good estimates of many of the underlying acceleration parameters even for half an orbital arc.