Trajectory uncertainties of incoming Comet Interceptor targets including non-gravitational forces

Comet Interceptor (CI), the next ESA comet mission, is targetting an as-yet unknown long-period or dynamically new comet for a fast fly-by, some time after mission launch in 2028 [1]. In order to plan the interception accurately, mission requirements currently call for a maximum uncertainty in the incoming comet’s position and velocity two months before closest approach of 1000 km and 1 m/s, respectively. Newly discovered comet trajectories will be subject, however, to significant uncertainties due to limited observational data and the effects of cometary outgassing-induced non-gravitational accelerations (NGAs). Hence, we have conducted numerical simulations to asses the capability of standard orbit-fitting techniques [2] to reach the required accuracy. We generate synthetic observations, with Vera Rubin LSST-level astrometric accuracy, of various reference trajectories, before fitting orbits to them and propagating the determined orbit and their uncertainty (covariance matrix) forwards to the time of intercept. Fitted-orbit heliocentric positions are then compared to those of the reference trajectories. Both gravitational and non-gravitational reference trajectories are tested, with NGAs provided either by the standard Marsden model [3], or by Monte Carlo sampling of a suite of plausible thermophysical model accelerations [4]. Orbit determination uses either a purely gravitational or  Marsden-style model, with both symmetric [3] and asymmetric [5] versions tested. Here we will present the results of the orbit fitting, which demonstrate the difficulty in constraining incoming comet trajectories, as well as presenting an analysis of the physical interpretability of the fitted Marsden NGA terms as compared to the thermophysical model accelerations.

Fig 1. Difference at perihelion between reference trajectories perturbed by a suite of Monte Carlo sampled thermophysical models, and orbits fitted to them with various models (Case A: pure-gravity, Case B: symmetric Marsden, Case C: asymmetric Marsden). Each point represents the median over the suite of divergences from the reference orbits and its standard deviation, incorporating astrometric data up-until that time. Mission goal is 1000 km uncertainty at 60 days.

[1] Jones G. H., et al., 2024, Space Science Reviews, 220, 9

[2] Dirkx D., et al., 2025, in EPSC-DPS Joint Meeting 2025. pp EPSC–DPS2025–673, doi:10.5194/epsc-dps2025-673

[3] Marsden B. G., Sekanina Z., Yeomans D. K., 1973, AJ, 78, 211

[4] Attree N., et al., 2019, A&A, 630, A18

[5] Yeomans D. K., Chodas P. W., 1989, AJ, 98, 1083