High-fidelity comet 67P ephemeris and predictions based on Rosetta data

Because of observational biases and outgassing, comets are notoriously challenging in terms of trajectory estimation and ephemeris predictions. We used tracking data collected by ESA's Rosetta mission at 67P/Churyumov-Gerasimenko to derive 10-m level constraints on the distance between the Earth and 67P in 2014 and 2016, which, together with high-quality ground-based astrometry, provide unprecedented information on a comet's trajectory. Based on these data, we reconstruct the trajectory of 67P from 2012 to 2018 by modeling nongravitational perturbations with a rotating jet model, informed by measured physical properties of the comet. Using this trajectory reconstruction as reference, we characterize the astrometric error of publicly available optical observations of 67P and show how their quality significantly degrades with brightness and proximity to the Sun. Moreover, we assessed the accuracy of predictions made with different nongravitational perturbation models and data treatments. While each comet is unique and it is difficult to provide a general recipe, the reliability of an ephemeris can benefit from the availability of high-quality data with quantified uncertainties and can be gauged by comparing different ephemeris solutions and tracking a solution's stability and predictive performance as new observations are added to the fit.