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

Because of observational biases and outgassing, comets are notoriously challenging interms of trajectory estimation and ephemeris predictions. We used tracking data collected by ESA’s Rosetta mission at 67P/Churyumov-Gerasimenko to derive 10-meterlevel 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 of67P from 2012 to 2018 by modeling nongravitational perturbations with a rotating jetmodel, informed by measured physical properties of the comet. Using this trajectoryreconstruction as reference, we characterize the astrometric error of publicly availableoptical observations of 67P and show how their quality significantly degrades withbrightness and proximity to the Sun. Moreover, we assessed the accuracy of predictionsmade with different nongravitational perturbation models and data treatments. Whileeach comet is unique and it is difficult to provide a general recipe, the reliability of anephemeris can benefit from the availability of high-quality data with quantified uncertainties and can be gauged by comparing different ephemeris solutions and tracking asolution’s stability and predictive performance as new observations are added to the fit.