The gravitational orbit of a comet is affected by the sublimation of water molecules by the nucleus when the comet approaches perihelion. This outgassing triggers a non-gravitational force (NGF) that significantly modifies the orbit of the comet. Up to now, modelling of this effect is mostly based on an empirical model defined in the early 70s that uses a simplified outgassing model. Attempts have been made to use advanced anisotropic thermal models both to calculate the NGF taking several

more »

... vational constraints into account and to retrieve the nucleus's mass and density, but (i) this approach is restricted to a handful of cometary nuclei that are sufficiently well known to allow this type of modelling, and (ii) the authors usually do not fit the astrometric measurements directly but rather non-gravitational parameters calculated with the above-mentioned empirical model. We present a new model for non-gravitational forces with the aim of revisiting the problem of NGF calculation and nucleus density determination. Our model is closer to the nucleus outgassing physics with only a few free parameters. The amplitude of the perturbation depends on several parameters describing the comet activity that can be constrained by visible, infrared, and radio observations of the coma and the nucleus of the comet. It also depends on the nucleus mass, which can in turn be determined by modelling the effect of the NGF on the orbit of a comet. The method is based on the decomposition of the surface of the nucleus in several elements located at different latitudes. The contribution of each surface element to the overall NGF is fitted from the astrometric measurements, together with the density of the nucleus. This method is the only one available so far to estimate the density of cometary nuclei from ground-based observations. This method is tested on the well-known comet 19P/Borrelly. The density found for these comet is between 150 and 600 kg m −3 .