A New Model Of Non-gravitational Forces On Comets

International audienceThe 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) which significantly modifies the orbit of the comet. The amplitude of the perturbation depends on several parameters which can be constrained by visible, infrared and radio observations of the coma and nucleus of the comet. It depends also on the nucleus density, which can in turn be determined by modeling the effect of the NGF on the orbit of a comet. This method is the only method available so far to estimate the density of cometary nuclei from the ground. Up to now, the modeling of this effect is mostly based on an empirical model defined in the early 70's which uses a simplified isotropic outgassing model. Attempts have been made to use advanced anisotropic thermal models to calculate the NGF taking into account several observational constraint and to retrieve the nucleus density, but: (i) this approach is restricted to a handful of cometary nuclei which are sufficiently well-known to allow this type of modeling, and (ii) the authors usually don't fit directly the astrometric measurements 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. The method is based on the separation of the surface of the nucleus in several strips located at different latitudes. The contribution of each strip to the overall NGF is fitted from the astrometric measurements together with the density of the nucleus. This new method will be used to interpret future astrometric measurements of these pristine objects with GAIA