Estimating the nucleus density of Comet 19P/Borrelly

The nucleus bulk density of Comet 19P/Borrelly has been estimated by modeling the sublimation-induced non-gravitational force acting upon the orbital motion, thereby reproducing the empirical perihelion advance (i.e., the shortening of the orbital period). The nucleus has been modeled as a prolate ellipsoid, covered by various surface activity maps which reproduce the observed water production rate. The theoretical water production rate of active areas has been obtained by applying a sophisticated thermophysical model. This model takes into account net sublimation of ice and thermal reradiation from the surface, solid state conductivity, sub-surface sublimation and recondensation, mass and heat transport by diffusing gas, layer absorption of solar energy, a full treatment of local time-dependent illumination conditions, and a detailed consideration of nucleus/coma interaction mechanisms. The outgassing properties of the modeled nucleus are physically consistent with the gas kinetic structure of the innermost coma since the molecular backflux and surface gas density required in the thermophysical model (as functions of the nucleus surface temperature and the sub-surface temperature profile) have been obtained from Direct Simulation Monte Carlo modeling of inelastic intermolecular collisions in the cometary Knudsen layer. The calculation of local normal forces acting on the nucleus due to outgassing has been made within the same framework—recoil and/or impact momentum transfer to the nucleus caused by sublimating molecules and by recondensing and/or scattered coma molecules is therefore evaluated in accordance with local nucleus/coma conditions. According to this model, the density is found to be 100-300 kg, depending on the applied spin axis orientation and surface activity map. This range can be narrowed down to 180-300 kg by also requiring that the empirical changes (per orbital revolution) of the argument of perihelion and the longitude of the ascending node are reproduced.