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B.J.R. Davidsson (ESA/ESTEC, Noordwijk, The Netherlands), P.J. Gutiérrez (Laboratoire d'Astrophysique de Marseille, France)
The nucleus density of Comet 19P/Borrelly has been estimated by modeling the sublimation-induced non-gravitational forces acting upon the orbital motion, thereby reproducing the empirical perihelion advance. 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 properties of the modeled nucleus develop in physical consistency with the innermost coma since the molecular backflux and surface gas density needed as input in the thermophysical model (as functions of the local 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\,\mathrm{kg\,m-3}, depending on the applied spin axis orientation and surface activity map. This range can be narrowed down to 180-300\,\mathrm{kg\,m-3} by also requiring that the empirical secular changes in the argument of perihelion and the longitude of the ascending node are reproduced.
The author(s) of this abstract have provided an email address for comments about the abstract: bdavidss@rssd.esa.int
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.