A model of cometary gas and dust production and nongravitational forces with application to P/Halley

A program that computes gas and dust production rates and idealized nongravitational force components has been developed and applied to the case of Comet Halley. We use a modified form of our earlier comet model (F.P. Fanale and J.R. Salvali[(1984) Icarus 60, 476–511] to which coma effects and a section on nongravitational forces have been added. The possibility of grain cohesion is also included. These models are used together with observations from 1910 and semiempirically derived data to investigate the effects of obliquity and thermal conductivity of the near thermal conductivity of the nucleus on gas and dust production. The results indicate that the thermal conductivity of the nucleus is of the order of 10⁵ ergs/cm-s-°K, which implies that the ice near the surface is in the crystalline form. A general method is presented for calculating the radii of cometary nuclei using theoretically derived and semiempirically derived nongravitational force components. This method is used to calculate possible radii for Comet Halley that depend on the model variation chosen. The method used and the results presented herein should have greater significance and value when the observational data from Halley's current perihelion passage become available.