Improved gas-kinetic treatment of cometary water sublimation and recondensation: application to comet P/Halley

Abstract

The effects of nucleus ice sublimation and homogeneous recondensation inthe fluid expansion region, processes which are associated with thesolid-gas phase transition and are assumed to control cometary activity,are investigated. The present gasdynamic models reduce the gasvelocities and terminal dust velocities by 5-10 percent with respect toprevious treatments. A formula is developed for predicting the momentumloss coefficient required to determine the nongravitational forcesexerted on the nucleus. The classical homogeneous nucleus theory isapplied, showing that water vapor expands in an initially supersaturatedstate, with the nucleation rate being smaller than in the case ofadiabatic expansion.