Use of long-term models for analysis of comet Encke's motion

This paper is focused on the problem of the reactive non-gravitational effects change in the cometary motion caused by the comet's activity fading. The modern researches have been reviewed. The necessity of a new model compatible with Marsden's model of non-gravitational forces is shown. Modifications of Marsden's model for the cases in which the main factors of the parameters change are the deposition of the significant non-volatile mass and the growth of the superficial crust are developed. They contain besides the usual Marsden's parameters two and three additional values correspondingly. The developed equations have been used to explain the known change of Marsden's non-gravitational parameters for the comet Encke during 225 years of its observational interval. Although the model with significant non-volatile mass contains less free values, it explains the characteristic features of the comet's non-gravitational parameters behavior better. The model can be further applied for the comet's observed astrometric positions fitting.