ENCOUNTER FREQUENCY OF HALLEY-LIKE COMETS WITH THE PLANETS

We present the results of a numerical study on encounter frequencies o f fictitious Halley-like comets with the planets in a dynamical model of the solar system, in which we take into account the gravitational f orces of the Sun and the planets Venus through Neptune. The change of the orbital elements during a close approach with a planet was careful ly monitored with the aid of a thoroughly tested numerical integration method with automatic step size control. We computed the encounter fr equencies of the comets' orbits using two different ''spheres of influ ence'' and compared the results. In both cases, it turned out that til e encounter frequency of the fictitious Halley-like comets with Jupite r and Saturn is about a factor 10 to 100 higher than for the other pla nets. Concerning the changes of the semi-major axes and inclinations o ur results show that an increase and decrease of these elements is equ ally probable after an encounter.