DYNAMICS OF COMETS IN THE OUTER PLANETARY REGION .2. ENHANCED PLANETARY MASSES AND ORBITAL EVOLUTIONARY PATHS

We study the effect of increasing the masses of the outer planets to s peed up the numerical integration of orbital evolutions of fictitious comets subject to planetary close encounters. We use Opik's theory to model encounters on crossing orbits, and find that the distribution of energy perturbations, and especially of its tails, is changed signifi cantly depending on the range of initial conditions considered and on the enhancement factor used for the planetary masses. We then analyze nearly tangent, low-velocity encounters making use of a simple argumen t based on the opening of Hill's surfaces of zero velocity, again find ing that the energy perturbation distribution depends on the mass enha ncement factor. To check these results we compare the outcomes of inte grations done with masses enhanced by the factors 10 and 40 to those o f integrations done with the red masses, coming once more to the same conclusion; we put these findings in perspective, discussing the mure general issue of the dynamical paths followed by potential short-perio d comets in the region of the outer planets. Results concerning the or bital evolution of comets obtained by integrations performed with enha nced masses should be considered with caution.