Tuorla Observatory, University of Turku, Finland
(*) Astronomical Observatory, Uppsala University, Sweden
New comets come from the Oort Cloud (Oort 1950) to the inner Solar System at a steady rate. Usually they arrive in nearly parabolic orbits, and suffer changes in their orbit, then return to a large distance. Sometimes the new orbit is much more strongly bound to the Sun than the old one and the comet may even become a short period comet. Everhart (1972) estimates that all known short period comets could originate from the Oort Cloud via several strong perturbations of this kind while Joss (1973), Duncan et al. (1988) and Wetherill (1991) see problems with the idea. In particular, the inclinations of the Jupiter family comets are only around 10 degrees, in contrast to models where the captured Oort Cloud comets have considerably higher inclinations. Even though the capture process is inclination dependent (Valtonen & Innanen 1982; Valtonen et al. 1992), the dependence does not appear strong enough to explain the properties of the Jupiter family (Fernandez and Gallardo 1993). Therefore it has been suggested that there may be another source region for the short period comets, the so-called Kuiper Belt (Kuiper 1951), which consists entirely of low inclination comets just outside our planetary system. As yet all observed Kuiper Belt objects are far larger than typical cometary nuclei, but the population is expected to include small members as well. The problem is rather if the present flux of objects entering into Neptune-crossing orbits suffices to guarantee a significant influx into the Jupiter family via multi-planet captures (Morbidelli et al. 1996).
Here we concentrate on the traditional model of capture from the Oort Cloud. We improve the method of calculation of captured comet populations by combining large numbers of accurate orbit calculations with Monte Carlo methods and compare the resulting statistics with the known populations of comets (Fernandez and Ip 1991). As a quite satisfactory model is found using the capture from the Oort Cloud alone, it does not appear necessary to assume a large contribution of captures from the Kuiper belt. In any case, the numerical methods developed in this work are not suitable for the study of the Kuiper Belt problem.
We may say that the low-inclination distribution of short period comets (especially the Jupiter family) is mainly produced by strong encounters with all major planets.