The capture of Halley-type and Jupiter-family comets from the near-parabolic flux

Emel'yanenko V.V., Bailey M.E.

Armagh Observatory, Armagh, United Kingdom

We have considered the transformation of comets from the near-parabolic flux to short-period orbits under the perturbations of Jupiter, Saturn, Uranus, and Neptune for 5 Gyr. We have developed a combined analytical and numerical scheme which includes all essential features of the dynamical evolution (mean-motion resonances, secular oscillations, secular resonances, and close encounters with planets).

In particular, we have considered the evolution of 5 sets of 20000 randomly oriented near-parabolic orbits with initial inclinations i uniformly distributed in cos i and perihelia q uniformly distributed in each of five ranges: 0 The inclination-averaged probability for a nearly parabolic orbit to evolve into a Halley-type orbit is 0.01 for orbits with initial perihelion distances 00.06 respectively. The transfer probability for nearly parabolic orbits with initial perihelia in the range 10.5 The probability of capture to the Jupiter family increases for smaller initial values of semi-major axis. During a very long interval of time, the action of secular resonances can transform perihelia to the near-planetary regions in which objects are captured into short-period orbits. We stress that the probability of capture into the Jupiter family from low-inclination orbits in the inner part of the Oort cloud with q>10 AU is substantially larger than that from corresponding near-parabolic orbits with 4 Our results also show that the number of Halley-type objects arising from the observed near-parabolic cometary flux of all inclinations and absolute magnitudes brighter than H10=7, is at least 400 times larger than the number of known Halley-type comets. The physical evolution of comets is therefore crucial to understanding the terrestrial-planet impact rate due to bodies in Halley-type orbits. High-inclination, intermediate-period comets and hitherto undiscovered asteroid-like remnants circulating in Halley-type orbits may dominate the impact rate of kilometre-sized objects, unless they disintegrate into streams of small bodies - boulders and dust - during their dynamical evolution. In this case, the flux of small bodies in high-inclination inner solar system orbits may itself pose a significant environmental hazard to civilization.