CAPTURE OF HALLEY-TYPE COMETS FROM THE NEAR-PARABOLIC FLUX

The dynamical transfer of comets from nearly parabolic to short-period orbits is investigated, considering perturbations by the major planet s Jupiter, Saturn, Uranus and Neptune, for 5 Gyr. The combined analyti cal and numerical scheme includes all the essential features of the dy namical evolution, namely mean-motion resonances, secular oscillations , secular resonances, and close encounters with planets. The orbital e volution of similar to 10(5) randomly oriented near-parabolic orbits i s considered, with initial inclinations i and perihelion distances CI uniformly distributed respectively in cos i and each of the five range s 0 < q < 4 au, 4 < q < 6 au, 6 < q < 10.5 au, 10.5 < q < 18 au and 18 < q < 31 au. The objects which eventually evolve into Halley-type orb its primarily originate from initial orbits of small perihelion distan ce, in contrast to those that evolve to Jupiter-family orbits. Most Ha lley-type comets originate from orbits with q in the range 0 < q < 4 a u, with the majority coming from q < 2 au. The inclination-averaged pr obability for evolution from a nearly parabolic orbit with 0 < q < 4 a u into a Halley-type orbit, assuming an isotropic distribution of init ial inclinations, is about 0.01. When we include nongravitational forc es (for example, taking typical values for Halley-type, short-period, and nearly parabolic comets), this figure increases to 0.02, 0.04 and > 0.06 respectively. The probability for nearly parabolic orbits with initial perihelia in the range 10.5 < q < 18 au to evolve into Halley- type orbits is about 0.0002, again assuming an isotropic distribution of inclinations. However, the new-comet flux in the outer planetary re gion is expected to be mush higher than that in the inner Solar system , so the outer Solar system flux may be a significant additional sourc e of Halley-type comets. Our results show that the number of Halley-ty pe objects arising from the observed nearly parabolic cometary flux wi th absolute magnitudes brighter than H-10 = 7 and q < 4 au is hundreds of times greater than the number of known Halley-type comets. The res olution of this discrepancy must lie in more observations and a deeper understanding of the physical evolution of comets, which together bec ome the key issues for understanding the number of Halley-type objects and the terrestrial-planet impact rate due to both active and inactiv e objects in Halley-type orbits.