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 0
0.06 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.