Dynamics and orbital evolution of Oort Cloud comets

(J.Q. Zheng, M.J. Valtonen, S. Mikkola and H. Rickman
Tuorla Observatory, University of Turku, Finland.
Astronomical Observatory, Uppsala University, Sweden.)

   Investigators generally conjecture a steady flux of new comets from the Oort
cloud through the inner Solar System. Due to gravitational perturbations by major
planets these objects may escape, become long period comets (LPCs) or short
period comets (SPCs).
   The most striking characteristic of the observed SPCs is the great
preponderance of low inclination prograde orbits, in comparison with the roughly
random distribution of orbital inclinations of the long period comets. To explain
the origin of SPCs some investigators suggest a disk-like source (Kuiper belt or
inner Oort cloud) while others insist capture mechanism.
   The most direct way to attack the problem of orbital evolution of Oort cloud
comets is to integrate the orbits of large numbers of comets up to the age of the
Solar System. But the required computational effort is overwhelming unless the
problem is somehow simplified.
   In view of the difficulty with direct integrations, we develop a Monte Carlo
method based on cross-sections obtained with accurate orbit integrations using
a KS-regularization code. Our starting point is isotropic directional distribution
of Oort cloud comets and an assumed annual flux of new comets. Using the cross
-sections for changes of orbital elements we repeat the encounter process
with four major planets and obtain new orbital elements for the comets until they 
become short period ones. We calculate the steady state population these objects
in captured orbits and record the orbital elements when they are such that the
comets might be observed. It seems that many enough captures occur in order to 
explain the present populations of the various families of short period comets.
   In conclusion, we find it entirely possible that the SPCs are a captured sub-
population of the OOrt cloud comets, and it is not necessary to  assume existence
of additional comet sources to keep up the populations of Jupiter family and the
Halley type comets.