ORBITS OF SHORT-PERIOD COMETS CAPTURED FROM THE OORT CLOUD

Oort cloud comets occasionally obtain orbits which take them through t he planetary region. The perturbations by the planets are Likely to ch ange the orbit of the comet. We model this process by using a Monte Ca rlo method and cross sections for orbital changes, i.e. changes in ene rgy, inclination and perihelion distance, in a single planet-comet enc ounter. The influence of all major planets is considered. We study the distributions of orbital parameters of observable comets, i.e. those which have perihelion distance smaller than a given value. We find tha t enough comets are captured from the Oort cloud in order to explain t he present populations of short period comets. The median value of cos i for the Jupiter family is 0.985 while it is 0.27 for the Halley typ es. The results may explain the orbital features of short period comet s, assuming that the active lifetime of a comet is not much greater th an 400 orbital revolutions.