On the observed excess of retrograde orbits among long-period comets

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Abstract
The distribution of orbital inclinations of the observed long-period comets is analysed. An excess of retrograde orbits is found which increases with the perihelion distance, except for the range 1.1 < q < 2 AU where a pre-dominance of direct orbits appears. However, selection effects in the observations may account for this discrepancy. In addition, it is shown that the inclination distribution of the dynamically young long-period comets (with semi-major axes a > 103AU) has the same behaviour as the total sample. It is thus suggested that the excess of retrograde orbits among long-period comets is related to an already existent excess among the incoming new comets (i.e. comets driven into the planetary region by stellar perturbations). Using theoretical considerations and a numerical model it is proposed that an important fraction of the so-called new comets are actually repeating passages through the planetary region. Nearly a half of the new comets with q > 2 AU may be repeating passages. An important consequence of the presence of comets repeating passages among the new ones is the production of an excess of retrograde orbits in the whole sample.

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