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A numerical simulation of the Oort cloud is used to explain the observedorbital distributions and numbers of Jupiter-family and Halley-typeshort-period comets. Comets are given initial orbits with perihelion distancesbetween 5 and 36 AU, and evolve under planetary, stellar and Galacticperturbations for 4.5 Gyr. This process leads to the formation of an Oort cloud(which we define as the region of semimajor axes a>1000 AU), and to a flux ofcometary bodies from the Oort cloud returning to the planetary region at thepresent epoch. The results are consistent with the dynamical characteristics ofshort-period comets and other observed cometary populations: the near-parabolicflux, Centaurs, and high-eccentricity trans-Neptunian objects. To achieve thisconsistency with observations, the model requires that the number of cometsversus initial perihelion distance is concentrated towards the outer planetaryregion. Moreover, the mean physical lifetime of observable comets in the innerplanetary region (q<2.5 AU) at the present epoch should be an increasingfunction of the comets' initial perihelion distances. Virtually all observedHalley-type comets and nearly half of observed Jupiter-family comets come fromthe Oort cloud, and initially (4.5 Gyr ago) from orbits concentrated near theouter planetary region. Comets that have been in the Oort cloud also return tothe Centaur (5<q<28 AU, a<1000 AU) and near-Neptune high-eccentricity regions.Such objects with perihelia near Neptune are hard to discover, but Centaurswith characteristics predicted by the model (e.g., large semimajor axes, above60 au, or high inclinations, above 40 degrees) are increasingly being found byobservers. The model predicts that the mean physical lifetime of all comets inthe region q<1.5 AU is less than approximately 200 revolutions.

A model for the common origin of Jupiter family and Halley type comets