The fundamental role of the Oort cloud in determining the flux of comets through the planetary system
A model of the Oort cloud has been developed by accounting for planetary, stellar and Galactic perturbations using numerical symplectic integrations covering 4.5 Gyr. The model is consistent with the broad dynamical characteristics of the observed cometary populations injected from the Oort cloud into different regions of the Solar system. We show that the majority of observed high-eccentricity trans-Neptunian objects, Centaurs and short-period comets have visited the Oort cloud (a > 1000au) during their dynamical history. Assuming from observations that the near-parabolic flux from the Oort cloud with absolute magnitudes H10 < 7, perihelion distances q < 5au and a > 104au is approximately 1 comet per year, our calculations imply a present Oort cloud population of ~5 × 1011 comets with H10 < 10.9. Roughly half this number have a > 104au. The number of comets reaching the planetary region from the Oort cloud (a > 1000au) is more than an order of magnitude higher per unit perihelion distance immediately beyond Neptune than in the observable zone q < 5au. Similarly, the new-comet flux from the Oort cloud per unit perihelion distance is a few tens of times higher in the near-Neptune region than in the observable zone. The present number of high-eccentricity trans-Neptunian objects (q > 30au and 60 < a < 1000au) originating from the Oort cloud is in the approximate range 1-3 × 1010, depending on details of the initial model. A substantial fraction of these have a > 200au and/or q > 40au, and they are found mostly to originate from initial orbits with 25 < q < 36au. Similarly, the number of Centaurs produced from the Oort cloud, where we define Centaurs to have 5 < q < 28au and a < 1000au, is smaller by a factor of 20-30. About 90 per cent of these Centaurs have a > 60au. Objects that have visited the Oort cloud represent a substantial fraction of the Jupiter-family comet population, achieving short-period orbits by a process of gradual dynamical transfer, including a Centaur stage, from the outer Solar system to near-Earth space. A similar mechanism produces Halley-type comets, in addition to the well-known diffusion process operating at small perihelion distances.