The Keplerian map for the restricted three-body problem as a model of   comet evolution
  We examine the evolution of highly eccentric, planet-crossing orbits in the restricted three-body problem (Sun, planet, comet). We construct a simple Keplerian map in which the comet energy changes instantaneously at perihelion, by an amount depending only on the azimuthal angle between the planet and the comet at the time of perihelion passage. This approximate but very fast mapping allow us to explore the evolution of large ensembles of long-period comets. We compare our results on comet evolution with those given by the diffusion approximation and by direct integration of comet orbits. We find that at long times the number of surviving comets is determined by resonance sticking rather than a random walk. 
Leonid Malyshkin and Scott Tremaine