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Session 25 - Solar System.
Oral session, Monday, January 13
The observed distribution of long-period (> 200 yr) comet orbits has proved difficult to reconcile with theory. In particular, the fraction of observed comets which appear to have made more than one perihelion passage since leaving the Oort cloud is much smaller than that predicted by simple dynamical models of the Solar System (the "fading problem").
In this paper, the evolution of long-period comets is examined through direct numerical integration of over 20,000 individual comets from the Oort cloud to their final demise. The comets evolve within a model Solar System consisting of the Sun, the four giant planets and the Galactic tide. To this model, non-gravitational forces and a (hypothetical) solar companion object or circumsolar disk are added both separately and in combination, and the effects of the heliopause, the solar wind, radiation pressure and drag are also considered. None of these influences seem capable of producing a distribution of long-period comet orbits matching observations: the comets' observable lifetimes are too long.
The possibility of fading or break-up of the nucleus is then examined, and a number of simple fading laws are explored. Two scenarios are found to provide reasonable matches with observations. The first is one in which the fraction of observable comets remaining goes like m^-0.6 \pm 0.1, where m is the apparition number, and which may have implications regarding the comets' mass distribution. The second is one in which approximately 95% of comets live for only a short time (\sim 5 orbits) and the remainder indefinitely. This scenario might be explained by a division of the Oort cloud comets into fragile ("snowy") and robust ("icy") objects.
Program listing for Monday