The effect of non-gravitational forces on the median inclination of short-period comets

The numbered Jupiter family comets (orbital periods P < 20 yr) have a median orbital inclination of about 10 degrees.0 +/- 1 degrees.1. In this paper, we integrate the orbits of these comets into the future, under the influence of both typical non-gravitational forces and planetary perturbation, using a Bulirsch-Stoer integrator. In the case where non-gravitational forces were not acting, the median inclination of those comets that remained on P < 20 yr orbits increased at the rate of (1.92 +/- 0.12) x 10(-3) deg yr(-1) for the first 3600 yr of the integration. During this time the population of the original family decreases, such that the half-life is about 13 200 +/- 800 yr. The introduction of non-gravitational forces slows down the rate of increase in inclination to a value of around (1.23 +/- 0.16) x 10(-3) deg yr(-1). This rate of increase in inclination was found to be only weakly dependent on the non-gravitational parameters used during the integration. After a few thousand years, the rate of change in inclination decreases, and after 20 000 yr the inclinations of those initial Jupiter family members that still have orbits with P < 20 yr become constant at about 20 degrees.0, independent of whether non-gravitational forces are acting or not. The presently known Jupiter family of comets is losing members at the rate of one in every 67 yr. To maintain the family in equilibrium, Jupiter has to capture comets at a similar rate, and these captured comets have to be of low inclination to compensate for the pumping up of inclinations by gravitational perturbation.