Abstract

We study the dynamical evolution of the near-Earth Jupiter family comets (NEJFCs) that came close to or crossed the Earth's orbit at the epoch of their discovery (perihelion distances q(disc) 1.3 AU). We found a minimum in the time evolution of the mean perihelion distance (q)over bar> of the NEJFCs at the discovery time of each comet (taken as t = 0) and a past-future asymmetry of (q) over bar in an interval -1000 yr, +1000 yr centred on t = 0, confirming previous results. The asymmetry indicates that there are more comets with greater q in the past than in the future. For comparison purposes, we also analysed the population of near-Earth asteroids in cometary orbits (defined as those with aphelion distances Q > 4.5 AU) and with absolute magnitudes H 18. We found some remarkable differences in the dynamical evolution of both populations that argue against a common origin. To further analyse the dynamical evolution of NEJFCs, we integrated in time a large sample of fictitious comets, cloned from the observed NEJFCs, over a 20 000 yr time interval and started the integration before the comet's discovery time, when it had a perihelion distance q > 2 AU. By assuming that NEJFCs are mostly discovered when they decrease their perihelion distances below a certain threshold q(thre) = 1.05 AU for the first time during their evolution, we were able to reproduce the main features of the observed (q) over bar evolution in the interval [-1000, 1000] yr with respect to the discovery time. Our best fits indicate that similar to 40% of the population of NEJFCs would be composed of young, fresh comets that entered the region q 2 AU a few hundred years before decreasing their perihelion distances below q(thre), while similar to 60% would be composed of older, more evolved comets, discovered after spending at least similar to 3000 yr in the q 2 AU region before their perihelion distances drop below q(thre). As a byproduct, we put some constraints on the physical lifetime tau(phys) of NEJFCs in the q 2 AU region. We found a lower limit of a few hundreds of revolutions and an upper limit of about 10 000-12 000 yr, or about 1600-2000 revolutions, somewhat longer than some previous estimates. These constraints are consistent with other estimates of tau(phys), based either on mass loss (sublimation, outbursts, splittings) or on the extinction rate of Jupiter family comets (JFCs).