Asteroid 5335 Damocles and its implications for cometary dynamics

Results are presented for the long-term evolution of an ensemble of orbits similar to that of the high-inclination, intermediate-period asteroid 5335 Damocles (formerly 1991 DA). This body has a chaotic orbit that crosses the orbits of Mars, Jupiter, Saturn, and Uranus, but is relatively stable for several x 10 exp 4 yr either side of the present because the orientation does not allow close encounters with the two largest planets. The short-term evolution is dominated by mean-motion resonances with Jupiter, leading to extended periods of almost constant semimajor axis, while secular perturbations and random close encounters with the major planets on time-scales greater than a few x 10 exp 4 yr cause occasional transitions to orbits of small perihelion distance. There is a substantial probability that the future orbit will cross that of the Earth, and Damocles may spend about 25 per cent of its lifetime on such an orbit. Although Damocles has been classified as an asteroid, it seems likely to be an inactive cometary nucleus with dynamical characteristics typical of Halley-type comets. It is suggested that Damocles may be the first of a significant population of such bodies to be discovered which make a major contribution to the flux of Earth impactors.