SECULAR RESONANCES AND THE DYNAMICS OF MARS-CROSSING AND NEAR-EARTH ASTEROIDS

We study by numerical integration the secular dynamical behavior of a sample of 32 Mars-crossing and near-Earth asteroids. We divide the planet-crossing asteroid population into two groups, with semimajor axis a >2 AU and <2 AU. In the former region (a>2 AU), a computer-assisted analytical theory allows us to select 24 objects which are inside or close to the main secular resonances. The numerical computation of their evolution shows that the global dynamical picture is characterized by the existence of ''fast tracks'' (strong and rapid changes in the eccentricity due to resonances) and ''slow tracks'' (random walk in semimajor axis due to close approaches to the inner planets). The nu(6) secular resonance appears to be a common and very effective fast-track mechanism. A solar collision is the most probable end-state of bodies on the fast-track orbits, but hyperbolic ejections following Jupiter encounters are also observed. In the inner region (a<2 AU), no analytical theory can be applied and we integrate eight orbits of NEAs which had shown secular trends in eccentricity and/or inclination in previous (shorter) integrations. Here also evolutions mainly affected by resonances and by close encounters can be distinguished, although the interplay of the two kinds of effects appears to be complex. The existence of secular resonances in this region is pointed out for the first time, but further investigations are needed to understand their locations and strengths. (C) 1995 Academic Press, Inc.