DYNAMICAL EVOLUTION OF NEAS - CLOSE ENCOUNTERS, SECULAR PERTURBATIONSAND RESONANCES

We discuss the main mechanisms affecting the dynamical evolution of Ne ar-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integ ration the only perturbing planet is the Earth. So the evolution is do minated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost cons tant the perihelion distance and the Tisserand invariant. In the secon d integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and nu(6) (secular) resonances substantially ch ange the eccentricity but not the semimajor axis. The third, most real istic, integration including all the three planets together shows a co mplex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of t he Tisserand invariant. This shows that simplified 3-body or 4-body mo dels cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some ''fast -track'' dynamical routes from low-eccentricity to very eccentric, pla net-crossing orbits.