Dynamical behaviour of Near-Earth asteroids in the terrestrial planet region : the role of secular resonances

In this paper, we present the last results on the dynamics of Near-Earth asteroids with orbital semimajor axes smaller than 2 AU and we concentrate particularly on the dynamics of the secular resonances with planets. Many numerical integrations of NEA orbits show that the secular resonances are present in this region and that their effects can play a major role in the orbital evolutions and dynamical lifetimes of small bodies. They can either provide a protection mechanism from close approaches, or a transport mechanism from a region of the phase space to another. Moreover the recent location of all the linear secular resonances, by means of a semi-analytical method, confirms that all of them are present in this region and some can even overlap. This shows that the region of the inner planets is dynamically very complex and therefore, that a dynamical model considering only the effects of close encounters with planets to study the evolution of NEAs may be oversimplified and should take into account the secular perturbations and resonances which appear to be numerous and relevant in the region where NEAs evolve.