DYNAMICAL EVOLUTION OF 2 NEAR-EARTH ASTEROIDS TO BE EXPLORED BY SPACECRAFT - (433)-EROS AND (4660)-NEREUS

We have studied numerically the dynamical evolution of the two near-Ea rth asteroids (433) Eros and (4660) Nereus, to be explored by planned space missions in the next decade, over a time scale of 2 Myr. For eac h real asteroid, we integrated a set of several initially ''indistingu ishable'' orbits (clones), obtained either by small variations of the nominal initial conditions, or by using different computers (introduci ng different round-off errors). As for Nereus, its orbital evolution i s typically dominated by Earth encounters, causing a random walk in se mimajor axis. Protection mechanisms related to secular and mean motion resonances are often temporarily at work, but are not likely to incre ase much Nereus' lifetime. According to our results, it is unlikely th at this (C-type) NEA comes from a comet-like initial orbit, or at leas t that such a transition has occurred in the last approximate to 10 My r. As for Eros, 5/8 clones always stay in the Mars-crossing, Amor-like region, but 3 of them become Earth-crossers within the 2 Myr of our i ntegration time span, and indeed one of them was observed to hit the E arth after 1.14 Myr. Therefore an Earth impact by this 20-km sized NEA is unlikely but not impossible in the next 10(6) - 10(8) yr. Eros pro bably reached its present orbit by a ''slow-track'' evolution dominate d by Mars encounters, and is a comparatively old fragment originated b y a large-scale collision occurred in the main belt.