THE DANGEROUS BORDER OF THE 5 2 MEAN MOTION RESONANCE/

After discovering that asteroid (2953) Vysheslavia, a Koronis family m ember. has a typical dynamical lifetime of the order of 10 Myr only ag ainst ''falling'' into the 5:2 Kirkwood gap and ending up in a hyperbo lic orbit (Milani and Farinella, 1995. Icarus 114, 209-212), we decide d to repeat the same study for two other asteroids: 1991 UA(2) and 199 3 FR58, lying also very close to the outer edge of the same resonance. The orbital elements of these bodies were not yet accurate enough and we appealed to observers, both professional and amateur, to obtain mo re astrometric data. Eight groups in three different countries carried out observations, and as a result the Minor Planet Center could deriv e updated, accurate orbits for both objects. Here we report on the pre liminary results of our study. Whereas the long-term behavior of 1991 UA(2) looks fairly regular and stable, 1993 FR58 exhibits a typical '' stable chaos'' behavior; nevertheless, it does not end up in the reson ance within a time span of 50 Myr. On the other hand, its fictitious c lones, placed just 3-4 x 10(-3) AU closer to the resonance, undergo la rge irregular semimajor axis changes, fall into the resonance and esca pe on a timescale <1 Myr. (C) 1998 Elsevier Science Ltd. All rights re served.