Risk of dust particle impacts during the Rosetta asteroid fly-bys

On its course to the comet 67P/Churyumov­Gerasimenko, Rosetta will fly past two main belt asteroids: 2867 Steins and 21 Lutetia. In order to explore the possible risks of impact between the Rosetta probe and particles orbiting 21 Lutetia, the asteroid's dynamical environment is studied. Stable retrograde, nearly equatorial orbits, for small particles (1 mm) can reach about 180 asteroidal radii on the X À Y directions. Stable inclined orbits, for small particles (1 mm) reach 50 asteroidal radii on the Z direction. The importance of the solar radiation pressure and of the solar gravitational perturbations in affecting the behavior of distant orbits is shown. In particular, the stability zone for 1 cm particles are much larger than for 1 mm-sized particles, due to the reduced relative importance of the solar radiation pressure, while particles smaller than 1 mm should be almost completely swept away by this perturbation.