Environments of the Rosetta target asteroids

The ROSETTA spacecraft will fly-by a few asteroids during its course to the final cometary target. The candidate asteroids presently are 140 Siwa (C-type) and 4979 Otawara. An alternative interesting trajectory reaching four asteroids (5331 Erimomisaki, 905 Universitas, 1515 Perrotin and 4247 Grahamsmith) has also been found by ESA. With the limited data presently available on these bodies we calculated some approximate quantities which may be useful to select the fly-by trajectories of the ROSETTA probe. Some first information are provided by the classical quantities such as the radius of the Hill sphere. Then, exploiting the results of Hamilton and Burns (1991), we determined the orbital stability zone, i.e. the union of all the numerically integrated orbits showing long-term stability, for each of the above asteroids. As was suggested by Hamilton and Burns (1991) the particular shape of these zones would suggest to have the spacecraft's close approach out of the orbital plane of the asteroids. The results of a large set of numerical integrations are then presented, strengthening and broadening the conclusions of the analytical calculations. The stability zone appears to be severely limited by the effect of the solar radiation pressure which sweeps away small particles from the asteroid vicinity. In the case of 140 Siwa the borders of the zone (for mm-sized particles) stretch to about 420 RA in the X - Y plane and 162 RA in the Z direction, mainly due to the apocenter distance reached by particles placed on initial highly eccentric orbits.