Compositional type characterization of Rosetta asteroid candidates

The final selection of the two Rosetta target asteroids will be made in a successive phase of the Rosetta project development, when the engineering parameters will be frozen. In this paper we present spectroscopic observations of the possible Rosetta candidates and we discuss the results obtained, particularly the definition of their compositional type. We examine the possibility to select some more ``primitive'' candidates. On the basis of its size and spectral type, we suggest including the asteroid 140 Siwa as one of the asteroid targets of the Rosetta mission.