Rotational variation of the spectral slope of (21) Lutetia, the second asteroid target of ESA Rosetta mission

The ESA Rosetta mission, launched in 2004 March, will flyby the second asteroid target (21) Lutetia in 2010 July. This asteroid is quite different from (2867) Steins, encountered by Rosetta in 2008 September. Lutetia is in fact a much larger asteroid, approximately 100 km of diameter, as compared to the 5 km of Steins and also its surface composition seems fairly different. A wide international ground-based observational campaign has been carried out and is still going on to obtain information on the object. In this context, we observed Lutetia four times spectroscopically in the visible region totally covering its rotational period. In this paper we have compared all our observations, in order to try to shed more light on its nature. Moreover, an analysis of the geometric configuration of Lutetia during the several observations has also been performed. Our paper points out small variations of reflectance over the surface, possibly due to a large crater. However, the nature of Lutetia remains still elusive, probably because it could be a transition object between X and C taxonomic classes, pointing out to the crucial values of the forthcoming flyby to clarify the situation. Therefore, all the information we have gathered and here discussed have been very useful also to better define the observational strategy of the asteroid by Rosetta.