Visible spectral properties of asteroid 21 Lutetia, target of Rosetta Mission

Rosetta spacecraft was successfully launched on March 2nd 2004 after its launch postponement in January 2003. Owing to this delay all the original targets, comet and asteroids, have been changed. In particular, on its 10 years journey to the new comet 67P/Churyumov-Gerasimenko, the spacecraft will pass close to the asteroid 21 Lutetia. In this paper we present three visible spectra of 21 Lutetia covering its rotational period. We have not found big surface composition variations and the overall behavior resembles that of a C-type asteroid. However, we discovered two main features which are present on the three spectra at around 0.43 and 0.51 \u3bcm probably due to aqueous alteration and porphyrins respectively. Based on observations performed at ESO, program n. 71.C-0157, P.I. M. Lazzarin