Asteroid 21 Lutetia : UBVRI Light Curves from OSIRIS-NAC

We report on new observations aimed at characterizing asteroid 21 Lutetia, one of the targets of the Rosetta space mission. The asteroid was imaged with the OSIRIS-NAC camera during the cruise phase of the Rosetta spacecraft, at a phase angle of 22.36o, and was continuously monitored over 14 hr (on 2 and 3 January 2007), thus covering one rotational period and a half. From accurate photometric calibrations with the solar analog star, 16 CygB, we have obtained high quality light curves of 21 Lutetia in the U, B, V, R and I photometric bands. We confirm a sidereal period of 8.17 hr. No spectral variations with the rotation have been found. The resulting reflectivity spectrum is in a very good agreement with that of Barucci et al. (2005). Combining with existing observations, we have obtained its phase function.