Photometric and spectroscopic observations of asteroid (21) Lutetia three months before the Rosetta fly-by

Context. On its journey to comet 67P/Churyumov-Gerasimenko, the International Rosetta Mission (ESA) was planned to fly-by two asteroids: (2867) Steins and (21) Lutetia. Although classified as an M-type asteroid because of its high albedo, its reflectance spectrum in the near and mid-infrared region, suggests a primitive composition, more typical of C-type asteroids. Results from ground-based observations are indicative of compositional variegation and of at least one significantly large crater on the surface of this asteroid. Aims. We analyse photometric and spectroscopic data of the asteroid, obtained from ground-based observations, to support the data taken by the spacecraft. Methods. We obtained uvbyIRi′ photometric measurements covering the complete rotational period of the asteroid (about 8 h), using both the BUSCA instrument at the 2.2 m telescope in Calar Alto Observatory (CSIC-MPIA), and the 1 m telescope at Lulin Observatory (Taiwan, NCU). We also obtained visible and near-infrared spectra, covering the range 0.4-2.5 μm, with CAFOS at the 2.2 m (Calar Alto) and NICS at the 3.6 m telescope TNG ("El Roque de los Muchachos" Observatory). The spectroscopic data were taken at different rotational phases to search for any significant inhomogeneities in the surface of the asteroid. Results. The simultaneous photometric lightcurves in five filters obtained with the BUSCA instrument, and the lightcurves obtained at Lulin Observatory reveal a brightness variation around a rotational phase 0.1. We took visible and near-infrared spectra at that rotational phase, and a different rotational phase for comparison. Differences in the visible spectral slope among the spectra are indicative of a crater as the most likely cause of this variation.