Photometric and spectroscopic observations of asteroid (21) Lutetia three months before the Rosetta fly-by
Lara, L. M.; Lin, Z. -Y.; Duffard, R.; de León, J.
Spain
Abstract
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.