Near infra-red spectroscopy of the asteroid 21 Lutetia - I. New results of long-term campaign

Aims. Investigation of the physical nature of the asteroid 21 Lutetia, target of Rosetta mission, is required for the completion of its ground-based science and in the frame of its future fly-by. Monitoring this object is essential in preparing the future encounter with the spacecraft. Methods. The asteroid was observed with SpeX/IRTF in the spectral region 0.9-4.0 mu m, in remote observing mode from Meudon, in March 2003 and August 2004. Results. The new spectrum in the range 0.9-2.5 mu m confirms the previous results (Birlan et al. 2004), for a neutral trend with a large shallow band around 1 mu m. The spectral region around 3 mu m is usually considered as a tracer of aqueous alteration of the surface. The 3 mu m band in Lutetias' spectrum is shallower than those of hydrated asteroids, and the 2.9 vs. 3.2 ratio reveals a value close to the CV-CO meteorites. The band around 3.1 mu m, if it exists in the spectrum of 21 Lutetia, is different from the one present in the spectrum of 1 Ceres, and is lower than 0.5%.