2006A&A...454..677B

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. The asteroid was observed with SpeX/IRTF in the spectral region 0.9-4.0µm, in remote observing mode from Meudon, in March 2003 and August 2004. The new spectrum in the range 0.9-2.5µm confirms the previous results (Birlan et al., 2004NewA....9..343B), for a neutral trend with a large shallow band around 1µm. The spectral region around 3µm is usually considered as a tracer of aqueous alteration of the surface. The 3µm 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 3.1µm, 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%.