A&A 454, 677-681 (2006)
DOI: 10.1051/0004-6361:20054460

Near infra-red spectroscopy of the asteroid 21 Lutetia

I. New results of long-term campaign

M. Birlan¹, P. Vernazza², M. Fulchignoni², M. A. Barucci², P. Descamps¹, R. P. Binzel³ and S. J. Bus<sup>4

1</sup>  Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE), Observatoire de Paris, 77 avenue Denfert-Rochereau, 75014 Paris Cedex, France
    e-mail: Mirel.Birlan@imcce.fr
²  LESIA, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
    e-mail: [Pierre.Vernazza; Marcello.Fulchignoni; Antonella.Barucci]@obspm.fr
³  Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139, USA
    e-mail: rpb@mit.edu
⁴  Institute for Astronomy, 640 North A'ohoku Place, Hilo, HI 96720, USA
    e-mail: sjb@ifa.hawaii.edu

(Received 2 November 2005 / Accepted 7 March 2006)

Abstract
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 m, in remote observing mode from Meudon, in March 2003 and August 2004.
Results.The new spectrum in the range 0.9-2.5 m confirms the previous results (Birlan et al. 2004), for a neutral trend with a large shallow band around 1 m. The spectral region around 3 m is usually considered as a tracer of aqueous alteration of the surface. The 3 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 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%.

Key words: minor planets, asteroids -- techniques: spectroscopic -- methods: observational

© ESO 2006

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