Near infra-red spectroscopy of the asteroid 21 Lutetia

I. New results of long-term campaign

¹ 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 μm, in remote observing mode from Meudon, in March 2003 and August 2004.

Results.The new spectrum in the range 0.9-2.5 μm confirms the previous results (Birlan et al. 2004), for a neutral trend with a large shallow band around 1 μm. The spectral region around 3 μm is usually considered as a tracer of aqueous alteration of the surface. The 3 μm 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 μm, 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%.