Surface properties of Rosetta's targets (21) Lutetia and (2867) Steins from ESO observations ^*

¹ Observatório Nacional (COAA), rua Gal. José Cristino 77, São Cristóvão, CEP20921-400 Rio de Janeiro RJ, Brazil e-mail: carvano@on.br
² LESIA – Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon Principal Cedex, France
³ Laboratoire Cassiopée, Observatoire de la Cote d'Azur, BP 4229, 06304 Nice Cedex 04, France
⁴ Université de Paris 7 Denis Diderot, France
⁵ Astrophysics Research Centre, Physics Building, Queen's University Belfast, Belfast BT7 1NN, UK
⁶ NASA Jet Propulsion Laboratory [MS 183-301], 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 11 June 2007
Accepted: 8 October 2007

Abstract

Aims. The aim of this work is to constrain the size, composition and surface properties of asteroids (2867) Steins and (21) Lutetia, targets of the Rosetta mission. Rosetta is en route to rendezvous with comet 67P/Churyumov-Gerasimenko.

Methods.Thermal-Infrared N-band observations for Lutetia and Steins were obtained using, respectively, TIMMI2 on the ESO 3.6-m telescope at La Silla and VISIR at the UT3 VLT telescope on Cerro Paranal; visible light curves for Steins were obtained using NTT+SUSI2, while R-band photometry for Lutetia was obtained with the 2.0-m Faulkes Telescope North on Haleakala. For Steins, the NEATM model was used to constrain its visible geometric albedo and beaming parameter. A detailed thermophysical model was implemented and used to analyze our set of observations of Lutetia as well as previous reported measurements.

Results. The visible photometry of Steins was used along with data from the literature to yield a slope parameter of . Problems during the observations led to the loss of measurements on two of the three N-band filters requested for Steins. Using the remaining data and the polarimetric albedo recently published, we were able to constrain the thermal beaming parameter as , which is more similar to near-Earth asteroids and suggests either high thermal inertia or a very rough surface. For Lutetia, the best fit visible geometric albedo obtained with our model and the reported observation is p_v=0.129, significantly lower than that obtained if one applies the same model to previously reported measurements. The discrepancy cannot be explained solely by assuming inhomogeneities in the surface properties and we suggest that the most plausible explanation is the presence of one or more large craters on the northern hemisphere. For both sets of measurements, the implied single scattering albedo of Lutetia is compatible with laboratory measurements of carbonaceous chondrite meteorites.