Asteroid 2867 Steins

II. Multi-telescope visible observations, shape reconstruction, and rotational state

¹ Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS/Université de Provence, Technopôle de Marseille-Etoile, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France e-mail: philippe.lamy@oamp.fr
² Department of Mathematics and Statistics, University of Helsinki, PO Box 68, 00014, Finland
³ School of Mathematics and Physics, Queen's University, Belfast, UK
⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MS 183-301, Pasadena, CA 91109, USA
⁵ LESIA, Observatoire de Paris, 92195 Meudon Principal Cedex, France
⁶ Observatorio National de Rio de Janeiro, Brazil
⁷ Institut de Mécanique Céleste, 75014 Paris, France
⁸ Université de Paris 7 Denis Diderot, France
⁹ Astronomical Observatory, Adam Mickiewicz University, 60-286 Poznan, Poland
¹⁰ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 USA
¹¹ Konkoly Observatory, PO Box 67, 525, Hungary
¹² Palmer Divide Observatory, Colorado Springs, CO 80908, USA

Received: 5 November 2007
Accepted: 25 May 2008

Abstract

Context. Asteroid 2867 Steins is the first target of the Rosetta space mission with a flyby scheduled in September 2008.

Aims. An early physical characterization is needed to optimize the flyby parameters and the science operations, and to maximize the scientific return. The aim of this article is to characterize the shape and rotational state of this asteroid.

Methods. We compile a set of 26 visible light curves whose phase angle coverage extends from to , and perform their simultaneous inversion relying on convex modeling.

Results. The full three-dimensional solution for asteroid 2867 Steins is rather spherical with axial ratios and . The rotational state is characterized by a sidereal period of  h, and the pole direction defined by its ecliptic coordinates and has an uncertainty of about . It is therefore almost exactly perpendicular to the ecliptic plane, and the viewing geometries are thus restricted to only about Steins' equator. Consequently, the shape model is not strongly constrained, and the polar flattening has an uncertainty of about 10%. Inversion is basically scale-free, and absolute scaling comes from a measurement of its thermal emission with the Spitzer Space Telescope (Lamy et al. 2008, A&A, 487, 1187), yielding overall dimensions of , , and km.