Asteroid 2867 Steins. II. Multi-telescope visible observations, shape reconstruction, and rotational state

International audienceContext: 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 7.5° to 41.7°, 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 a/b=1.17 and a/c=1.25. The rotational state is characterized by a sidereal period of 6.04681 ± 0.00002 h, and the pole direction defined by its ecliptic coordinates lambda ≈ 250° and beta ≈ - 89° has an uncertainty of about 5°. It is therefore almost exactly perpendicular to the ecliptic plane, and the viewing geometries are thus restricted to only ±20° 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 5.73 ± 0.52, 4.95±0.45, and 4.58 ± 0.41 km