Asteroid (2867) Steins: Shape, topography and global physical properties from OSIRIS observations

The Rosetta spacecraft flew by Asteroid (2867) Steins on 5 September 2008, allowing the onboard OSIRIS cameras to collect the first images of an E-type asteroid. We implemented several three-dimensional reconstruction techniques to retrieve its shape. Limb profiles, combined with stereo control points, were used to reconstruct an approximate shape model. This model was refined using a stereophotoclinometry technique to accurately retrieve the topography of the hemisphere observed by OSIRIS. The unseen part of the surface was constrained by the technique of light curves inversion. The global shape resembles a top with dimensions along the principal axes of inertia of 6.83 x 5.70 x 4.42 km. It is conspicuously more regular than other small asteroids like (233) Eros and (25143) Itokawa. Its mean radius is R-m = 2.70 km and its equivalent radius (radius of a sphere of equivalent volume) is R-v = 2.63 km. The north pole is oriented at RA = 99 +/- 5 degrees and Dec = 59 +/- 5 degrees, which implies a very large obliquity of 172 degrees and a retrograde rotation. Maps of the gravitational field and slopes were calculated for the well-imaged part of the asteroid. Together with the shape, they helped characterizing the most prominent topographic features identified at the surface of (2867) Steins: an equatorial ridge restricted to the extremities of the long axis, a large crater having dimensions of 2100 x 1800 m in the southern hemisphere, and an elongated hill in the northern hemisphere. We conjecture that the equatorial ridge was formed by centrifugal acceleration as the asteroid was spun up by the Yarkovsky-O'Keefe-Radzievskii-Paddack effect. (C) 2012 Elsevier Inc. All rights reserved.