Imaging Asteroid (2867) Steins with OSIRIS onboard Rosetta

The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS observed the E-type asteroid (2867) Steins during the fly-by of ESA's Rosetta spacecraft. Observations over a large phase angle range (from near 0 to 140) by the scientific camera system OSIRIS revealed the illuminated hemisphere of the asteroid's diamond-like shaped body with a mean radius of 2.7 km and a projected surface at zero phase angle of 5.3 x 3.9 km2. A large crater (diameter 2 km) is evidence of an almost disastrous impact and implies that Steins is not a solid rock. More than 30 craters or crater-like features with diameters > 150 m are identified. 7 round concavities are arranged along a line pointing radially away from the big impact crater. The shape and volume of asteroid Steins is derived from models based on the images of both OSIRIS cameras and earlier observations of the photometric light curves. Its resemblance to a spinning top suggests that it was influenced by the YORP effect making it the first optical observation of such a body. Analysis of the images provide the disk integrated albedo, reveal a strong opposition effect, and photometric properties of the surface showing very little variegation. Its very uniform, bright surface suggests that this asteroid is homogeneously formed out of the igneous (magmatic) minerals found in enstatite achondrite meteorites that are produced in melts requiring temperatures of more than 1000 C. Consequently (2867) Steins is a fragment of the interior of a large parent body.