Rosetta Fly-by of Asteroid 2867 Steins: Phase Function from the OSIRIS Imaging System

The ESA Rosetta mission performed a fly-by of the E-type asteroid Steins on September 5, 2008. The geometry of the fly-by was optimized to reach the zero phase angle aspect a couple of minutes before the closest approach. A fast monitoring sequence was implemented to investigate the phase function of the asteroid in the narrowband OI filter, centered at 630 nm, with the Wide Angle Camera of the OSIRIS imaging system. A set of 21 images of 30 ms exposure time were acquired for monitoring the opposition surge in the phase angle range from 0 to 11 degrees with a step of 0.5-1 degrees. In addition, several OI images were taken in the spectrophotometric sequences allowing us to reach a phase function coverage up to 140 degrees. In this work we present the Steins phase function and its interpretation. The derived values of the albedo, of the linear slope, and of the Hapke G parameter match those of E class asteroids.