Determination Of The Light Curve Of Rosetta Target Asteroid 2867 Steins With The Osiris Narrow Angle Camera Onboard Rosetta

Abstract

On its way to comet 67P/Churyumov-Gerasimenko, the European Space Agency’s Rosetta spacecraft is going to fly by Asteroids 2867 Steins in Sept. 2008 and 21 Lutetia in July 2010. While 21 Lutetia is a large main belt asteroid with a long history of investigation, relatively little is known about 2867 Steins. In particular, knowledge of the rotation period and the orientation of the rotation axis are valuable for the flyby preparations. OSIRIS is the scientific camera system onboard Rosetta, consisting of a narrow angle camera (NAC) and a wide angle camera (WAC). In March 2006, the NAC observed 2867 Steins continuously for 24 hours, acquiring 238 images, with a clear filter. The camera performed flawlessly. The data obtained from spacecraft are unique in terms of continuous temporal coverage and phase angle. Indeed, the NAC observations were taken at a phase angle of 42 deg., compared to a maximum phase angle of 30 deg. when seen from Earth. The distance between Rosetta and 2867 Steins was 1.06 AU. The OSIRIS data show a double peaked light curve with an amplitude of ~0.23 magnitudes and a synodic rotation period of 6.052 ± 0.035 hours, in good agreement with ground based data by Hicks et al. (2004, IAU Circular 8315) and Weissman et al. (2006, ACM Conference). The continuous observations over four rotations completely exclude the possibility of period ambiguities. There is no indication of deviation from a principal axis rotation state. The observations are being combined with Earth-based data sets to derive the orientation of the spin axis and pole position for 2867 Steins. We acknowledge funding from the national space agencies ASI, CNES, DLR, the Spanish Space Program (Ministerio de Educación y Ciencia), SNSB, and ESA.