Lightcurve of Asteroid 2867 Steins
Asteroid 2867 Steins is one of two asteroids (the other being 21 Lutetia) that Rosetta will fly by on its way to comet 67 P/Churyumov-Gerasimenko. This plot shows the variation of the asteroid's apparent magnitude as measured by the OSIRIS Narrow Angle Camera on 11 March 2006 from a distance of 159 million kilometres (1.06 AU). The data were gathered during a single 24 hour period and show the asteroid has a spin period of just over six hours (6.052), in good agreement with ground based observations.
This is the most accurate lightcurve of the asteroid obtained so far. The difference between minimum and maximum brightness is ~0.23 in magnitude. The asymmetry of the lightcurve suggests an irregular shape of the asteroid.
Further analyses of the data and correlation with ground based observations will allow for determining the orientation of the asteroid's spin axis.
Image from Küppers, M. et al.  Determination of the light curve of the Rosetta target asteroid (2867) Steins by the OSIRIS cameras onboard Rosetta, A&A 462, L13-L16, 5 Jan 2007, DOI: 10.1051/0004-6361:20066694