Photometric observations of Rosetta target asteroid 2867 Steins

Asteroid 2867 Steins is one of two flyby targets of ESA's International Rosetta Mission, launched in March, 2004. We obtained CCD observations of Steins on April 14-16, 2004 at Table Mountain Observatory, California, in order to characterize the asteroid physically, information that is crucial for planning the Steins flyby. This study includes the first detailed analysis of the physical properties of Steins from time-series R-filter data along with V- and I-filter photometric measurements. We found a mean R-filter absolute magnitude of 12.60 +/- 0.02 (for G = 0.15), corresponding to a mean radius of 3.57 +/- 0.03 km assuming an S-type reflectance of 0.20, or 2.24 +/- 0.02 km assuming an E-type reflectance of 0.40 (and G = 0.40). The observed brightness range of 0.29 +/- 0.04 mag suggests a lower limit on the axial ratio, a/b, of 1.30. We determined a synodic rotation period of 6.048 +/- 0.007 h, assuming a double-peaked lightcurve. We fitted the available R-filter photometry over the phase angle range of 11.08 - 17.07 degrees and found best-fit phase function parameters of G = 0.46(-0.20)(+0.32), and H = 12.92(-0.17)(+0.22). Derived colour indices for the asteroid are (V - R) = 0.58 +/- 0.03, and (R - I) = 0.44 +/- 0.03. These values are consistent with, though slightly redder than Hicks et al. (IAUC 8315). Barucci et al. (2005) identified Steins as an E-type based on visual and near-infrared spectra, but if that is correct, then it is an unusually red E-type asteroid.