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 \pm 0.02 ( for G=0.15 ) , corresponding to a mean radius of 3.57 \pm 0.03 km assuming an S-type reflectance of 0.20 , or 2.24 \pm 0.02 km assuming an E-type reflectance of 0.40 ( and G=0.40 ) .
The observed brightness range of 0.29 \pm 0.04 magnitudes suggests a lower limit on the axial ratio , a / b , of 1.30 .
We determined a synodic rotation period of 6.048 \pm 0.007 hours , 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 ^ { +0.32 } _ { -0.20 } , and H= 12.92 ^ { +0.22 } _ { -0.17 } .
Derived colour indices for the asteroid are ( V - R ) = 0.58 \pm 0.03 , and ( R - I ) = 0.44 \pm 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 .