Context: Asteroid 2867 Steins is the first target of the Rosetta space mission with a flyby scheduled in September 2008.
Aims: An early characterization is needed to optimize the flyby parameters and the science operations, and to maximize the scientific return.
Methods: We used the infrared spectrograph (IRS) of the Spitzer Space Telescope (SST) to obtain 14 spectra ranging from 5.2 to 38.0 mum, and to sample the rotational period of the asteroid. The observations were performed on 22 November 2005, when the asteroid was 2.13 AU from the Sun, 1.60 AU from the SST, and at a phase angle of 27.2°. They were interpreted using a standard thermal model incorporating the thermal inertia.
Results: The solution for a spherical shape leads to an effective radius r_n = 2.46 ± 0.20 km and a thermal inertia I = 150 ± 60 J K^-1 m^-2 s^-1/2, for a beaming factor between 0.8 and 1.0. The geometric albedo is then constrained by visible photometry to p_R = 0.31 ± 0.05 and p_V = 0.27 ± 0.04 when using a linear phase function. The H-G phase law, which includes an opposition effect, leads to larger values of the albedo, p_R(H-G) = 0.40 ± 0.07 and p_V(H-G) = 0.34 ± 0.06. The solution for our 3-dimensional shape model has overall dimensions of 5.73 ± 0.52 × 4.95 ± 0.45 × 4.58 ± 0.41 km.