Multi-color, rotationally resolved photometry of asteroid 21 Lutetia from OSIRIS/Rosetta observations

Context. Asteroid 21 Lutetia is the second target of the Rosetta space mission. Extensive pre-encounter, space-, and ground-based observations are being performed to prepare for the flyby in July 2010. Aims. The aim of this article is to accurately characterize the photometric properties of this asteroid over a broad spectral range from the ultraviolet to the near-infrared and to search for evidence of surface inhomogeneities. Methods. The asteroid was imaged on 2 and 3 January 2007 with the Narrow Angle Camera (NAC) of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) during the cruise phase of the Rosetta spacecraft. The geometric conditions were such that the aspect angle was 44 degrees (i.e., mid-northern latitudes) and the phase angle 22.4 degrees Lutetia was continuously monitored over 14.3 h, thus exceeding one rotational period and a half, with twelve filters whose spectral coverage extended from 271 to 986 nm. An accurate photometric calibration was obtained from the observations of a solar analog star, 16 Cyg B. Results. High-quality light curves in the U, B, V, R and I photometric bands were obtained. Once they were merged with previous light curves from over some 45 years, the sidereal period is accurately determined: P-rot = 8.168271 +/- 0.000002 h. Color variations with rotational phase are marginally detected with the ultraviolet filter centered at 368 nm but are absent in the other visible and near-infrared filters. The albedo is directly determined from the observed maximum cross-section obtained from an elaborated shape model that results from a combination of adaptive-optics imaging and light curve inversion. Using current solutions for the phase function, we find geometric albedos p(V) = 0.130 +/- 0.014 when using the linear phase function and p(V)(H - G) = 0.180 +/- 0.018 when using the (H - G) phase function, which incorporates the opposition effect. The spectral variation of the reflectance indicates a steady decrease with decreasing wavelength rather than a sharp fall-off.

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