Ultraviolet and visible photometry of asteroid (21) Lutetia using the hubble space telescope

Abstract

Context. The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. Aims. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ∼-1500 Å to ∼-7000 Å. Methods. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia∼-s size and shape were used to convert the absolute fluxes into albedos. Results. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 1200-8000 Å. There appears to be a steep drop in the albedo (by a factor of ∼-2) for wavelengths shorter than ∼-3000 Å. Nevertheless, the far ultraviolet albedo of Lutetia (∼-10%) is considerably larger than that of typical C-chondrite material (∼-4%). The geometric albedo at 5500 Å is 16.5 ± 1%. Conclusions. Lutetia∼-s reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ∼-10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010. © 2010 ESO.