Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope ^*

¹ Johns Hopkins University Applied Physics Laboratory, Space Department, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA e-mail: hal.weaver@jhuapl.edu
² Johns Hopkins University, Department of Physics and Astronomy, 3100 N. Charles Street, Baltimore, MD 21218, USA
³ Southwest Research Institute, Department of Space Studies, Suite 300, 1050 Walnut Street, Boulder, CO 80302-5150, USA
⁴ Space Telescope Science Institute, 3900 San Martin Drive, Baltimore, MD 21218, USA
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁶ Service d'Aéronomie, Réduit de Verrières – BP3, Routes des Gatines, 91371 Verrières le Buisson Cedex, France
⁷ Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁸ Starfire Optical Range, Air Force Research Laboratory, Kirtland Air Force Base, Albuquerque, NM 87117, USA
⁹ Southwest Research Institute, Space Division, 1050 Walnut Street, Boulder, CO 80302-5150, USA

Received: 23 December 2009
Accepted: 27 June 2010

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.