The size and albedo of Rosetta fly-by target 21 Lutetia from new IRTF measurements and thermal modeling

¹ DLR Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany e-mail: michael.mueller@dlr.de
² Institute for Astronomy, 640 North A'ohoku Place, Hilo, HI 96720, USA
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-65, Cambridge, MA 02138-1516, USA
⁴ Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, HI 96743, USA
⁵ Cornell University, 206 Space Science Bldg., Ithaca, NY 14853, USA

Received: 1 July 2005
Accepted: 11 October 2005

Abstract

Recent spectroscopic observations indicate that the M-type asteroid 21 Lutetia has a primitive, carbonaceous-chondrite-like (C-type) surface composition for which a low geometric albedo would be expected; this is incompatible with the IRAS albedo of . From new thermal-infrared spectrophotometric measurements and detailed thermophysical modeling we infer that Lutetia has a diameter of  km and a geometric albedo of , in excellent agreement with the IRAS value. We can thus rule out a low albedo typical of a C-type taxonomic classification. Furthermore, we find that Lutetia's thermal properties are well within the range expected for large asteroids; we find no evidence for unusually high thermal inertia.