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

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 0.221 +/- 0.020. From new thermal-infrared spectrophotometric measurements and detailed thermophysical modeling we infer that Lutetia has a diameter of 98.3 +/- 5.9 km and a geometric albedo of 0.208 +/- 0.025, 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.