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We obtained N- and Q-band observations of the Apollo-type asteroid 25143Itokawa during its close Earth approach in July 2004 with TIMMI2 at the ESO 3.6m telescope. Our photometric measurement, in combination with already publisheddata, allowed us to derive a radiometric effective diameter of 0.32+/-0.03 kmand an albedo of 0.19 +0.11/-0.03 through a thermophysical model. Thiseffective diameter corresponds to a slightly asymmetrical and flattenedellipsoid of the approximate size of 520(+/-50) x 270(+/-30) x 230(+/-20) m,based on the Kaasalainen et al. (2005) shape model. Our studies show that thethermal observations lead to size estimates which are about 15% smaller thanthe radar results (Ostro et al. 2005), slightly outside the stated radaruncertainties of +/-10%. We determined a rather high thermal inertia of 750Jm-2s-0.5K-1. This is an indication for a bare rock dominated surface, a thickdust regolith can be excluded as well as a metallic surface. From our data weconstructed a 10.0 micrometer thermal lightcurve which is nicely matched inamplitude and phase by the shape and spin vector solution in combination withour TPM description. The assumed S-type bulk density in combination withradiometric size lead to a total mass estimate of 4.5 +2.0/-1.8 x 10^10 kg.Comment: 9 pages, 7 figures, accepted for publication by A&A 29/Aug/200

Thermal infrared observations of the Hayabusa spacecraft target asteroid 25143 Itokawa