Thermal infrared observations of the Hayabusa spacecraft target asteroid 25143 Itokawa

We obtained N - and Q -band observations of the Apollo-type asteroid 25143 Itokawa during its close Earth approach in July 2004 with TIMMI2 at the ESO 3.6 m telescope. Our photometric measurement, in combination with already published data, allowed us to derive a radiometric effective diameter of $0.32 \pm 0.03$ km and an albedo of 0.19$^{+0.11}_{-0.03}$ through a thermophysical model. This effective diameter corresponds to a slightly asymmetrical and flattened ellipsoid of the approximate size of 520( ± 50) $\times$ 270( ± 30) $\times$ 230( ± 20) m, based on the Kaasalainen et al. (2005, Proceedings of the 1st Hayabusa Symposium, ASP Conf. Ser., submitted) shape model. Our studies show that the thermal observations lead to size estimates which are about 15% smaller than the radar results (Ostro et al. 2005, Met. Plan. Sci., submitted), slightly outside the stated radar uncertainties of ± 10%. We determined a rather high thermal inertia of 750 J m -2 s -0.5 K -1 . This is an indication for a bare rock dominated surface, a thick dust regolith can be excluded as well as a metallic surface. From our data we constructed a 10.0 μ m thermal lightcurve which is nicely matched in amplitude and phase by the shape and spin vector solution in combination with our TPM description. The assumed S-type bulk density in combination with radiometric size lead to a total mass estimate of $4.5^{+2.0}_{-1.8} \times 10^{10}$ kg.