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T.G. M\"{u}ller (ISO Data Centre, ESA Astrophysics Division, Villafranca del Castillo, Spain), J.S.V. Lagerros (Astronomiska Observatoriet, Uppsala, Sweden; Five College Radio Astronomy Observatory, LGRC-619, University of Amherst, USA)
ISO observed in its 29 month lifetime more than 40 asteroids at wavelengths between 2 and 200 \mu m during special observing programmes in great details. Since August 1999 all ISO observations are publically available at {\tt http://www.iso.vilspa.esa.es/}. A detailed analysis of selected bright asteroid observations, photometric, spectroscopic and polarimetric measurements, lead to considerable improvements in the understanding and description of the thermal emission of Solar System objects.
The observations are compared to a thermophysical model of the asteroids. The model is capable of predicting the absolute flux and state of polarization of the thermal emission from mid-infrared to microwave wavelengths. Current information on the shape and spin state of the asteroids are used in order to calculate model thermal lightcurves. The conduction of heat into the surface material in general lowers the mid-infrared flux from main-belt asteroids. The small scale surface roughness enhances the emission towards the solar direction, but randomizes the scattering planes which lowers the expected degree of polarization.
We find thermal inertias for main belt asteroids between 5 and 25 Jm-2s-0.5K-1, and surface roughnesses described by r.m.s. slopes between 0.4 and 0.8. The signature of the surface regolith, the grain sizes and the surface material can be seen at longer wavelengths beyond 50 micron, where the emissivity changes with wavelength and in some cases drops to about 0.6 in the FIR. At shorter wavelength we find clear Silicate emission features on a 5-15% level above the thermal continuum.
The combination of our thermophysical modelling ideas and the high quality ISO data on asteroids resulted in the description of the thermal emission from 5 to 200\mu m with unprecedented accuracy.
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The author(s) of this abstract have provided an email address for comments about the abstract: tmueller@iso.vilspa.esa.es