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Asteroids have been used extensively as calibration sources for the Infrared Space Observatory (ISO) and are planned to be used by future groundbased, airborne and space-based projects in the thermal infrared (IR) and in the sub-millimetre. We summarize the general IR observational parameters with a focus on space observatories and discuss brightness variations, apparent velocities and background influences. During the ISO mission ten well- studied asteroids were used for the photometric calibration of ISOPHOT, but additionally the bright asteroids turned out to be of great interest for many technical tests and calibration aspects. We evaluated the dierent applications, like testing the photometry of the spectrometers, validation of relative spectral response functions, determination of beam proles or colour correction tests. The description of the asteroids' thermal emission has been obtained by a recent thermophysical model (TPM). The important model aspects are size, albedo, shape together with the spin vector, a beaming model, thermal inertia and a wavelength-dependent emissivity. With a large sample of observational data provided by three dierent ISO instruments we had for the rst time the possibility to study the thermal emission of several asteroids in detail. The intercomparison between results from dierent instruments allowed us to distinguish between observational errors and model shortcomings. It turned out that the accuracy of TPM predictions is in many cases strongly related to the limited knowledge of the asteroid shapes. The concepts of beaming, thermal inertia and wavelength dependent emissivities were nicely conrmed for a wide range of observing and illumination geometries under many aspect angles for dierent asteroids. The TPM predictions for Ceres, Pallas and Vesta are accurate within 5% over the full wavelength range from 5 to 200m, for Hygiea and a few other asteroids the predictions and observations agree within 10 to 15%. We found similar emissivity behaviour for the four large asteroids over the full ISO wavelength range. Up to now, no clear spectral features have been seen in the asteroid far-IR spectra.

Asteroids as calibration standards in the thermal infrared for space observatories