5 Conclusions

In this paper we present the first results on mid-infrared wavelength range of five asteroids. ISO gave the unique opportunity to observe asteroids at this wavelength range and we obtained photometric data up to 60 $\mu$m and low resolution spectra between 5.8 and 11.6 $\mu$m. On the basis of these data we computed albedos and diameters of the observed asteroids. The obtained results are in agreement with the estimations computed on the basis of the IRAS data.

The interpretation of the obtained spectra is neither easy nor unique. A tentative interpretation has been suggested by comparing the observed spectral behaviours with the emissivities of meteorites which are our best available analogs of asteroidal surface materials. The theories about the thermal history of C-type asteroids seems to be confirmed by the match between the spectrum of 511 Davida and the emissivity of the CM-type carbonaceous chondrite meteorite Murchison. While the tentative analogy between the spectrum of Frigga and the emissivity of Winona is consistent with the still debated taxonomic classification of this asteroid.

Problems relative to the calibration of these faint objects due to non-standard instrument configurations have affected the quality of the ISO data, especially the point at 25 $\mu$m. For this reason and due to the difficulty in interpreting the surface spectra, the obtained results and the suggested interpretations cannot be conclusive. This work can be considered a support for future observations of asteroids in these wavelengths by space platform projects, planned to be operative in next future (e.g. the Space Infrared Telescope Facility SIRTF).

Acknowledgements

J.R.B., V.M., and L.C. whish to thank ASI and MIUR for supporting the experimental work.