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P. Tanga, D. Loreggia (Oss. Astronomico di Torino), D. Hestroffer (Bureau des Longitudes), M. Lattanzi, A. Cellino (Oss. Astronomico di Torino), F. Guglielmetti (STScI), M. Di Martino, V. Zappala (Oss. Astronomico di Torino)
Four main belt asteroids and one trojan (Hektor) have been recently observed by the Fine Guidance Sensor astrometer (FGS#3) of the Hubble Space Telescope. The targets were chosen on the basis of their possible binary nature deduced both from theoretical considerations (Leone et al., 1984) and from light curve morphology (Cellino et al. 1985). The FGS#3 has been used in the past to measure Mira-type stars and it was shown that it is capable of allowing the measurement of stellar diameters with a precision of 2 mas, provided a very high S/N ratio (Lattanzi 1997). This corresponds to about 3-4 km at typical main-belt distances. The tranfer function observed on two orthogonal axis was compared with that obtained by two numerical models, based on different methods, in order to derive precise constraints on the asteroid sizes and scattering processes. The results have been compared to the IRAS indirect diameter measurements. The estimated rotational properties (pole and rotation period) and the changes observed over time in the measured sizes have allowed to derive new constraints on the shape of the observed objects, as well as to test their possible binary nature.