5 Conclusions

The direct detection of binary asteroids by long-range observations requires the exploitation of the highest resolutions currently available. For the first time, we have been able to use the FGS astrometer of the HST to study moving targets of the Solar System. The restricted sample of targets was selected mainly on the basis of models derived from photometric observations. Even though no detached binary object was discovered, this criteria has proven to be valuable in selecting targets that present significant shape features.

Due to the rotation of the asteroids around their axis, not only can the sky-projected profile be measured, but it is also possible to infer information about the fully three-dimensional shape, provided that some pole solution can be found to be consistent with FGS data. Moreover, wrong pole solutions can be rejected and nearly-contact binary asteroids revealed. Results are expected to be even better using the upgraded and more sensitive FGSR#1. Depending on the geometry of the observations, the lengths of the principal axes of the bodies cannot all be determined with the same precision, but observations at different epochs would efficiently overcome such limitations.

The inversion procedures applied here have proved to be solid, being able to provide best-fit ellipsoidal figures and to test alternative, more complex models. Some evolved binary structures cannot be completely ruled out for some of the objects of our sample based on the available data, although single ellipsoidal shapes are generally a good first approximation of what we have observed. While we hope to obtain new HST/FGS data in the future, we are confident that more detailed analyses of data coming from different sources, including the data presented in this paper, will likely provide stricter constraints to the overall shapes and internal structures of large asteroids and to their past collisional evolution.

Acknowledgements

We are grateful to the FGS team at the STScI, in particular E. Nelan and D.C. Taylor. Important help concerning data reduction and modeling has been provided by D. Loreggia (OATo), F. Gugliemetti (currently at IMPRS, Munich) and J. Berthier (IMCCE). P. Descamps (IMCCE) kindly put at our disposal "MOVIS'', the shape modelling and visualization software. D. Hestroffer acknowledges funding from the OATo, and P. Tanga the support of the H. Poincaré fellowship of the Observatoire de la Côte d'Azur.