1 Introduction

At the beginning of last century, by analyzing the asteroid (433) Eros, André made the hypothesis that asteroids could have satellites (André 1901). Since then, however, we know that Eros is actually an elongated single-body. But, since the discovery of Dactyl, the moon of Ida, by the Galileo space probe, it has been established that some asteroids do have satellites. Merline et al. (1999) reported the first ground-based detection of an asteroid satellite using adaptive optics ( AO). These fundamental findings have motivated a systematic search for possible satellites of main-belt asteroids using AO imaging-systems and discoveries are now accumulating (Merline et al. 1999, 2000; Brown et al. 2001; Merline et al. 2001). However, one should distinguish among different types of asteroid binaries: whether the system is composed of similarly-sized bodies such as (90) Antiope or whether a small moon is orbiting around a larger attracting asteroid such as (22) Kalliope. The events or physics underlying the formation of such systems are surely different, and they may also be different for Earth-crossing asteroids which can be affected by larger tidal evolution. An accurate orbit analysis is of great importance for studies of the internal structure, formation and evolution of such systems (Marchis et al. submitted). In this respect the asteroid (216) Kleopatra is of particular interest: should its binary nature or dumbbell-shaped nature be confirmed, it would provide additional constraints on collisional-evolution and binary-formation models.

Direct or indirect imaging of Kleopatra in the past showed it to be a very elongated body but was not able to conclusively prove or disprove its binary nature (Dunham 1992; Mitchell et al. 1995; Storrs et al. 1999). For instance, the direct imaging data from the HST/WFPC1 of Storrs et al. (1999) had limited resolution, and artifacts, due to the restoration technique used, arose in the image reconstruction (Storrs et al. 2000). Analysis of recent observations with the HST Fine Guidance Sensor interferometer provide a bi-lobated shape-model (Tanga et al. 2001), consistent with the images obtained from the Keck telescope (Hammergren et al. 2000). At present, the best resolution model obtained for Kleopatra is given by radar observations (Ostro et al. 2000). However the radar data do not rule out the presence of an empty gap between the two lobes and could be consistent with a binary model.

We present results from observations using the ADaptive Optics Near Infrared System ( ADONIS, Marchis et al. 1999; Roddier 1999) installed on the 3.6 m ESO telescope at La Silla (Chile), on October 25, and December 3, 1999. The observations and restored images using the MISTRAL technique are presented in Sect. 2. Simulations of observations with higher resolution, e.g. with the VLT/NAOS instrument are presented in Sect. 3.

Figure 1: Observed and restored images obtained at the ESO-La Silla 3.6 m telescope. Upper panel: October 15, 1999 run; lower panel: December 3, 1999 run. The restored images are shown on a linear scale.