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a b s t r a c tTo take full advantage of the September 2008 opposition passage of the M-type Asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO) imaging with the W.M. Keck II telescope to capture unprecedented high resolution images of this unusual asteroid. Our AO observations with the W.M. Keck II telescope, combined with Spitzer/IRS spectroscopic observations and past stellar occultations, confirm the value of its IRAS radiometric radius of 67.5 km as well as its dog-bone shape suggested by earlier radar observations. Our Keck AO observations revealed the presence of two small satellites in orbit about Kleopatra (see Marchis, F. et al. Circ. 8980). Accurate measurements of the satellite orbits over a full month enabled us to determine the total mass of the system to be 4.64 ± 0.02 Â 10 18 kg. This translates into a bulk density of 3.6 ± 0.4 g/cm 3 , which implies a macroscopic porosity for Kleopatra of $30-50%, typical of a rubble-pile asteroid. From these physical characteristics we measured its specific angular momentum, very close to that of a spinning equilibrium dumbbell.

The existence of binary asteroidal systems has been confirmed observationally during the past decade with spacecraft exploration 3 , ground-based imaging 4 , radar observations 4 , and light curve measurements 5 . Most recently, the discovery of two moons orbiting around the irregular rubble pile asteroid 87 Sylvia 6 with adaptive optics system observations confirms that collision and disruption is the main formation mechanism for multiple main-belt systems. The system 617 Patroclus, the only binary Trojan known 1 , was discovered with the Hokupa'a Gemini 8-m adaptive optics system 7 under excellent seeing conditions. Because of their faintness (with a magnitude in the visible spectrum of m v >15.5), Trojan asteroids cannot be directly observed by most of the adaptive optics systems. Using related techniques such as stellar appulse 8 and LaserGuide Star (LGS) observations 9 at the Lick 3-m telescope, our group failed to detect any new companions of Trojans, indicating that the proportion of multiple systems in the Trojan population larger than 40 km in diameter is less than 4% (ref. 9). The study of a