Triplicity and physical characteristics of Asteroid (216) Kleopatra

To 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. [2008a]. (3749) Balam. In: Green, D.W.E. (Ed.), IAU Circ. 8928; Marchis, F., Descamps, P., Berthier, J., Emery, J.P. [2008b]. S/2008 ((216)) 1 and S/2008 ((216)) 2. In: Green, D.W.E. (Ed.), IAU 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 × 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.