216 Kleopatra

Physical characteristics

Size and shape

Kleopatra is a relatively large asteroid, with a mean (volume-equivalent) diameter of 120±2 km and an unusually elongated shape.

The initial mapping of its elongated shape was indicated by stellar occultation observations from eight distinct locations on 19 January 1991. Subsequent observations with the ESO 3.6 m Telescope at La Silla, run by the European Southern Observatory, were interpreted to show a double source with two distinct lobes of similar size. These results were disputed when radar observations at the Arecibo Observatory showed that the two lobes of the asteroid are connected, resembling the shape of a ham-bone. The radar observations provided a detailed shape model that appeared on the cover of Science Magazine. Later models suggested that Kleopatra was more elongate and the most recent models using radar delay-Doppler imaging, adaptive optics, and stellar occultations provide dimensions of 267 × 61 × 48 km.

Satellites

In 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Maunakea Observatories, but the effort was fruitless. In September 2008, Franck Marchis and his collaborators announced that by using the Keck Observatory's adaptive optics system, they had discovered two moons orbiting Kleopatra. In February 2011, the minor-planet moons were named Alexhelios () (outer) and Cleoselene () (inner), after Cleopatra's twin children, Alexander Helios and Cleopatra Selene II. The outer and inner satellites are about 8.9 ± 1.6 and 6.9 ± 1.6 km in diameter, with periods of 2.7 and 1.8 days, respectively.

Mass, density, and composition

The presence of two moons provides a way to estimate Kleopatra's mass, although its irregular shape makes the orbital modeling a challenge. The most recent adaptive-optics observations and modeling provides a mass of Kleopatra of (1.49±0.16)×10⁻¹² M_☉, or (2.97±0.32)×10¹⁸ kg, which is significantly lower than previously thought. When combined with the best volume estimate for Kleopatra, this indicates a bulk density of 3.38±0.50 g/cm³.

These recent bulk density results call into question the canonical view of Kleopatra as a pure metallic object. Kleopatra's radar albedo suggests a high metal content in the southern hemisphere, but is similar to the more common S- and C-class asteroids along the equator. One way to reconcile these observations is to hypothesize that Kleopatra is a rubble-pile asteroid with significant porosity in dynamic equilibrium.

Origin

One possible origin that explains Kleopatra's shape, rotation, and moons is that it was created by an oblique impact perhaps 100 million years ago. The increased rotation would have elongated the asteroid and caused Alexhelios to split off. Cleoselene may have split off later, around 10 million years ago. Kleopatra is a contact binary – if it were spinning much faster, the two lobes would separate from each other, making a true binary system.