216 Kleopatra


216 Kleopatra is a metallic, ham-bone-shaped asteroid and trinary system orbiting in the central region of the asteroid belt, approximately in diameter. It was discovered on 10 April 1880, by Austrian astronomer Johann Palisa at the Austrian Naval Pola Observatory, in what is now Pula, Croatia. The M-type asteroid has a shorter than average rotation period of 5.4 hours. It was named after Cleopatra, the famous Egyptian queen. Two small minor-planet moons were discovered in 2008, and later named Alexhelios and Cleoselene.

Orbit and classification

Kleopatra is a non-family asteroid from the main belt's background population. It orbits the Sun in the central asteroid belt at a distance of 2.1–3.5 AU once every 4 years and 8 months. Its orbit has an eccentricity of 0.25 and an inclination of 13° with respect to the ecliptic. The body's observation arc begins at Leipzig Observatory on 20 April 1880, ten days after to its official discovery observation at Pola Observatory.

Physical characteristics

Kleopatra is a relatively large asteroid, measuring 217 × 94 × 81 km. Calculations from its radar albedo and the orbits of its moons show it to be a rubble pile, a loose amalgam of metal, rock, and 30–50% empty space by volume, likely due to a disruptive impact prior to the impact that created its moons.
Kleopatra has an unusual shape. Initial 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.

Moons

In 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but the effort came up empty. 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. The outer and inner satellites are about 8.9 ± 1.6 and 6.9 ± 1.6 km in diameter, with periods of 2.32 ± 0.02 and 1.24 ± 0.02 days, respectively.
In February 2011, the minor-planet moons were named Alexhelios and Cleoselene , after Cleopatra's children Alexander Helios and Cleopatra Selene II.

Origin

It is believed that Kleopatra's shape, rotation, and moons are due to 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.