Kleopatra

The dog-bone asteroid

Kleopatra is one of the strangest-looking objects in the asteroid belt: about 276 km long but under 100 km wide, with two lobes joined by a thinner middle.^[1] Its volume-equivalent diameter is only about 120 km.^[2]

The elongation first showed up in a 1991 stellar occultation timed from eight locations. Later radar observations at Arecibo showed the two lobes are connected, like a ham-bone, and the resulting shape model made the cover of Science.^[2][5]

Alexhelios and Cleoselene

A 1988 search for moons came up empty. Then in September 2008, Franck Marchis and colleagues announced two moons found with the Keck Observatory’s adaptive optics.^[2]

In 2011 they were named after Cleopatra’s twin children: Alexhelios (outer, about 8.9 km, 2.7-day orbit) and Cleoselene (inner, about 6.9 km, 1.8-day orbit).^[2]

Metal — but how much?

Kleopatra is an M-type asteroid, a class often linked to metal.^[4] Its moons allow its mass to be measured: (2.97 ± 0.32) × 10¹⁸ kg, lower than earlier thought, and a bulk density of 3.38 ± 0.50 g/cm³.^[2]

That density questions the old picture of a pure metal body. Radar suggests metal-rich material in the south but more ordinary rock along the equator. One explanation is a porous rubble pile with a lot of metal mixed in.^[2][6]

Spun into shape

One idea explains shape, spin and moons at once: an oblique impact, perhaps 100 million years ago, spun Kleopatra up, stretched it out and flung off Alexhelios; Cleoselene may have split off around 10 million years ago.^[2]

Kleopatra is a contact binary — if it spun much faster, the two lobes would separate and become a true binary system.^[2][7]

Orbit

Kleopatra belongs to the background population of the central belt rather than any family. It orbits between about 2.1 and 3.5 AU every 4.7 years, tilted 13°.^[2][1]

Holding a piece of an asteroid

No meteorite is known to come from Kleopatra, but iron and stony-iron meteorites do come from metal-rich asteroids. Our meteorite buying guide explains the main types and how to buy a genuine specimen.^[4]

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