How Kleopatra got its dog-bone shape

From a 1991 occultation to Arecibo radar and adaptive optics — and a theory linking its shape, spin and moons.

Published 2026-10-23

Kleopatra was discovered on April 10, 1880, but its remarkable shape was not revealed until more than a century later.^[2]

1991: shadows on the ground

On 19 January 1991, Kleopatra passed in front of a star. Observers at eight locations timed the star’s disappearance, and the resulting outline was strongly elongated.^[2][8]

One body or two?

Observations with ESO’s 3.6 m telescope at La Silla were interpreted as a double source with two lobes of similar size. Then radar observations at Arecibo showed that the lobes are connected, resembling a ham-bone. That radar shape model appeared on the cover of Science.^[2][5]

Today’s picture

JPL lists dimensions of about 276 × 94 × 78 km, with a volume-equivalent diameter around 120 km.^[1][2] The object spins once every 5.4 hours.^[1]

An impact theory

One proposed origin: an oblique impact roughly 100 million years ago. The added spin would have stretched Kleopatra and caused the moon Alexhelios to split off. Cleoselene may have broken away later, about 10 million years ago.^[2]

On the edge

Kleopatra is a contact binary. If it were spinning much faster, the lobes would separate and become a true binary asteroid.^[2][7]

Numbered citations link to the sources list on the Kleopatra overview page.