Alexhelios and Cleoselene: Kleopatra’s twins
Two small moons discovered with Keck in 2008 and named after Cleopatra’s children.
Published 2026-10-23
For an asteroid named after Cleopatra VII, the names of its moons were almost inevitable.[2]
The search
In 1988 astronomers looked for satellites or dust around Kleopatra with the UH88 telescope on Maunakea and found nothing.[2] Twenty years later, in September 2008, Franck Marchis and colleagues announced two moons, found using the adaptive optics system at the Keck Observatory.[2]
The names
In February 2011 they were named after Cleopatra’s twins, Alexander Helios and Cleopatra Selene II:[2]
- Alexhelios — the outer moon, about 8.9 ± 1.6 km across, orbiting in about 2.7 days.[2]
- Cleoselene — the inner moon, about 6.9 ± 1.6 km across, orbiting in about 1.8 days.[2]
What the moons tell us
Moons act as scales. Their orbits give Kleopatra a mass of about 2.97 × 1018 kg, though the irregular shape makes modelling hard.[2] Combined with the volume, the density is about 3.38 g/cm³ — lower than solid metal, implying a porous interior.[2]
Johann Palisa’s find
Kleopatra was discovered by Johann Palisa at the Austrian Naval Observatory in Pola. He found 122 asteroids and remains the most successful visual asteroid discoverer.[3]
Numbered citations link to the sources list on the Kleopatra overview page.