An international team of astronomers report that the rubble-pile asteroid Kleopatra gave birth to two moons sometime in the last 100 million years. The report also details the precise orbits of the moons, and yields information on the asteroid’s density.

The astronomers used small telescopes as well as the Keck II telescope to reach their conclusions. “Our observations of the orbits of the two satellites of 216 Kleopatra imply that this large metallic asteroid is a rubble pile, which is a surprise,” says Franck Marchis of the University of California, Berkeley. “Asteroids this big are supposed to be solid, not rubble piles.”

To date, all asteroids found with moons are porous rubble-piles made from chunks of rock, and Kleopatra, at 217 kilometres long, is one of just a few such asteroids which could implicate the method of planet formation. Marchis and colleagues say that collisions between two asteroids are just as likely to break up both bodies as to coalesce into one large asteroid, making planet formation a slow process. Rubble pile asteroids, however, would merge more easily during a collision. “If a large proportion of asteroids in the early Solar System were rubble-pile, then the formation of the core of planets would be much faster,” says Marchis.

Kleopatra likely coalesced from the remains of a rocky, metallic asteroid that had been broken up during a previous collision with another asteroid any time during the Solar System’s history. But according to co-author Pascal Descamps of the Observatoire de Paris, Kleopatra was set spinning faster by an oblique impact around 100 million years ago. This would have caused the asteroid to elongate and throw out the most distant moon while the inner moon could have been born as recently as 10 million years ago.

Each of the moons are about eight kilometres across, and by precisely charting their orbits, the astronomers could derive key characteristics of the asteroid itself, including calculating its density to be 3.6 grams per cubic centimetre. Since the bulk of the asteroid is iron, which has a density of about five grams per cubic centimetre, then the asteroid must be between 30 and 50 percent empty space, the report concludes. The new observations from Keck II also confirm Kleopatra’s dog bone-shape form.

The asteroid’s namesake Kleopatra, the last pharaoh and queen of Egypt, also had twins, and the moons will thus be named Cleoselene and Alexhelios.