Washington: A team of French and American astronomers has found how the asteroid Kleopatra gave birth to two moons probably spawned by the asteroid sometime in the past 100 million years.
The team also confirms earlier reports that the asteroid is shaped like a dog bone.
“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,” said Franck Marchis, a research astronomer at the University of California, Berkeley.
“Asteroids this big are supposed to be solid, not rubble piles.”
The outermost moon is named Alexhelios and the innermost moon is Cleoselene. Kleopatra, about 217 kilometers long, is one of several large asteroids recently found to be composed of rocky rubble held together by mutual gravitational attraction.
Marchis said that the collisions between two asteroids are 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,” he said.
The team believes that asteroid probably coalesced from the remains of a rocky, metallic asteroid smashed to smithereens after a collision with another asteroid, which could have occurred any time since the origin of the solar system 4.5 billion years ago.
Based on Descamps’ theory of binary asteroid formation, the rubble pile was set spinning faster by an oblique impact 100 million years ago. The spinning asteroid would have elongated and slowly thrown out the most distant moon. The inner moon was likely shed more recently, perhaps 10 million years ago.
The team also charted the orbits of the two small moons, each about 8
kilometers across, providing enough information to derive the mass of the asteroid.
The team continues to observe large asteroids in search of ones with moons, which allows them to calculate the density and determine the prevalence of porous, rubble pile asteroids.
The study appears in the February issue of the journal Icarus.