University of California
This image shows orbital tracks for Alexhelios and Cleoselene, the two tiny moons that circle the bone-shaped asteroid Kleopatra.
Kleopatra, a dog-bone shaped asteroid named after the last pharaoh of ancient Egypt, is a pile of rubble that spawned twin moons about 100 million years ago, astronomers announced in a new study.
The discovery stems from detailed observations of 135-mile-long Kleopatra with the Keck II telescope in Hawaii made in 2008 that confirmed the asteroid's dog-bone shape and the presence of two moons, each about 5 miles wide.
The scientists charted the orbits of the moons to determine the asteroid's mass. This combined with other data on the asteroid's size and shape allowed them to determine its density: a low 3.6 grams per cubic centimeter.
"That implies that somehow this asteroid has a large portion of void in its interior," Franck Marchis, an assistant research astronomer with the University of California at Berkeley, told me today.
In other words, the asteroid is a pile of rubble, not a solid chunk of rock as such large asteroids were once thought to be. Instead, Kleopatra joins a growing list of large asteroids that appear to be rubble piles, said Marchis, who is also a planetary scientist at the SETI Institute in Mountain View, California.
"We now know that asteroids can have a large porosity," Marchis said. "They are not primitive bodies; they are most likely bodies that are the result of a catastrophic disruption."
He and his colleagues suggest that the rubble pile coalesced from the remains of a smashup between rocky, metallic asteroids, which happened sometime after the origin of the solar system 4.5 billion years ago.
The moons could be leftover fragments from this collision, though Marchis and his colleagues now think they are pieces of the rubble pile asteroid that broke off during another impact that sent the asteroid spinning faster and faster.
This spinning would have elongated the asteroid and thrown off the moon. The team's calculations suggest the outermost moon was ejected about 100 million years ago; the inner moon about 10 million years ago.
The moons, Cleoselene and Alexhelios, are named after the twins born to their namesake, Cleopatra VII, and Mark Antony, a politician and general from the Roman republic: Cleopatra Selene II and Alexander Helios.
The names are in keeping with a tradition started when Marchis and his colleagues discovered the triple asteroid system 87 Sylvia in 2005 and named its two moons Romulus and Remus because Rhea Silvia was the mother of Romulus and Remus, the twins who founded Rome.
"And so just to keep this tradition, and because Cleopatra had twins as well, we decided to call them Alex Helios and Cleopatra Selene II, and we shortened them to Alexhelios and Cleoselen," Marchis said.
The outermost moon is Alexhelios and the innermost moon is Cleoselene. In Greek mythology, Helios and Selene represented the sun and moon, respectively.
The study of rubble pile asteroids may also help astronomers understand how planets could form more quickly than they would if their cores are formed by larger bodies glomming together.
"We know that planets from by collision of planetesimals. What we don't know is what was the composition of these planetesimals," Marchis explained. "We think that observing asteroids will give us direct information about the formation of the planetesimals, from which the planets formed."
If an asteroid is a rubble pile, when it collides with another asteroid, it is likely to be obliterated into small fragments that will then accrete into a larger body. This process is faster than the accretion of solid body asteroids, he said.
"So the question right now is how many of the asteroids that we have been observing, the 500,000 of them that we know, are rubble piles?" he said. "We don't know that yet. We just know the more measurements we have of the mass and density of asteroids, the more of them are in fact rubble piles."
Findings are reported in the February issue of the journal Icarus.
John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).