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How Kleopatra gave birth to twins

  • Asteroid Kleopatra is a rubble pile shaped like a dog bone, 217km long
  • Found to have twin 8km-wide moons that orbit it
  • Moons have been named after queen Cleopatra’s twins

ASTRONOMERS STUDYING two mini-moons orbiting an asteroid called Kleopatra have confirmed that the dog bone-shaped asteroid is probably a pile of rocky rubble instead of a solid body.

The French and American team, which includes Franck Marchis (University of California, Berkeley) and Pascal Descamps (Institut de Mecanique Celeste et de Calculs des Ephemerides, Observatoire de Paris), report their findings in the journal Icarus.

Kleopatra was discovered in 1880. Observations made in 2000 showed it to have an unusual, elongated shape reminiscent of a dog bone.

Subsequent radar observations confirmed the shape, but Marchis and his colleagues wanted higher resolution images to determine whether the two lobes of the dog bone are touching or are two separate bodies, and also to calculate its density.

The video above is a rotation of a computer model produced from the radar data.

Using the Keck II telescope in Hawaii, in 2008 the astronomers obtained the best images yet and confirmed that the asteroid is a double-lobed body. They also discovered the two small moons.

Space rubble

The team charted the orbits of the moons (diameters 3 and 5 kilometres), from which they could calculate the mass of the asteroid. Given its size, shape and mass, the astronomers then calculated the asteroid’s density—3.6 grams per cubic centimetre. (As a comparison, Earth’s average density is 5.5 grams per cubic centimetre.)

If the bulk of the asteroid is made of iron—a common component with a density of about 5 grams per cubic centimetre—then it must be between 30 and 50 percent empty space, the team concludes.

“Our observations of the orbits of the two satellites of … Kleopatra imply that this large metallic asteroid is a rubble pile, which is a surprise,” said Marchis, who is also a planetary scientist at the SETI Institute. “Asteroids this big are supposed to be solid, not rubble piles.”

Asteroid Kleopatra and its two tiny moons

Asteroid Kleopatra (overexposed in the left-hand image) and its two tiny moons. The image on the right has been processed to reduce the glare and more easily show the moons, which are now called Alexhelios and Cleoselene.

Kleopatra, about 217 kilometres long, is one of several large asteroids recently found to be composed of rocky rubble held together by mutual gravitational attraction. Others are Sylvia (280 kilometres in diameter), Antiope (86km), Hermione (190km) and 22 Kalliope (166km). Each of these has one or more moons, or in the case of Hermione, is itself a double asteroid.

How to grow a planet

The proportion of large asteroids in the Solar System that are rubble piles is unknown. But the fact that, so far, all multiple asteroids are porous collections of gravitationally bound chunks could have implications for how planets form, Marchis said.

Astronomers think planets are built up by rocks and asteroids crashing into each other and merging, with the resulting bodies gradually growing bigger and bigger.

But collisions between two asteroids are just as likely to smash both bodies to pieces ars they are to coalesce into a single large one, potentially making planet formation a slow process.

Rubble pile asteroids, however, would merge more easily during a collision.

Asteroid Kleopatra

Radar image of dog bone-shaped Kleopatra. The asteroid is thought be made of rocky rubble held together under its own weak gravitational field.

“If a large proportion of asteroids in the early Solar System were rubble-pile, then the formation of the cores of planets would be much faster,” Marchis said.

The twins leave home

Kleopatra 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 a theory of “binary asteroid” formation, the rubble pile would have been set spinning faster by another, oblique impact 100 million years ago. The spinning asteroid would have slowly elongated and eventually split off the most distant of its moons.

The inner moon was likely shed more recently, perhaps 10 million years ago.

The International Astronomical Union’s Committee on Small Body Nomenclature has accepted the proposal of Marchis and his collaborators to name the moons after Cleopatra’s twin children—Cleopatra Selene II and Alexander Helios.

The outermost moon has been named Alexhelios and the innermost moon Cleoselene. In Greek mythology, Helios and Selene represented the Sun and Moon, respectively.

Adapted from information issued by the University of California, Berkeley / NSSDC / NASA / Stephen Ostro et al. (JPL) / Arecibo Radio Telescope / NSF.

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