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Kleopatra’s Twins Reveal Asteroid’s True Nature

Before succumbing to her legendary death-by-snake in 30 B.C., Cleopatra VII, last queen of Egypt, gave birth to twins.

Alexander Helios and Cleopatra Selene II were born in 40 B.C., two of the eight children sired by Roman general Mark Antony during his lifetime.

As it happens, the asteroid 216 Kleopatra also had twins: Two small moons were recently found orbiting the space rock.

Kleopatra the asteroid was discovered in 1880, waaaay out in the so-called main belt, between the orbits of Mars and Jupiter. (See an interactive solar system.)

Using Earth-based telescopes in Chile and Puerto Rico, scientists discovered in 2000 that Kleopatra has two lobes connected by a long midsection, reminiscent of a dog’s bone—albeit one that’s 135 miles (217 kilometers) long.


Red Rover, Red Rover, let Kleo come over?

Radar picture courtesy Stephen Ostro et al. (JPL), Arecibo Radio Telescope, NSF, NASA

The radar images that revealed her curves also told scientists that Kleopatra is most likely made of metal, with a loose, gravelly surface texture. But the asteroid’s interior structure remained a mystery.

In 2008 higher resolution pictures taken with a telescope in Hawaii confirmed the dog-bone shape and uncovered the two orbiting offspring, each about five miles (eight kilometers) wide.

Just as children’s behavior can say a lot about a parent, the motions of these moons can be used to decipher the properties of the ancient asteroid.

Writing in this month’s issue of the journal Icarus, astronomers based in California, Texas, and France carefully charted the orbits of the two small moons.

Given the moons’ paths, sizes, shapes, and masses, the team could determine that Kleopatra must have a density of 3.6 grams per cubic centimeter.


Kleopatra and her twins, as seen by the Keck II telescope.

—Picture courtesy UC Berkeley

Assuming the asteroid really is metallic, that means it’s not very dense for its size … somewhere between 30 and 50 percent empty space.

In other words, it seems mighty Kleopatra is just a porous pile of rubble.

(Related: “Shaking Asteroid Sorts, Instead of Sheds, Its Rubble.”)

This makes perfect sense, the study authors say. Kleopatra likely has its odd shape because it’s a product of multiple collisions.

To start, a solid metallic asteroid was turned to smithereens when it had a violent clash with another of its kind millions to billions of years ago. The debris coalesced into a rubble pile asteroid.

A second, angled impact sent the hunk of loose material spinning rapidly about a hundred million years ago, causing the body to stretch into its current shape and, over time, to cast off the moons.