Astronomers see 'dog bone' asteroid

Slowly spinning in space and shaped like a dog bone

By BBC News Online science editor Dr David Whitehouse

Astronomers have obtained extraordinary radar images of a giant, dog bone-shaped asteroid.

The radar images are the first ever of an asteroid in the main asteroid belt between Mars and Jupiter.

Getting images of Kleopatra from Arecibo was like using a Los Angeles telescope the size of the human eye's lens to image a car in New York

Steven Ostro, Nasa

Discovered in 1880, and designated 216 Kleopatra, the asteroid is a leftover from an ancient and violent collision. It measures about 217 km (135 miles) long by about 94 km (58 miles) wide. Mostly made of metal, it is a strong reflector of radar signals.

"With its dog bone shape, Kleopatra has the most unusual shape we've seen in the Solar System," says Steven Ostro of Nasa's Jet Propulsion Laboratory, who led a research team using the 305-m (1,000-ft) Arecibo telescope.

"Kleopatra could be the remnant of an incredibly violent collision between two asteroids that did not completely shatter and disperse all the fragments," Ostro says.

Metal cores

The astronomers used the mighty Arecibo telescope to bounce radar signals off the space rock. They decoded the echoes and transformed them into images.

Transmitted signals took about 19 minutes to make the trip to the asteroid and back

"The Arecibo radar instrument now has the capability to image small near-Earth asteroids, and some main-belt asteroids such as Kleopatra, with resolutions far exceeding that of optical telescopes, including the Hubble Space Telescope," says Donald Campbell of Arecibo observatory.

The images were obtained when Kleopatra was about 171 million km (106 million miles) from Earth. The transmitted signal took about 19 minutes to make the trip to the asteroid and back.

"Getting images of Kleopatra from Arecibo was like using a Los Angeles telescope the size of the human eye's lens to image a car in New York," Ostro says.

Kleopatra is one of several dozen asteroids whose spectrum suggests they contain metal. They were once heated, melted and changed into a body that contained a core, mantle and crust, much like the Earth. Unlike Earth, these asteroids cooled and solidified throughout, and then were subjected to violent collisions that exposed their metallic cores.

Separate lobes

"The radar-based reconstruction of Kleopatra's shape shows the object's two lobes connected by a handle, forming a shape that resembles a distorted dumbbell, or dog bone," says R Scott Hudson of Washington State University.

It measures about 217 km (135 miles) long by about 94 km (58 miles) wide

"The shape may have been produced by the collision of two objects that had previously been thoroughly fractured and ground into piles of loosely consolidated rubble. Or, Kleopatra may once have been two separate lobes in orbit around each other with empty space between them, with subsequent impacts filling in the area between the lobes with debris," he adds.

Michael Nolan of Arecibo comments that the radar observations "indicated the surface of Kleopatra is porous and loosely consolidated, much like surface of the Moon, although the composition is different."

He says: "Kleopatra's interior arrangement of solid metal fragments and loose metallic rubble, and the geometry of fractures within any solid components, are unknown. What is clear is that this object's collision history is extremely unusual."

Ostro voices his amazement "that nature has produced a giant metallic object with such a peculiar shape." He says: "We can think of some possible scenarios, but at this point none is very satisfying. The object's existence is a perplexing mystery that tells us how far we have to go to understand more about asteroid shapes and collisions."