ARECIBO, P.R. -- Astronomers using the world's most powerful radar system, the massive Arecibo telescope in Puerto Rico, have obtained radar images of a giant, dog bone-shaped asteroid, an apparent leftover from an ancient, violent cosmic collision. The new radar images are the first ever made of an asteroid in the main belt between Mars and Jupiter.
The asteroid, named 216 Kleopatra, measures about 217 kilometers (135 miles) long and about 94 kilometers (58 miles) wide, roughly the size of the state of New Jersey. Its strong reflection of radar signals and its coloring indicate it is mostly metal, possibly nickel-iron alloy. Kleopatra was discovered in 1880, but until now its shape was unknown.
"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 (JPL), who led a team of astronomers observing Kleopatra using the 305-meter (1,000-foot) telescope at Arecibo Observatory, part of the National Astronomy and Ionosphere Center (NAIC), operated by Cornell University for the National Science Foundation. "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.
The team's findings appear in the latest issue (May 5) of the journal Science. Ostro's team includes Michael Nolan and Jean-Luc Margot of Arecibo Observatory; Donald Campbell of Cornell, associate director of NAIC; R. Scott Hudson of Washington State University; Daniel Scheeres of the University of Michigan, Ann Arbor; Christopher Magri of the University of Maine at Farmington; and Jon Giorgini and Donald Yeomans of JPL.
The astronomers used the telescope to bounce radar signals off Kleopatra. With sophisticated computer analysis techniques, they decoded the echoes, transformed them into images and assembled a computer model of the asteroid's shape. The Arecibo telescope underwent major upgrades in the 1990s, which dramatically improved its sensitivity. "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 Cornell's Campbell.
The images were obtained when Kleopatra was about 171 million kilometers (106 million miles) from Earth. The transmitted signal took about 19 minutes to make the round trip to Kleopatra 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 coloring suggests they contain metal. These objects were once heated, melted and differentiated into a structure containing a core, mantle and crust, much as the Earth was formed. Unlike Earth, these asteroids cooled and solidified throughout, and many underwent massive collisions that exposed their metallic cores. In some cases, those collisions launched fragments that eventually collided with Earth, becoming iron meteorites like the one that created Meteor Crater in Arizona. "But we don't need to worry about Kleopatra -- it will never hit Earth," Ostro says.
"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 Hudson. "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."
Nolan 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."
The Kleopatra radar observations were supported by NASA's Office of Space Science, Washington, D.C. JPL is managed for NASA by the California Institute of Technology in Pasadena, Calif.