The University Record, June 5, 2000

U-M engineer on team that discovers bone-shape of asteroid Kleopatra

By Janet Harvey-Clark
College of Engineering

Scheeres holds a model of the asteroid 216 Kleopatra. Photo by Bill Wood, U-M Photo Services
First there was Pluto. Now a team of astronomers has given the “dog” a bone. The team, which included an assistant professor from the Department of Aerospace Engineering, has collected radar images of a metallic, dog bone-shaped asteroid the size of New Jersey, an apparent leftover from an ancient, violent cosmic collision.

The asteroid, named 216 Kleopatra, is a large object in the main asteroid belt between Mars and Jupiter. It measures about 217 kilometers (135 miles) long and about 94 kilometers (58 miles) wide. 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,” said Steven Ostro of NASA’s Jet Propulsion Laboratory, who led a team of astronomers observing Kleopatra with the 305-meter (1,000-foot) telescope of the Arecibo Observatory in Puerto Rico.

Daniel J. Scheeres, assistant professor of aerospace engineering, used the measured shape of the asteroid to evaluate conditions on its surface—namely, what “local gravity” is (between 3 and 10 milliG), how fast an object must be thrown from the asteroid to “escape” (between 40 and 200 m/s depending on where you are on the asteroid), and what the surface “slope” over the asteroid is (less than 40 degrees everywhere), making for a very relaxed surface with no steep cliffs or hillsides. Scheeres also evaluated whether or not the asteroid is really two separate “ends” in orbit about each other, discovering that it isn’t.

“Based on these computations,” Scheeres said, “we can infer that the asteroid is covered with a loose ‘soil’ (called regolith) consisting of debris from previous, smaller collisions of Kleopatra with other main belt asteroids.”

The astronomers used the telescope to bounce radar signals off of 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 and made it feasible to image more distant objects.

In fact, these new radar images are the first ever made of a main belt asteroid. They were obtained when Kleopatra was about 171 million kilometers (106 million miles) from Earth. Traveling at the speed of light, 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 said.

Kleopatra is one of several dozen asteroids whose coloring suggests they contain metal. Kleopatra’s strong reflection of radar signals indicates it is mostly metal, possibly nickel-iron alloy. 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, those 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 said.

“It is amazing that nature has produced a giant metallic object with such a peculiar shape,” Ostro added. “We can think of some possible scenarios, but at this point none are 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 team’s findings appeared in the May 5 issue of the journal Science, with an image of the asteroid on the cover. In addition to Scheeres, Ostro’s team included Hudson; Nolan and Jean-Luc Margot of the Arecibo Observatory; Donald Campbell of Cornell University; Christopher Magri of the University of Maine at Farmington; and Jon Giorgini and Donald Yeomans of the Jet Propulsion Laboratory.

The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, operated by Cornell University for the National Science Foundation. 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.