Bow-wowing them with radar

At the end of a lecture that he gave at the Asteroids 2001 meeting in Santa Flavia, Italy, last month, astronomer Steve Ostro reached into his backpack and threw a bunch of dog bones into the audience. The crowd couldn’t get enough.

216 Kleopatra: A 7-centimeter-long plastic replica. Design Cast Studios

The highest-resolution radar image of an asteroid reveals that 1999 KW4 consists of two orbiting bodies. The smaller body is about 400 meters in diameter. Margot/Caltech

No, the scientists weren’t hungering for something other than Sicilian cuisine. It’s just that they wanted to take home their very own plastic replica of 216 Kleopatra, an asteroid shaped like a classic dog bone.

Radar images taken last year revealed the asteroid’s puzzling shape. Astronomers still don’t have a clue to how it formed, notes Ostro of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

By measuring the shape, rotation, and location of asteroids, radar can determine the orbits of these primitive rocks–and whether any might be on a collision course with Earth, Ostro says (see story, “A Rocky Bicentennial,” in this issue).

Unlike other astronomical studies, which record sunlight reflected from a solar system body or the light generated by a star or galaxy, radar relies on an artificial source of radiation.

Scientists beam precisely tuned radio waves at an asteroid. By measuring the time it takes for the waves to bounce back to radio telescopes on Earth, researchers can determine the rock’s location. A shift in the signal’s frequency indicates the asteroid’s rate of rotation down to one-tenth of a millimeter per second–the speed of the minute hand on a kitchen clock. The intensity of the reflected waves indicates the asteroid’s composition.

Using this information, scientists can create animations of asteroids and even generate three-dimensional replicas that you can hold in your hand–such as Ostro’s model of Kleopatra.

Using the 70-meter Goldstone antenna in California’s Mojave Desert, Ostro and his colleagues imaged an asteroid when it came within 5 million kilometers of our planet in May. The radar measurements revealed that asteroid 1999 KW4 consists of a smaller body orbiting a larger one.

It’s the third binary asteroid among near-Earth objects imaged by radar.

Jean-Luc Margot of the California Institute of Technology in Pasadena and his collaborators, including Ostro, then examined the rock at higher resolution with the recently upgraded 305-m radio telescope at the Arecibo Observatory in Puerto Rico. For the first time, they used radar to discern asteroidal features as small as 7.5 m–about the length of a stretch limousine. The ultrasharp picture, which Margot unveiled at the meeting, shows a crater not readily seen in other images, he says.

“The radar data are absolutely spectacular,” says Andrew F. Cheng of Johns Hopkins Applied Physics Laboratory in Laurel, Md., a leader of a robotic mission that landed on the asteroid 433 Eros last February. “I look at what those guys have been able to do [with radar], and it’s so much easier than sending a spacecraft,” he says.

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