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Researchers find evidence for mechanismthat creates near-Earth binary asteroids

About one in six of all near-Earth asteroids are binaries – in other words, two bodies that travel in close companionship as they orbit the sun. A new study reveals that binaries most likely form when a single asteroid swings close to Earth, is ripped apart by the planet's tidal attraction, and eventually reforms into separate bodies.

In a refereed article to be released Thursday, April 11 on the Science Express Web site of the journal Science, California Institute of Technology astronomer Jean-Luc Margot and his co-authors report detailed information on the near-Earth asteroid currently assigned the rather unpoetic name 2000 DP107, and also on four other binary asteroids. 2000 DP107 comprises two bodies that are about three kilometers apart, the larger of the two being about 800 meters in diameter and the other about 300 meters. Using particularly detailed radar data, the study is a description of the system and explains how both these particular bodies and near-Earth binaries in general can be formed.

Near-Earth asteroids were formed between Mars and Jupiter, like all other asteroids, but are kicked into elliptical orbits by the gravitational influence of Jupiter and occasionally pass near Earth. An Earth-crossing orbit is one in which the asteroid actually crosses the path that Earth follows around the sun, which means the two bodies could eventually collide.

Margot, a postdoctoral researcher in the Division of Geology and Planetary Science at Caltech, led the observations in October 2000 that uncovered 2000 DP107's binary nature, just months after the asteroid was first discovered by MIT researchers. The current study, of which Margot is lead author, employs data obtained from the 70-meter Goldstone NASA tracking telescope and the Arecibo Observatory's radio telescope in Puerto Rico, which is funded by the National Science Foundation with additional support from NASA and operated by Cornell University, to yield a much more detailed picture of the two orbiting bodies and their dynamics.

Other details from the radar data show that the two bodies are probably in a tidal lock, which means that a person standing on the larger body would always see the same face of the smaller body, but a person on the smaller body would see the larger body spinning. This is exactly like the tidal lock of the Earth-moon system.

Further, the research suggests that the tidal force applied to an asteroid by a larger planet can be the cause of its breaking apart. The process, known as "spin and fission," means that a body approaching Earth is made to change its spin rate. Specifically, the tidal force tends to make an asteroid passing nearby spin at the orbital rate, which can increase rather substantially in a close approach to a planet. This increase in spin rate, coupled with the tidal pull itself, can cause a loosely-bound, gravel-like accumulation such as the near-Earth asteroids, to sling off material. Later, the weak gravitational attraction of the particles allows the material to reform in a second body.

But the most important issue raised by the paper is that near-Earth binaries are so common, says Jet Propulsion Laboratory researcher Steve Ostro, one of the authors. "The discovery of the existence and substantial abundance of binary asteroids in Earth-crossing orbits is a major one," says Ostro, an expert on the radar characterization of asteroids. "Presumably, binary asteroids have hit Earth in the past, and will do so in the future."

"Of course, the most important thing to know about any (potentially hazardous asteroid) is whether it is two objects or one, and this is why we want to observe these binaries with radar whenever possible."

"The use of radar allows precise measurements of asteroid densities, a very important indicator of their composition and internal structure," says Margot.

"Getting (near-Earth asteroid) densities from radar is dirt-cheap compared with getting a density with a spacecraft," Ostro explains.

In addition to Margot and Ostro, the other authors are Michael Nolan of the Arecibo Observatory; Lance Benner, Raymond Jurgens, Jon Giorgini, and Martin Slade, all of JPL; and Donald Campbell of Cornell University.

The article will be available Thursday on the Science Express Web site at

Contact: Robert Tindol (626) 395-3631

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