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Astronomers find lots of asteroids with twins or moons

LOS ANGELES (AP) – Astronomers have discovered a bumper crop of asteroid pairs – space rocks locked in an orbital dance with partners of equal or smaller size.

The latest discovery was announced Wednesday, when radar images showed that asteroid 1999 KW4 is two objects separated by about a mile, something that had been suspected for the past year.

In three time-lapse radar images of the asteroid pair, a small moon just one-quarter of a mile across can be seen whipping clockwise around a companion three times as large.



”Some day people will go to a binary asteroid and what an interesting sky they will see,” said Steven Ostro, of the National Aeronautics and Space Administration’s Jet Propulsion Laboratory, and leader of the team that imaged the pair.

That boosts to roughly 10 the number of binary asteroids imaged since the spacecraft Galileo spotted the first, 243 Ida and its tiny moon Dactyl. In addition, seven or so suspected pairs have been found.




While the tally is still small, it is certain to grow as astronomers refine the techniques used to spy the miniature planetary systems.

”We certainly expect to detect more. There’s no question about that,” said Jean-Luc Margot of the California Institute of Technology.

Among the population of near-Earth asteroids alone, Czech astronomer Petr Pravec estimates that as many as one in six boasts a companion. More lurk in the main belt of asteroids found between Mars and Jupiter.

Pravec said the growing number of pairs makes the study of near-Earth asteroids all the more important, especially if scientists are going to entertain ways to defend the planet from potential impacts.

”If some of them are on a collision course with the Earth in the future, it will be more difficult to divert them than if they were a single asteroid,” Pravec said.

The asteroid pairs found so far share little more than their diversity.

Pairs like 90 Antiope are nearly twins, each 50 miles or so across. Some, like 2000 DP107, are also of about equal size, but just hundreds of feet in diameter. Others are far more lopsided, like the case of 87 Sylvia, which at 176 miles across dwarfs its little moon, just 5 percent as large.

”They’re all unique in their own right,” JPL’s Ostro said. ”We don’t know how any of them formed. We don’t know any of their orbital histories and we don’t know the future of the systems. So our ignorance cuts across any difference between the individual binary systems.”

Collisions between asteroids may have formed many of the binary asteroids, meaning each little moon is, literally, a chip off the old block. In other cases, close passage by the planet Earth may have subjected asteroids to the strong tug of our planet’s gravity, pulling off material and dumping it in orbit.

”That would mean there is a constant replenishment of these systems because we keep having close encounters by these objects,” Margot said.

In the case of 1999 KW4, the objects may be the remnants of an extinct comet. Orbital observations will allow astronomers to determine the mass, density, composition and porosity of each member of the pair.

”That tells us an awful lot about these things without having to go there,” said Bill Merline, a senior research scientist at the Southwest Research Institute in Boulder, Colo., who has discovered three binary asteroids.

Observations of paired, or doublet, craters on the Earth and other bodies led astronomers to suspect that binary asteroids existed.

On Earth, the craters – all of equal age – are too large and too far apart to have been formed by a single asteroid breaking up in the atmosphere. The odds of two asteroids hitting the Earth in the same location and at the same time are slim – unless they were paired before impact. But the first binary asteroid was not seen until 1993, when Galileo spotted Ida and Dactyl while en route to Jupiter.

Not all asteroid moons orbit asteroids. The two moons of Mars, Phobos and Deimos, are probably asteroids captured in orbit by the Red Planet’s gravitational tug.


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