NASA finds lots of asteroids have twins or moons

The Associated Press
Friday June 01, 2001


LOS ANGELES — Astronomers are finding a new batch of binary asteroids – space rocks locked in an orbital dance with a partner. 

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

Radar images show a small moon just one-quarter of a mile across running clockwise around a companion three times as large. 

The discovery boosts to roughly 10 the number of binary asteroids imaged by radar since 1993 when the spacecraft Galileo spotted the first, 243 Ida and its tiny moon Dactyl. Another seven suspected pairs haven’t been confirmed. 

The small tally is expected to grow as astronomers refine the techniques used to view the miniature planetary systems. 

“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. 

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

Czech astronomer Petr Pravec said the study of near-Earth asteroids is becoming more important – especially if scientists are going to entertain ways to defend the planet from potential asteroid 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 vary in their size and relationship. 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 moon, just 5 percent as large. 

Collisions may have formed many of the binary asteroids, meaning each little moon is, literally, a chip off the old block. In other cases, passing close to Earth may have pulled off material, dumping it into a mini-orbit. 

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.