A near-Earth asteroid is made of two motley parts that dance around each other like a miniature Earth and Moon, a new study finds.

In May 2001, the asteroid 1999 KW4 passed within about 3 million miles (4.8 million km) of Earth.

Scientists bounced radar off the asteroid's surface and, by measuring the strength and lag time of the returning signals, were able to calculate many of its physical properties.

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The radar imaging shows that Alpha, KW4's larger component, is about one mile (1.6 km) wide and essentially a floating pile of rubble held together by gravity; about 50 percent of it is empty space.

The smaller piece, Beta, is about a quarter of Alpha's size and elongated, like a peanut.

Beta orbits Alpha every 17 hours from a distance of about 1.5 miles (2.5 km).

"They are so close together that when one rotates, it affects the other's movements," said study team member Daniel Scheeres of the University of Michigan.

Near break-up speed

The findings, detailed in the Oct. 13 issue of the journal Science, also reveal that Alpha is spinning close to its break-up speed.

It makes one complete revolution about once every three hours; if it spun any faster, material from its equator would fly off into space, the researchers say.

• Click here to view animation of the two parts of KW4 orbiting one another.

As with binary stars, scientists were able to calculate the properties of KW4 from a distance based on how its separate parts gravitationally affect each other.

To get the same kind of detailed information from a single-body asteroid, a spacecraft would have to observe it from close orbit.

NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft did just this with Asteroid 433 Eros in 2001, as did the Japanese spacecraft Hayabusa with asteroid Itokawa last winter.

Some of the information gathered from KW4 could be applied to other asteroids, said Eugene Fahnestock, a study team member at the University of Michigan who helped simulate KW4's motions based on the radar data.

"A lot of the things you can tell will inform our general understanding of the internal structure of all asteroids, not just binary asteroids," Fahnestock told SPACE.com.

Propelled by sunlight

Scientists think that KW4's two pieces once belonged to a larger asteroid that broke apart during a perilously close pass by the Earth.

Another possibility is that sunlight shining on the precursor asteroid caused it to spin so fast it broke in two.

Because of their odd shapes, asteroids can sometimes act like solar sails, catching sunlight the way sailboats catch wind.

KW4 is classified as a potentially hazardous asteroid because it approaches relatively close to Earth compared to other asteroids.

However, the latest observations show that there is no chance that KW4 will hit Earth within the next 1,000 years, Scheeres said.

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