Dancing Asteroid Mapped in Motion

Dancing Asteroid Mapped in Motion
Still images from a computer simulation of the motions of the two-part, near-Earth asteroid 1999 KW4. Scientists mapped the asteroid in 2001 using radar.
(Image: © E.M. DeJong, S. Suzuki, and S.J. Ostro at JPL, and D.J. Scheeres and E.G. Fahnestock from Univ. of Michigan)

Anear-Earth asteroid is made of two motley parts that dance around each otherlike a miniature Earth and Moon, a new study finds.

In May2001, the asteroid 1999KW4 passed within about 3 million miles (4.8 million km) of Earth.Scientists bounced radar off the asteroid's surface and, by measuring thestrength and lag time of the returning signals, were able to calculate many ofits physical properties.

The radarimaging shows that Alpha, KW4's larger component, is about one mile (1.5 km)wide and essentially a floating pile of rubble held together by gravity; about50 percent of it is empty space.

The smallerpiece, 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).

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

Nearbreak-up speed

The findings,detailed in the Oct. 13 issue of the journal Science, also reveal thatAlpha is spinning [animation]close to its break-up speed. It makes one complete revolution about once everythree hours; if it spun any faster, material from its equator would fly offinto space, the researchers say.

As with binarystars, scientists were able to calculate properties of KW4 [image]from a distance based on how its separate parts gravitationally affect eachother.

To get thesame kind of detailed information from a single-body asteroid, a spacecraftwould have to observe it from close orbit. NASA's NearEarth Asteroid Rendezvous (NEAR)Shoemaker spacecraft did just this with Asteroid 433Eros in 2001, as did the Japanese spacecraft Hayabusawith asteroid Itokawalast winter.

Some of theinformation gathered from KW4 could be applied to other asteroids, said Eugene Fahnestock, astudy team member at the University of Michigan who helped simulate KW4's motionsbased on the radar data.

"A lotof the things you can tell will inform our general understanding of theinternal structure of all asteroids, not just binary asteroids," Fahnestocktold SPACE.com.

Propelledby sunlight

Scientiststhink that KW4's two pieces once belonged to a larger asteroid that broke apartduring a perilously close pass by the Earth.

Anotherpossibility is that sunlight shining on the precursor asteroid caused it tospin so fast it broke in two. Because of their odd shapes, asteroids cansometimes act like solar sails, catching sunlight the way sailboats catch wind.

KW4 isclassified as a potentiallyhazardous asteroid (PHA) because it approaches relatively close to Earthcompared to other asteroids. However, the latest observations show that thereis no chance that KW4 will hit Earth within the next 1,000 years, Scheeressaid.

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