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.
The radar
imaging 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; 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 [animation]
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.
As with binary
stars, scientists were able to calculate properties of KW4 [image]
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 (PHA) 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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