[meteorite-list] Observations of Double Asteroid Stress Arecibo Radar's Vital Role in Identifying Threats in Earth's Vicinity

[Ron Baalke]

http://www.news.cornell.edu/stories/Nov06/kw4.arecibo.html

News from Chronicle Online

Observations of double asteroid stress Arecibo radar's vital role in 
identifying threats in Earth's vicinity

By Lauren Gold (LG34 at cornell.edu)
Chronicle Online
November 15,2 006

Researchers using the Arecibo Observatory's powerful radar have made 
the most detailed observations ever of a binary near-Earth asteroid 
(NEA) -- two clusters of rubble circling each other -- offering new 
clues about how such systems formed, the properties they share and 
the dynamics of their motion.

The observations, made by Steve Ostro, senior research scientist at 
the NASA/Caltech Jet Propulsion Laboratory in Pasadena (who earned 
his master's degree in engineering physics at Cornell), Jean-Luc 
Margot, assistant professor of astronomy at Cornell, and their 
colleagues, describe asteroid (66391) 1999 KW4 (called KW4). Their 
report appears in the latest issue (Nov. 24) of the journal Science. 
The double asteroid also appears on the cover.

KW4, they say, is actually a pair of light, porous clusters of rubble 
that circle each other as they orbit from a point closer to the sun 
than Mercury and then outward -- occasionally passing very close to 
Earth along the way. The bodies were discovered in 1999 but were not 
known to be binary until they were observed in May 2001, when they 
came within about 2.98 million miles of Earth -- their closest pass 
until 2036.

The researchers used antennas at Arecibo and NASA's Goldstone Deep 
Space Network -- the only telescopes with the radar capability for 
such observations. Arecibo, in Puerto Rico, is managed by the 
National Astronomy and Ionosphere Center at Cornell for the National 
Science Foundation.

KW4 is a valuable source of information for planetary scientists 
studying the formation and evolution of NEAs -- as well as for 
researchers studying how to mitigate the potential threat they pose 
to Earth. KW4 is classified a Potentially Hazardous Asteroid, but 
data show that its path will not intersect Earth's for at least 1,000 
years.

Unlike single asteroids, many of whose physical properties are 
impossible to determine from Earth-based observations, binaries can 
reveal information about their mass and density by their interaction 
with each other. The researchers were able to reconstruct the orbit, 
mass, shape and density of KW4's two components, Alpha and Beta. They 
found an oddly shaped pair of dance partners, with Alpha, by far the 
larger (1.5 kilometers, or a little less than one mile, in diameter) 
of the two, spinning as fast as possible without breaking apart, and 
the smaller and denser Beta wobbling noticeably as it orbits its 
partner.

"It's the first time we have very detailed high-resolution images 
that allowed us to derive the shape of both components," said Margot. 
Viewed pole-on, Alpha looks circular; but from the side it looks more 
like a squashed diamond with rounded edges, showing a distinct ridge 
at the equator. A particle on Alpha's surface will be pulled toward 
the equator -- which means, strangely, that the body's highest point 
is also its lowest.

The study also involved the most precise tracking of an irregularly 
shaped binary system's motion -- information vital in learning how 
the two asteroids formed.

"The overwhelming majority of these binaries have primary components 
whose spins are very near the maximum of what they can sustain. It's 
a distinctive feature," said Margot. That indicates the systems could 
have been a single asteroid -- or pieces of a larger asteroid -- that 
were sent spinning by a close encounter with another body or by the 
effects of sunlight.

The system's orbit has brought it within about 9.3 million miles of 
Earth or closer dozens of times in the last several millennia, but 
not near any other planet.

As a whole, the Arecibo/Goldstone data on KW4 take the understanding 
of NEAs to a new level of precision, say researchers. The study also 
highlights the value of both telescopes involved: NASA's Goldstone, 
which is more steerable, and Arecibo, whose radar is an order of 
magnitude more powerful.

"They are complementary and both are essential," said Margot. 
Goldstone can track objects over a longer time period, but "you 
couldn't do it at this level of precision without the Arecibo data."