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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 15 Nov 2006 14:00:16 -0800 (PST)
Message-ID: <200611152200.OAA03305_at_zagami.jpl.nasa.gov>

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."
Received on Wed 15 Nov 2006 05:00:16 PM PST