[meteorite-list] First Meteorites Recovered on Earth from an Asteroid Tracked in Space (2008 TC3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 25 Mar 2009 17:22:40 -0700 (PDT)
Message-ID: <200903260022.RAA20761_at_zagami.jpl.nasa.gov>

http://www.sciam.com/article.cfm?id=asteroid-meteorite-sudan-fireball

Rock Science: First Meteorites Recovered on Earth from an Asteroid
Tracked in Space

Fragments in the Sudanese desert make up an "asteroid trifecta":
discovery, prediction and recovery

By John Matson
Scientific American
March 25, 2009

Last October, asteroid monitors at the Catalina Sky Survey at the
University of Arizona in Tucson picked up a small object on an immediate
collision course with Earth. The asteroid was too small to present a
real threat - just a few meters across, it stood little chance of
penetrating the atmosphere intact. Indeed, it exploded in a
stratospheric fireball over northern Sudan less than 24 hours later - an
event witnessed by people on the ground as well as the pilots of a KLM
airliner- conforming well to astronomer's predictions for its trajectory.

But the asteroid, dubbed 2008 TC3, was
nonetheless a momentous discovery: Among the countless small objects
that strike Earth's atmosphere every year, none had ever been detected
and tracked before it impacted. Now the Sudan bolide has yielded yet
another first: Researchers report in Nature today
<http://www.nature.com/nature/journal/v458/n7237/full/nature07920.html>
that they have recovered 47 meteorites from the object in the Nubian
Desert. And lead author Peter Jenniskens, a meteor astronomer at the
SETI Institute in Mountain View, Calif., says that another search completed
earlier this month, after the paper was submitted, has upped the
meteorite count to about 280.

Astronomer Donald Yeomans, manager of NASA's Near-Earth Object Program
office <http://neo.jpl.nasa.gov/> at the Jet Propulsion Laboratory in
Pasadena, Calif., calls 2008 TC3 "a perfect asteroid trifecta,"
referring to "pre-impact discovery, successful impact prediction, and
successful sample return." (Yeomans did not contribute to the recovery
research, but his office played a leading role in tracking the
asteroid's entry <http://www.cfa.harvard.edu/mpec/K08/K08T50.html>.)

The find allows astronomers to connect the chemical composition of the
meteorite to its orbit and reflectance in the sky during tracking. "The
holy grail of asteroid science is to uniquely link a specific meteorite
and its detailed composition to a specific asteroid type," Yeomans says.
"And that has now been done without an expensive sample-return mission."

This object, which the study's authors call Almahata Sitta (Arabic for
Station Six, a train station in the desert where eyewitnesses saw the
fireball and that served as the researchers' base camp), appears to
belong to a rare class of bodies called F-class asteroids, which
constitute just 1.3 percent of all asteroids.

Chemically speaking, Almahata Sitta is a meteorite whose specific
composition is unique among meteorite collections. It is a fragile,
porous ureilite (a relatively rare kind of olivine- and
pyroxene-bearing meteorite)
containing graphite and nanodiamonds, among other materials. Its
fragility, Jenniskens says, helps explain why it broke apart so high in
the atmosphere.

With the benefit of the object's rarity as an F-class body and its
orbit, tracked backward through time, the researchers established a
possible link to a larger F-class asteroid, the 1.6-mile-
(2.6-kilometer-) diameter 1998 KU2, which may have originated from the
same parent body as Almahata Sitta.

"The orbit of the asteroid, by just tracking it for 20 hours, is 10,000
times better than anything you can get from just observing a fireball
Jenniskens says. "What's neat about this is that the big asteroid allows
you to extend back in time the evolutionary history." He notes that
scientists might be able to pinpoint the specific region of the asteroid
belt that 2008 TC3 came from with more F-class asteroids from the same
parent body.

Even the brief amount of time 2008 TC3 was tracked provided an excellent
lead on where to look- and the desert surface provided an ideal surface
for turning up the dark fragments. "The entry trajectory was very
precisely known," Jenniskens says.

The first samples were found in early December by a 45-person search
team from the University of Khartoum. (Three scientists from that
university and one from the University of Juba in Sudan are among the
co-authors of the study.) "We had many eyes and hands," Jenniskens says,
trying "to find these."
Received on Wed 25 Mar 2009 08:22:40 PM PDT