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Meteorites in Africa Traced to Asteroid "Parent"

Anne Minard
for National Geographic News
March 25, 2009
 
For the first time, astronomers have been able to track down meteorites from an asteroid spotted before it broke up in Earth's atmosphere—and the space rocks are among the rarest known.

Automated telescopes at the Catalina Sky Survey in Arizona first sighted the truck-size asteroid 2008 TC3 barreling toward Earth in early October 2008.

Astronomers around the world soon trained several telescopes on the asteroid, tracking it for about 20 hours until it disappeared into Earth's shadow just before impact.

The asteroid was predicted to slam into the skies over Sudan and get blown to dust 23 miles (37 kilometers) over Earth. Meteor astronomer Peter Jenniskens decided to find out for sure.

"Meteorites have never been collected from something [that disintegrated] that high up in the atmosphere," said Jenniskens of the SETI (Search for Extraterrestrial Intelligence) Institute. "We didnt know whether we were going to find something."

On a long shot, Jenniskens and astronomer Muawia Shaddad of Sudan's University of Khartoum took students on a meteorite hunt.

The group struck gold, recovering dozens of fist-size or smaller black rocks made of "a material so fragile it was not previously represented in meteorite collections," Jenniskens said.

The unusual finds come from a rare type of asteroid and may offer new clues to the origins of the solar system.

"For the first time, we have a complete evidence chain," Michael Zolensky, cosmic mineralogist at NASA's Johnson Space Center in Houston, said during a press conference.

"This meteorite is the key to a much greater understanding of asteroids."

Cosmic Interruption

As predicted, 2008 TC3 had lighted up the sky over northern Sudan on October 6 at 5:45 a.m. local time, during the predawn hour of Arabic prayer.

Reports of sightings poured in from observers along the Nile in northern Sudan. (See a Sudan map.)

One man told Jenniskens that his prayers had been interrupted.

"The whole landscape lit up," Jenniskens said, "and he rushed outside and saw a big cloud."

Another man, fearing a rocket, had thrown himself on the ground.

The initial bolt of light woke up a napping train station attendant in time for a second, smaller blast. A few minutes later, he heard the thump-thump-thump of meteorites pounding the nearby desert.

Combing the Desert

In December, Jenniskens, Shaddad, and their students combed the remote desert area near the train station.

In all, a series of searches found 280 meteorites that had once been part of 2008 TC3.

The first 47 meteorites found—detailed this week in the journal Nature—represent about 9 pounds (4 kilograms) of the original asteroid, which weighed roughly 176,000 pounds (80,000 kilograms).

The space rocks were found scattered over an 18-mile (29-kilometer) distance, indicating that the surviving asteroid fragments had been ejected at high speed during the explosion.

Rare Finds

The meteorites are "black, flaky, dark, and crumbly looking," Jenniskens said.

Analysis shows they are lined with a mineral called olivine and contain microscopic diamonds, suggesting they are rare ureilite-type meteorites from an F-class asteroid, a type that makes up 1.3 percent of all known asteroids.

Most asteroids exist today as they did when they first formed 4.6 billion years ago. They still have a lot of their original components—fine grains, specs of cosmic dust, and little molten droplets.

Other asteroid types have gotten "so big and pummeled in collisions that they melted," Jenniskens said. "The iron sinks to the core, and other material floats on top."

The F-class asteroids are in between, melted in parts, but with other areas revealing the materials that made them.

"Our rock still has a lot of the original carbon there, highly cooked," Jenniskens said.

Also, asteroid colors change with distance from the sun. Inner belt asteroids tend to be brighter, and have spectral signatures suggesting different mineral composition than in the outer edge of the belt.

In the simplest scenario, those darker ones are more primitive, noted Lucy McFadden of the University of Maryland, College Park.

However, ureilite sort of contradicts that story, because we know they have been heated, and yet they are dark.

McFadden added that the asteroid appears to be a chunk of a former almost-planet that had to have been hundreds of kilometers in size.

Saving Millions

Observing the asteroid as it approached allowed ample time for astronomers to trace its orbit. They discovered that it shares a similar orbit to another, much larger F-class asteroid, 1998 KU2, which could be the original parent of 2008 TC3.

"These guys have managed for the first time to unequivocally connect the dots between an asteroid and its meteorite samples," said Harry McSween, a geologist and meteorite expert at the University of Tennessee in Knoxville.

The new information, he added, is equivalent to "a free asteroid sample return mission that would [have] cost NASA many millions."
 

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