Space 'Rosetta Stone' Unlike Anything Seen Before

Meteorite fragments of the first asteroid ever spotted in space before it slammed into Earth?s atmosphere last year were recovered by scientists from the deserts of Sudan.

These precious pieces of space rock, described in a study detailed in the March 26 issue of the journal Nature, could be an important key to classifying meteorites and asteroids and determining exactly how they formed.

The asteroid was detected by the automated Catalina Sky Survey telescope at Mount Lemmon , Ariz., on Oct. 6, 2008. Just 19 hours after it was spotted, it collided with Earth's atmosphere and exploded 23 miles (37 kilometers) above the Nubian Desert of northern Sudan.

Because it exploded so high over Earth's surface, no chunks of it were expected to have made it to the ground. Witnesses in Sudan described seeing a fireball, which ended abruptly.

But Peter Jenniskens, a meteor astronomer with the SETI Institute's Carl Sagan Center, thought it would be possible to find some fragments of the bolide. Along with Muawia Shaddad of the University of Khartoum and students and staff, Jenniskens followed the asteroid's approach trajectory and found 47 meteorites strewn across an 18-mile (29-km) stretch of the Nubian Desert.

"This was an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space," Jenniskens said.

Classification

Astronomers were able to detect the sunlight reflected off the car-sized asteroid (much smaller than the one thought to have wiped out the dinosaurs) while it was still hurtling through space. Looking at the signature of light, or spectra of space rocks is the only way scientists have had of dividing asteroids into broad categories based on the limited information the technique gives on composition.

However, layers of dust stuck to the surfaces of the asteroids can scatter light in unpredictable ways and may not show what type of rock lies underneath. This can also make it difficult to match up asteroids with meteorites found on Earth ? that's why this new discovery comes in so handy.

Both the asteroid, dubbed 2008 TC3, and its meteoric fragments indicate that it could belong to the so-called F-class asteroids.

"F-class asteroids were long a mystery," said SETI planetary spectroscopist Janice Bishop. "Astronomers have measured their unique spectral properties with telescopes, but prior to 2008 TC3 there was no corresponding meteorite class, no rocks we could look at in the lab."

Cooked carbon

The chemical makeup of the meteorite fragments, collectively known as "Almahata Sitta," shows that they belong to a rare class of meteorites called ureilites, which may all have come from the same original parent body. Though what that parent body was, scientists do not know.

"The recovered meteorites were unlike anything in our meteorite collections up to that point," Jenniskens said.

The meteorites are made of very dark, porous material that is highly fragile (which explains why the bolide exploded so high up in the atmosphere).

The carbon content of the meteorites shows that at some point in the past, they were subjected to very high temperatures.

"Without a doubt, of all the meteorites that we've ever studied, the carbon in this one has been cooked to the greatest extent," said study team member Andrew Steele of the Carnegie Institution in Washington, D.C. "Very cooked, graphite-like carbon is the main constituent of the carbon in this meteorite."

Steele also found nanodiamonds in the meteorite, which could provide clues as to whether heating was caused by impacts to the parent asteroid or by some other process.

Rosetta Stone

Having spectral and laboratory information on the meteorites and their parent asteroid will help scientists better identify ureilite asteroids still circling in space.

"2008 TC3 could serve as a Rosetta Stone, providing us with essential clues to the processes that built Earth and its planetary siblings," said study team member Rocco Mancinelli, also of SETI.

One known asteroid with a similar spectrum, the 2.6-km wide 1998 KU2, has already been identified as a possible source for the smaller asteroid 2008 TC3 that impacted Earth.

With efforts such as the Pan-STARRS project sweeping the skies in search of other near-Earth asteroids, Jenniskens expects that more events like 2008 TC3 will happen.

"I look forward to getting the next call from the next person to spot one of these," he said. "I would love to travel to the impact area in time to see the fireball in the sky, study its breakup and recover the pieces. If it's big enough, we may well find other fragile materials not yet in our meteorite collections."

• Video: Asteroid Hunting
• Asteroid Exploded in Earth's Atmosphere
• Images: Asteroids