Friday, August 14, 2009

First-Ever Asteroid Tracked From Space to Earth


For the first time, scientists were able to track an asteroid from space to the ground and recover pieces of it. The bits are unlike anything ever found on Earth.The asteroid was spotted entering Earth’s atmosphere over Sudan in October and was believed to have fully disintegrated, but an international team found almost 280 pieces of meteorite in a 11-square-mile section of Sudan’s Nubian Desert. The largest was the size of an egg. Lab analysis showed that the rocks belong to a rare class of asteroid that has never been sampled in such a pristine state, so it could fill some gaps in our understanding of the solar system’s early history.

"It’s the first time we’ve been able to track something through the air and watch it fly apart and then find pieces of it," microbial ecologist Rocco Mancinelli of SETI, a co-author of a study on the meteorite pieces Wednesday in Nature, told

Finding the meteorites was a long shot, but because the rocks would be so important, meteor astronomer Peter Jenniskens of SETI, lead author of the study, took a bus loaded with 45 students and staff from the University of Khartoum deep into the desert to hunt for them. A 10-hour bus ride and an 18-mile trek through the sand took them to the remote area where scientists thought the rocks, if they existed, would be. The group began sweeping the desert in a line and two hours later the first meteorite was found by a student.

"It was very, very exciting. Everybody was celebrating," Jenniskens said. "You have to remember how important it is to find a piece linked to an asteroid we have seen in space."

Scientists use asteroids to learn about the early solar system because they are among the oldest objects in the universe and can remained relatively unchanged from when they formed, providing a historical snapshot. It is estimated that hundreds of meteorites fall to Earth each year, but only a few end up in the hands of scientists.

Because asteroids are typically surrounded by a shroud of dust as they travel through space, they reflect light differently in flight than they do in the lab, making it difficult to connect meteorites found on
Earth with particular types of asteroids. But because the car-sized
Sudan asteroid was spotted 20 hours before it hit Earth’s atmosphere, scientists were able to determine that it was an unusual type of asteroid that falls between the two most common types.

For the first time, scientists can begin to connect the light signatures of asteroids in space to signatures of meteorites in the lab.

"This is like the first step toward a Rosetta Stone for classifying asteroids," said study co-author, cosmic mineralogist Michael Zolensky, at a press conference at NASA’s Johnson
Space Center Wednesday.

The team, led by Jenniskens, hopes the intermediate meteorites will reveal details about how planets formed in the early solar system.

"It gives a window on the past," Jenniskens told "You see a little piece of early history coming into focus."

The Sudan meteorites are from a rare class of asteroids known as ureilites, which contain a lot of carbon, much of it in the form of graphite, as well as diamonds produced by shock. The Sudan specimens show evidence of volcanic activity, which means they came from a parent body that was almost big enough to call a planet.

"It’s showing us that this asteroid had planet-like activity on it,"
said astronomer Lucy McFadden of the University of Maryland, who was not involved in the study. "We’re lucky that the Earth was in the right place and placed itself in front of this new meteorite."

But that planet shut down, lost its heat source and quit growing,
Zolensky said. This gives scientists a glimpse of a specific stage in the evolution of planets.

"What this does is give us first-hand knowledge of what happens when planetesimals form from one that fell apart and failed to become a planet," Mancinelli said. "It really tells you what happens when these rocks bang into each other and some actually stick to each other and form a planetesimal."

There’s nowhere else to find this sort of information, he said, because you need the planet forming process to stop before it becomes a full-fledged planet.

"This is highly unusual," Mancinelli said. "It is key to understanding the early solar system."

Space scientist Ted Bunch at Northern Arizona University studies these rare meteorites. "Of the tens of thousands of meteorites that have been found, there’s probably only 100 that are ureilites," he said.

Ureilites are interesting in that they have a very primitive composition, Bunch said. And the Sudan ureilite pieces are even more rare because they were picked up so soon after they fell. Meteorites that have been lying around on Earth for a long time can become contaminated.

"To see something which is pristine, the chance of contamination is pretty low," Bunch said. "Whatever you see in the stone is what came from outer space, with no contribution from Earth."



Image 1: The contrail left by the asteroid’s passage through the atmosphere.Credit: Muawia Shaddad.
Image 2: Typical meteorite fragment. Credit: Muawia Shaddad.
Image 3: This space-based view of the Nubian Desert shows altitude in kilometers
(in white circles) and meteor locations in red. Credit: NASA
Image 4: Students from the University of Khartoum line up to go meteorite hunting in the Nubian desert.

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