Asteroid TC3 found in Nubian desert of N. Sudan

Photo: Peter Jenniskens, a scientist at the SETI Institute in Mountain View, Calif., joined students of the University of Khartoum at the location of one of the larger finds from the first search campaign on Dec. 8, 2008. (NYT)

From New York Times:
Recovered Pieces of Asteroid Hold Clues to Early History
By KENNETH CHANG
March 25, 2009

Scientists who for the first time tracked an asteroid on a collision course with Earth, and watched as it exploded in the atmosphere, have now picked up some of the remnants on the ground.

The discovery and analysis of the meteorites, reported in Thursday’s issue of Nature, give scientists solid data on the composition of meteorites that originate from at least one type of asteroid, known as F-class.

Millions of asteroids, mostly small, whirl around the solar system, and over the years people have picked up tens of thousands of meteorites, the surviving rock fragments of asteroids that collide with Earth.

“But we don’t know where a single one of them comes from,” said Michael E. Zolensky, a cosmic mineralogist at the Johnson Space Center in Houston, during a NASA-sponsored news conference on Wednesday.

That changed when Petrus M. Jenniskens, a scientist at the SETI Institute in Mountain View, Calif., organized a search team to comb through a Sudan desert to look for pieces of an asteroid that had been spotted less than a day before it hit Earth last year.

“For the first time, we can dot the line between the meteorite in our hands and the asteroid astronomers saw in space,” said Dr. Jenniskens, the lead author of the Nature paper.

The 280 pieces, about 10 pounds in total, are of a rare type of meteorite known as ureilites. The hodgepodge of minerals in ureilites indicates they were heated up but not fully melted, suggesting that they were once part of a much larger asteroid that possessed planetlike geological processes.

Because ureilites are now linked to F-class asteroids, also rare, the hope is that scientists can now determine the history of asteroids, which contain some of the most primitive materials left over from the early solar system.

“It’s like the first step towards a Rosetta stone of understanding asteroids,” Dr. Zolensky said.

The cascade of discovery started when Richard Kowalski, working with the of the University of Arizona, spotted a moving white dot on his computer screen late Oct. 5 at an observatory on Mount Lemmon outside Tucson. He sent the coordinates to the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics.

A computer program at the center automatically calculates the orbits of reported projects, but it failed for the object Mr. Kowalski reported, because Earth’s gravity appeared to be greatly distorting its orbit. The next morning, when Timothy B. Spahr, the center’s director, took a closer look, the asteroid, designated 2008 TC3, looked as if it was being pulled directly into Earth.

Dr. Spahr notified Steven R. Chesley, a scientist in NASA’s Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. “For the first time ever, I saw an impact probability of 100 percent pop up on the computer screen,” Dr. Chesley said. “And this was, needless to say, the kind of thing that makes you sit up straight in the chair.”

Because the asteroid was dim, the astronomers knew that it was small, about the size of a car and 80 tons, and would not cause any significant damage. Notice quickly spread, and asteroid watchers, professional and amateur, pointed their telescopes toward it.

With hundreds of observations coming in during the day, the computers at the Jet Propulsion Laboratory refined the trajectory. “Our last pre-impact prediction was accurate to about a kilometer and a couple tenths of a second in the impact time,” Dr. Chesley said.

The asteroid disintegrated about 23 miles over the Nubian desert of northern Sudan about an hour before sunrise, 20 hours after Mr. Kowalski discovered it. It released the energy of one to two kilotons of TNT.

“We figured that probably was the end of the story,” Dr. Chesley said. The expectation was that none of 2008 TC3 survived the passage through the atmosphere.

But still, Dr. Jenniskens, an expert on meteor showers, wondered. “If we could find something, it would be tremendous,” he said. “So you have to try. It was really a long shot.”

In December, he flew to Sudan and organized a team of 45 students and staff members from the University of Khartoum to search through the desert for fragments of 2008 TC3. And they found the shiny black fragments that had come from space.

A version of this article appeared in print on March 26, 2009, on page A21 of the New York edition.