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For the first time, scientists have tracked an asteroid, watched it explode, and have now picked up some of the remnants on the ground.
Kenneth Chang reports in the NY Times that a report in the latest issue of Nature claims the discovery and analysis of the meteorites has given scientists their first solid data on the composition of at least one type of meteorite from at least one type of asteroid, known as F-class.
Millions of mostly small asteroids whirl around the solar system. Over the years, people have picked up tens of thousands of meteorites, the surviving fragments of asteroids that collide with earth.
“But,” says Michael E Zolonsky, a cosmic mineralogist at the Johnson Space Center in Houston, “we don’t know where a single one of them comes from.”
That has changed. Petrus M Jenniskens, a scientist at the SETI Institute in Mountain View, California, 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,” says Dr Jenniskens, the lead author of the paper.
The 280 pieces, about 10 pounds in total, are of a rare type of meteorite known as “ureilites.” These are made of a hodgepodge of minerals which indicate that the ureilites were heated, but not fully melted. This suggests that they were once part of a much larger asteroid that possessed planetlike geological processes.
Because ureilites are now linked to F-class asteroids, the hope is that scientists might now be able to 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,” says Zolensky.
It began when Richard Kowalski, working with the Catalina Sky Survey of the University of Arizona, spotted a moving white dot on his computer screen at an observatory on Mount Lemmon outside Tuscon late on October 5, 2008.
He sent the coordinates to the Minor Planet Center at the Harvard-Smithsonian Center for Astrophysics, where a computer program that automatically calculates the orbits of reported objects failed for this object. Earth’s gravity appeared to be greatly distoring its orbit.
The next morning, when the center’s director, Timothy Spahr, took a closer look, the asteroid, designated 2008 TC3, looked as if it were being pulled directly into Earth.
Spahr notified Steven R Chesley, a NASA scientist in the Near-Earth Object Program office in Pasadena California.
“For the first time ever, I saw an impact probability of 100 percent pop up on the computer screen, ” says Dr Chesley. “And this was, needless to say, the kind of thing that makes you sit straight up in the chair.”
Astronomers knew the asteroid was small because it was dim; it was about the size of a car and would not cause significant damage. Word spread quickly, and asteroid watchers, professional and amateur, all pointed their telescopes toward it.
With hundreds of observations coming in during the day, the computers at NASA’s laboratory in California were able to refine the trajectory. “Our last pre-impact prediction was accurate to about a kilometer and a couple tenths of a second in the impact time,” says Chesley.
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.
Says Chesney, “We figured that was probably the end of the story.” The expectation was that none of 2008 TC3 survived the passage through the atmosphere.
But still, Dr Jenniskens, who is an expert on meteor showers, said “If we could find something, it would be tremendous. So you have to try. It was really a long shot.”
In December, Jenniskens flew to Sudan and organized a team of 45 students and staff members from the University of Khartoum. The goal was to search throught the desert for fragments of 2008 TC3.
And they found the shiny black fragments that had come from space.
sole source: Kenneth Chang’s article in the NY Times on 3/26/09. www.nytimes.com
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