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First Ever Spectrum of an Asteroid Which Hit the EarthING web news release
25th March, 2009
Having less than 24 hours in which to act, astronomers using the William Herschel Telescope have been able to obtain the only spectrum of an asteroid which hit the Earth, establishing the first direct link between an asteroid and a meteorite.
The asteroid in question, 2008 TC3, was first sighted by US telescopes on 6th October 2008 and subsequent observations by an army of professional and amateur astronomers led to the discovery that it was due to enter the atmosphere the following morning. Only 4 and a half hours before the impact, UK astronomers were able to use the ISIS spectrograph to study its size and composition, and predict whether it would explode and break up in the atmosphere or reach the ground. 2008 TC3 eventually exploded and some small fragments were recovered in Sudan, East Africa. These results are published in the March 26th issue of Nature.
"This was the first ever predicted impact of an asteroid with the Earth," said Prof. Alan Fitzsimmons of Queen's University Belfast. "The faint observed brightness implied a small size, which in turn meant there was little advance warning. It was important to try and figure out what type of asteroid it was before impact, which would give us a better idea of how it would interact with the atmosphere, its likely size and where it came from."
Fellow researcher Dr. Henry Hsieh and PhD student Mr. Sam Duddy were undertaking another project at the William Herschel Telescope, and the astronomer scheduled to use the telescope that night, Dr. Gavin Ramsay from Armagh Observatory, appreciated the importance of the event and readily agreed to assist in observing the asteroid. Only 4 and a half hours before impact, they used the ISIS spectrograph to measure how light was reflected from the surface of the asteroid.
Sam Duddy explained "When we found we could observe the asteroid from the William Herschel Telescope it was an exciting couple of hours planning the details of the observations we would conduct. Actually performing the observations of an object that was certain to impact the atmosphere was a great but challenging experience."
"These observations were technically quite difficult since the object was moving fast across the sky," said Gavin Ramsay. "However, the William Herschel Telescope rose to the challenge magnificently and demonstrated just what a versatile telescope it is. There was a keen sense of excitement in the control room!"
As explained in the Nature article, some small fragments survived the high-altitude explosion that vaporised most of the asteroid. The lead author of the article, astronomer Dr. Peter Jenniskens of the SETI institute in California, teamed up with Dr. Muawia Shaddad and 45 students and staff of the University of Khartoum to search the Nubian Desert in Sudan for meteorites. In the first search campaign on 5th-8th December 2008, 15 meteorites were recovered over an area stretching 29 kilometres along the calculated approach path of the 4-meter sized asteroid. In later searches, a total of 4 kilograms of meteorites was found, which still accounts for only a small fraction of the 80 tonnes that crashed in the Earth's atmosphere.
"This asteroid was made of a particularly fragile material that caused it to explode at a high altitude of 37 kilometres , before it was significantly slowed down, so that the few surviving fragments scattered over a large area", explains Jenniskens. "The recovered meteorites were unlike anything in our meteorite collections up to that point."
After measuring how the meteorites reflected light, it was discovered that the spectra of the asteroid and meteorites agree well, which implies that the asteroid was not covered in dust and did not have much weathering from radiation in space. More importantly, the team found that 2008 TC3 was a rare type of asteroid, called F-class, corresponding to dark ureilite achondrite meteorites with a texture and composition unlike any other ureilite meteorites found on Earth.
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