This is the very first time that an asteroid that hit the Earth has been studied before entering our atmosphere, allowing the scientists to predict whether it would explode and break up in the atmosphere or reach the ground – which determines whether an asteroid poses any threat. The results of the international collaboration studying the asteroid are published in this week's (March 26th) issue of Nature.
The asteroid in question - 2008 TC3 - an 80 tonne, 4 meter asteroid with a rare composition, was first sighted by US telescopes on 6th October 2008. Subsequent observations by an international army of professional and amateur astronomers led to the discovery that it was racing towards our planet and was due to enter the atmosphere the following morning.
"This was the first ever predicted impact of an asteroid with the Earth and the very first time an asteroid of any size has been studied before impact," said Prof. Alan Fitzsimmons, from the 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 its size and where it came from. This event shows we can successfully predict the impact of asteroids even with a short warning time, and obtain the astronomical observations necessary to estimate what will happen when the asteroid reaches us."
The spectrum gathered by the UK astronomers allowed them to obtain information on the size and composition of the asteroid and to establish the first direct link between an asteroid and the individual meteorites produced as it breaks up in our atmosphere. Not only does this help to validate the whole process of remotely surveying asteroids but comparing the asteroid and meteorite data tells us that 2008 TC3 may have only spent a few million years existing in the Inner Solar system before it hit our planet.
The team that observed the asteroid were already at the telescope when they got the news of its approach. Only 4 and a half hours before impact, they were able to use the ISIS spectrograph on the William Herschel Telescope to measure how light reflected from its surface.
Sam Duddy from Queen's University Belfast explained, "When we found we could observe the asteroid from the 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 Dr. Gavin Ramsay from Armagh Observatory. "However, the William Hershel 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."
Some small fragments survived the high-altitude explosion that vapourised 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, 15 meteorites were recovered over an area 29 km long along the calculated approach path of the 4-meter sized asteroid. In later searches, a total of 4 kg of meteorites was found, which still accounts for only a small fraction of the 80 tonnes that crashed into the Earth's atmosphere.
"This asteroid was made of a particularly fragile material that caused it to explode at a high 37 km altitude, before it was significantly slowed down, so that the few surviving fragments scattered over a large area," explains Dr. Peter Jenniskens of the SETI institute in California. "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 before.
Prof. Richard Crowther of the Science and Technology Facilities Council (STFC) and Chair of the UN Working Group that deals with near earth object (NEO) threats said, "The search for and study of asteroids is extremely important as not all impacts are as harmless as this small one in October. Larger impacts of the size associated with the Tunguska event of 1908 occur every few hundred years and even larger impacts with asteroids and comets the size of mountains occur every few tens of millions of years. Any extra knowledge we can gain about asteroids will help us mitigate the potential effects of such impacts in the future."
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