April 6, 2009 Asteroid impact is a very real threat to the earth. While the statistics on the probability of such occurrences vary in certainty, it is generally accepted that objects 5-10m in diameter hit the earth once every year while the odds of a devastating asteroid strike is approximately one in ten chance of hitting per century. Clearly, with the potentially monumental costs of such a devastating strike occurring, it is in our best interest to know as much as we can about potential strikes. Any ability to see such a catastrophe in advance has the potential to be of civilization saving significance, which brings us to the unprecedented observations made of 2008 TC3, an asteroid that fell to Earth late last year.

Back in early October the observers at the Catalina Sky Survey near Tuscon, Arizona discovered a world first. It saw an object in outer space that, after a brief calculation of its orbit, had a 100% chance of hitting the earth. The sense of urgency was increased when they discovered it was going to strike the earth within 20 hours. Luckily the object, cheerfully named 2008 TC3, turned out to be only four meters across, so the worst case scenario was that it would rain down a few small rocks over a small area. While you wouldn't want one to accidental hit you as it was falling, it was of no significant risk to human life or the Earth. It was however our first chance to study an object in space that was also going to be observable after it had entered the Earth. It was a chance to test the accuracy of our space observations.

Understanding the importance of the results a team of astronomers at La Palma in the Canary islands rushed to begin tracking the asteroid. Racing against the clock they made use of the William Herschel Telescope to measure the spectrum of light that was reflected from the asteroid. Such spectrographic measurement provides essential information about the asteroid's composition and size which help us understand where the asteroid has come from. Despite the difficulty of taking such measurements on an object that was streaking across the heavens so close and so quickly, the team managed to finish taking the measurements with only four hours to spare before impact. The spectrum data gathered by the UK astronomers would allow them to establish the first direct link between an asteroid in sky and meteorites found on the ground.

By October 7th, 2008 TC3 had hit our atmosphere and burned up, out of the sight of all the astronomers that had previously been watching it. The object was torn apart in the atmosphere at an altitude of 37 kilometers, leaving many tiny fragments to burn up in the atmosphere, producing the smoky vapor trails that were recorded only on local residents' cell phones.

The next stage in the scientific measurement of the object would come from Dr. PeterJenniskens of the SETI institute in California, teaming up with Dr. Muawia Shaddad and 45 students and staff of the University of Khartoum. In December they searched a 29km area of the Nubian Desert in Sudan along the calculated approach path. In searching for the 80 tonne asteroid, they managed to collect only 4 kg of meteorites - a minuscule fraction.

The measurements taken by the Herschel Telescope in La Palma were now compared with the direct analysis of the meteorite shards. The two were found to be in close agreement, providing an excellent test of the reliability of spectral analysis. It also proved to settle the question of whether or not asteroids are covered in dust which could potentially skew spectrographic data. The meteorite turned out to be what is known as an F-class "ureilite.", which gave them an insight into the history of 2008 TC3. All ureilites are thought to have originated from a single primordial source, and structure of these meteorite fragments indicate that "it was a volcanically active body in which gas bubbles were trapped inside the porous rock."

Prof. Richard Crowther, Chair of the UN Working Group that deals with Near Earth Object threats said, "The search for and study of asteroids is extremely important as not all impacts are as harmless as this small one. 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.”

This exercise has shown that astronomers can observe a Near Earth Object in space before it strikes the Earth and predict the place of impact. Alan Fitzsimmons, from the Queen's University in Belfast, said "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.”

Stephen Saunders

Via Wired / STFC.

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