BY DR EMILY BALDWIN
Posted: 26 March, 2009
Nearly 280 meteorite fragments from the high altitude explosion of asteroid 2008 TC3 have been recovered from the Nubian Desert, the first time meteorites have been directly connected to an asteroid observed to strike the Earth.
On the morning of 7 October an asteroid exploded in the Earth’s upper atmosphere at an altitude of 37 kilometres, having been detected less than 24 hours beforehand. It was widely believed that the four metre-wide asteroid had fully disintegrated as it plunged through African skies, but 280 pieces have since been collected from the Nubian Desert.
University of Khartoum students and staff prepare to search for meteorite remnants.
The recovery effort was lead by Peter Jenniskens of the SETI Institute’s Carl Sagan Center, who teamed up with Mauwia Shaddad of the University of Khartoum. Together with students and staff from the university, the intrepid team scoured a 29 kilometre track corresponding to the asteroid’s incoming trajectory. The first fragment was found after just two hours of searching, and in total, almost 280 fragments were recovered, with the largest about the size of an egg.
“This was an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space,” says Jenniskens, the lead author on this week’s Nature cover story article that describes the findings.
Eyewitness reports say that the impact left a cloud of dust in the atmosphere, and experts were skeptical that any fragments would have reached the ground intact, since meteorites have never before been collected from such a high altitude explosion. Furthermore, laboratory analysis on the fragments reveals them to be more unusual than any other meteorite found on Earth to date.
Michael Zolensky, cosmic mineralogist at NASA’s Johnson Space Centre, described the properties of the meteorite at last night’s press conference. “It has large cavities and in places it has 40 percent porosity. These fragments are small pieces of a larger parent asteroid that was destroyed long ago, but despite its porosity was still strong enough to survive entering the atmosphere. We collected the strongest parts, so as porous as this was you can imagine the original asteroid was probably more porous still.” He adds: “Its texture is probably due to some volcanic or surface process on the asteroid.”
A typical example of a meteorite remnant linked to asteroid 2008 TC3, with a dark scruffy texture.
Meteorites are the key to the much greater understanding of asteroids, which provide important clues in unraveling the early history of the Solar System where collisions between planetary building blocks were commonplace. “The parent asteroid probably had planet-like activity on it, like volcanism, and these gaseous cavities were trapped inside,” says Lucy McFadden, professor of astronomy at the University of Maryland in College Park. “It’s a very exciting result.”
Much of the excitement surrounding 2008 TC3 is that this is the first time laboratory analysis of a meteorite has been married up to direct observations of the parent asteroid, and before it careered through our atmosphere, astronomers had the opportunity to track it and take measurements of properties such as the amount of sunlight reflected by the object, which yields key information on its mineralogical composition. Analysis placed 2008 TC3 into the category of F-class asteroids. “F-class asteroids were long a mystery,” says planetary spectroscopist Janice Bishop of the SETI Institute. “Astronomers have measured their unique spectral properties with telescopes, but prior to 2008 TC3 there was no corresponding meteorite class, no rocks we could look at in the lab.”
Image taken by a cellphone of the contrail left by 2008 TC3 during its decent. Image courtesy: Shaddad.
Studying the nature of F-class asteroids could also contribute to designing ways of protecting the Earth from possible future dangerous impactors. The fact that 2008 TC3 exploded so high in the atmosphere meant it was very weak, and if a similar, but much larger, kilometre-sized F-class asteroid was headed our way, then appropriate mitigation could be taken. But striking such a fragile asteroid with an atomic bomb, for example, would merely turn it into a swarm of deadly impactors, highlighting the importance in selecting the right defence technique for each asteroid.
But as smaller near-Earth asteroids are incovered, Jenniskens expects more incidents similar to 2008 TC3, which will help scientists to learn about the inventory of asteroids as well as advance studies in Earth impact protection. “I look forward to getting a call from the next person to spot one of these,” he says. “I would love to travel to the impact area in time to see the fireball in the sky, study its breakup and recover the pieces. If it’s big enough, we may well find other fragile materials not yet in our meteorite collections.”