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Great balls of fire

Astronomers discover and track incoming asteroid for the first time.

NASA's Near-Earth Object programme has tracked an incoming asteroid with unprecedented precision. Credit: Getty

A space rock a few metres across exploded over northern Sudan early in the morning of Tuesday 7 October. The small asteroid mostly disintegrated when it collided with Earth's atmosphere, but fragments may have reached the surface.

Such an event happens roughly every three months. But this is "the first time we were able to discover and predict an impact before the event", says Donald Yeomans, manager of NASA's Near-Earth Object (NEO) programme at the Jet Propulsion Laboratory (JPL) in Pasadena, California.


The story began on Sunday evening, when astronomers with the Catalina Sky Survey near Tucson, Arizona, discovered the incoming object, dubbed 2008 TC3. By the next morning, three organizations — NASA's NEO office, the Minor Planet Center in Cambridge, Massachusetts, and NeoDys in Pisa, Italy — confirmed that the asteroid was racing towards Earth. Calculations predicted the object would streak through the sky above a Sudanese village called Station Six, which has a population of about ten, Yeomans says.

Even as a network of astronomers monitored the incoming asteroid, there has been only one visual report so far of its re-entry. A pilot with the Dutch airline KLM, who was flying about 1,400 kilometres southwest of the predicted impact site, apparently saw a quick flash, says Edward Beshore of the University of Arizona in Tucson. No one else has reported seeing the incoming fireball — most likely because the meteoroid crossed into Earth's shadow 56 minutes before impact. Only those close to, or directly below, the impact site would have seen the fireball, Beshore says.

The asteroid impact fireball occurred at almost the exact time and place predicted by NASA models of its trajectory. Credit: NASA/JPL-Caltech

Still, astronomer Peter Brown of the University of Western Ontario in London, Canada, was able to confirm that the space rock hit the atmosphere at around 02:45 GMT on Tuesday, within minutes of the predicted time and at the predicted location. Brown used data from a Kenya-based array of seven microbarometers, which record atmospheric sound waves to monitor countries' compliance with the Comprehensive Nuclear Test Ban Treaty. The infrasound signals show that the asteroid hit the atmosphere with an energy equivalent to detonating one to two kilotonnes of TNT.

"We can infer from that energy that 2008 TC3 was about three metres in diameter," Brown says.

Meteorite makeup

Before impact, a team of astronomers using the William Herschel Telescope at La Palma on the Canary Islands obtained a spectrogram of the object. From that they deciphered its chemical composition — probably that of a carbonaceous chondrite, a rare carbon-rich type of meteorite. "This is preliminary, and we are doing more analysis," says Alan Fitzsimmons of Queen's University Belfast, who led the team.

But if it was in fact a carbonaceous chondrite, the meteorite may have survived partially intact and hit the ground in northern Sudan. "If fragments did survive, they'd be easy to find," Yeomans says. "But I don't suggest anyone go looking for them because this place, Station Six, isn't too far from Darfur."

Astronomers hope that more observations of the fireball will soon stream in, which could help them better understand how small bodies such as this enter and break up in Earth's atmosphere. "All this is a taster of what we'll be able to do when the next generation asteroid surveys get going in the coming decade, and hopefully they will get more of these incoming objects," says Fitzgerald.

On Monday, once Yeomans' office had confirmed the incoming asteroid, he called NASA headquarters in Washington DC, which publicized the impact about seven hours before it occurred. If, however, the incoming object had been 50 to 100 times bigger than it was, the warnings would have been very different. "We would have found out several days sooner," Yeomans says, and arrangements would have been made to get people out of the area of impact.

Currently the JPL centre lists 5,681 objects that could one day crash into Earth, of which 757 are large enough — a kilometre across, or larger — to do some serious damage. 2008 TC3 was never on the list, says Yeomans, because it was too small and dark to be discovered until it was practically upon Earth.

The experience "shows that the whole NEO project is maturing to the extent that astronomers can now discover, track and observe incoming asteroids", he says. "There are still a few kinks, a few processes that need to be smoother. But we passed this test."


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Yeager, A. Great balls of fire. Nature (2008).

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