Graphic: down to earth

Astronomers have detected for the first time an asteroid on a collision course with Earth. They were able to track it reaching the atmosphere and exploding in a fireball.

The four-metre-long object was seen 19 hours before it reached Earth, giving scientists an unprecedented opportunity to observe its final trajectory and fiery end.

Remnants of the 83-tonne asteroid, to the surprise of astronomers, escaped being entirely burnt up during entry into the atmosphere and landed over an 18-mile stretch of the Nubian Desert in northern Sudan.

Searches yielded 47 fragments of the asteroid ranging from 1.5 to 283 grams. The meteorites weighed a total of 3.95 kilograms (8½lb). The asteroid was the first to be tracked from space to the ground.

Peter Jenniskens, of the SETI Institute in California, said the chance to follow the asteroid had been “a once in a lifetime opportunity”.

Calculations showed the space rock was travelling at 27,739mph when it reached Earth on October 7 last year.

As it hurtled into the atmosphere the air density rose to the point where the pressure became too high for the asteroid and it exploded. “It was like running into a brick wall,” Dr Jenniskens said.

People at a railway halt in the Nubian Desert called Station 6 and in the town of Wadi Halfa reported seeing a “rocket-like fireball”. The pilot of a KLM flight over Africa also reported seeing flashes over the horizon.

Professor Alan Fitzsimmons, of Queen’s University, Belfast, one of the researchers who monitored the asteroid, said: “It’s incredibly significant. This was the first near-Earth object we’ve tracked on a collision course and we’ve been able to watch it come in. We’ve been able not only to predict where it was going to hit, but, because of the warning, we’ve been able to act fast enough to get information on the object before it reaches us.”

The asteroid is considered to have been small; objects of that size in space are hard to spot. Three or four of such a size strike Earth each year. Most shooting stars are debris, ranging from the size of peas to specks of dust.

Big asteroids, those that are more than half a mile long and would be likely to cause devastation if they hit the planet’s surface, are much easier for astronomers to see, and any headed for Earth would, researchers believe, be detected years or even decades before they arrived.

The asteroid that was tracked was designated 2008 TC3 when first seen on October 6 but was later called Almahata Sitta after the area in the desert where the meteorites landed.

It was classified as an F-class asteroid and the meteorites collected from the desert are the first of that type to have been found. Only 1.3 per cent of asteroids are F-class.

By backtracking along Almahata Sitta’s trajectory, researchers, reporting their findings in the journal Nature, suggested that the rock may have broken away from a huge F-class asteroid called 1998 KU2, which was more than 1.6 miles long.

Almahata Sitta was formed 4.5 billion years ago, at the same time as Earth and the rest of the solar system. For most of the time since then it orbited the Sun in the outer asteroid belt between Mars and Jupiter.

A collision knocked it into a different orbit that brought it closer to Earth and eventually, with the help of Earth’s gravity, into a collision course.

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