Mile-wide asteroid heading towards Earth poses greatest threat yet, scientists warn

An asteroid more than a mile wide is heading for earth, posing the greatest threat yet by an object approaching the planet, scientists have warned.

The asteroid – called 2002 NT7 – was spotted only three weeks ago, but could strike on 1 February 2019, the US space agency Nasa said. Itis the first asteroid to rank positive on Nasa's Palermo scale, which combines the urgency of the object's threat with its potential effect. All other known objects have had negative values.

Bennt Peiser of John Moores University in Liverpool said the 1.2 mile-wide object had become "the most threatening object" in the short history of asteroid detection.

Gerrit Verschuur, an astrophysicist and radio astronomer at Rhodes College in Memphis, Tennessee, said the impact would create a fireball so intense it would kill anyone who could see it, after which material thrown into the air would shower half the world with flaming debris. "It would be as if the sky itself had caught fire," he said.

The heat would set fire to forests and cities, after which dust would fill the atmosphere, obscuring the sun for a month. That in turn would kill plants and animals, so that only creatures that lived underground would have a strong chance of survival.

But there are still large uncertainties about the asteroid's orbit. Dr Andrew Coates of the Mullard Space Science Laboratory in London said: "Most likely it will not hit us, but it's the highest-risk of any object that we know of at the moment." But Donald Yeomans of Nasa told the BBC that the margin of error in the predicted orbit might be "several tens of millions of kilometres".

Professor David Hughes of Sheffield University's astronomy department said: "They have too few figures to be able to predict this yet. They've only seen it for three weeks, when its orbit takes three years. The errors in the orbit they've calculated are sure to be huge.

"The more important question, though, is what they would do if they find it is going to hit the earth. Do you blow it up? Deflect it? Finding out the best way to do that could take a decade."

Spaceguard UK, which aims to raise public awareness of the threat from asteroids, and other astronomers have called for more funding to study how "earth-crossing" asteroids might be deflected.

In 1908, a comet estimated to be about 230ft (70 metres) wide hit Tunguska in Siberia with the force of a 12-megaton atom bomb. Had it entered the atmosphere four hours earlier, it would have hit London. Such impacts could occur once every century, astronomers predict.

Preventing an impact would require a rocket to be launched while the asteroid was still distant, to deflect its path by hitting it or breaking it up by detonating an atomic weapon. But that would require years of forward planning and precise knowledge of the object's path.

The asteroid is one of about 450 objects known to pose a threat to the earth because their elliptical orbits – which can take decades to complete – intersect the Earth's.