Could we survive an asteroid hit?

A massive iron asteroid crashed into the Arizona plain 49,000 years ago, creating a crater more than a kilometre wide.

The blast sent millions of tonnes of rock flying into the air and whipped up winds far faster than any gusts you would find inside a hurricane.

The effects on life in the region were devastating - but what impact would a similar blast have on a modern city such as London?

Scientists in the US have created a website to help us assess the risks.

Researchers at the University of Arizona's Lunar and Planetary Laboratory say their web tool will predict the effects different types of impactors would have if they hit different places on the planet.

A relatively modest sized asteroid of 45m (150ft) in diameter is said to strike the Earth approximately every 1,000 years.

So if the next one did come down in the UK's capital, just how far would the devastation spread and how much time would those living and working in the city have to grab their bags and run?

Gigantic fireball

To use the site, you have to have an idea of what type of object you are expecting to hit the Earth, where it will come down, and at what kind of speed and angle.

Simply type in your distance from the predicted impact point and the Earth Effects Impact Program will tell you if you should expect to be buried under a pile of debris, known as ejecta, while trees are flattened all around, or whether the effects will be limited to just loud noise and rocking cars.

The site includes descriptions of how much thermal radiation is needed to ignite grass and how far away from the impact site humans would suffer second or third-degree burns.

An asteroid impact would leave a massive crater

Planetary scientist Jay Melosh, who helped develop the site, said: "The website is valuable for scientists because they don't have to spend time digging up equations and data needed to calculate the effects.

"Similarly, it makes the information available to reporters and other non-scientists who don't know how to make the calculations."

The Arizona asteroid, thought to have been made of iron and nickel and about 45m wide, hit the Earth at a 90 degree angle at a speed of some 20km/s (12 miles a second).

Inserting these figures into the online blast calculator for a similar sized asteroid landing, say, at Oxford Circus reveals how an enormous crater almost 3km (2 miles) wide would open up across central London. It would spread across what had been Oxford Street, Bond Street, Regent Street and Marble Arch, wiping out much of Soho and Mayfair.

The immediate area would be vaporised instantly - the shoppers and office workers thronging the streets would not stand a chance.

Rock thrown up by the blast could shower over an area of several square kilometres around the crater like bombs, smashing through the roofs of embassy buildings in Belgravia and skyscrapers in the City of London.

Just 0.04 seconds after the blast, a gigantic fireball, 0.8km (0.5 miles) across would appear, igniting trees, grass, paper and clothing, with thermal radiation causing extensive third-degree burns to anyone unlucky enough to be caught within 1km of the blast.

A fraction of a second later, shockwaves from the blast would reach 5.4 on the Richter Scale, rocking cars up to a km away, moving furniture, cracking walls and making the ground unstable to walk on. Widespread panic would probably ensue, with those who were able to running out of their houses into the streets.

Blown away

The shockwaves would gradually spread out from the impact zone, reaching the London outskirts and the M25 orbital motorway about a second later.

Finally, 3.3 seconds after the impact, winds reaching 4,000km (2,500miles) per hour would rush through the city, at an ear-piercing 128 decibels, causing buildings and bridges to collapse, steel-framed office blocks to buckle and blowing down 90% of the trees.

Seismic shocks would reach the M25 after about four seconds, rocking the ground, making it hard to stand up. Church bells would ring and furniture would be overturned.

Even this far from the impact point, the fireball would be visible, cars and lorries would be destroyed, glass would be shattered and trees blown down and stripped of leaves by winds of 136km (84 miles) per hour.

The fireball following the impact would still be visible by people almost 100km (60 miles) away in Sussex, Essex or Hertfordshire, though the effects here would be milder.

Although cars might still be rocked by seismic shaking 19 seconds after the blast, the winds would reach only 23km (14 miles) per hour and would be no louder than heavy traffic noise.