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Meteorite impact crater

(Photograph) - Photograph by Yann Arthus-Bertrand

Insulated by our oxygen-rich atmosphere and with gravity to keep our feet firmly on the ground, it's easy to forget that we live on a planet which is hurtling through the universe at incredible speed.

And the Earth is not the only thing going places. In the cosmic shooting gallery that is outer space, there are millions of asteroids and comets flying around, and sometimes their paths meet. Objects less than 50 metres in diameter generally burn up in our atmosphere, but every year dozens of small lumps of rock, ranging in size from a pea to a basketball, make it through, usually landing out of harm's way, at sea or far from centres of population. Very rarely, something huge breaches the Earth's natural defences and lands with a cataclysmic thud.

This is what happened here at Gosses Bluff, in Australia's Northern Territory, 142 million years ago. This three-mile wide, 150-metre high crater is the remnant of the central uplift of the crater. The original rim, which has eroded, was four times as wide, 10 times as high and laid waste to about 150 square miles of land. Globally, this may only represent a dent in the Earth's surface, one of only 120 terrestrial impact sites to have been identified. But each of these had devastating consequences at the time, generating huge temperatures, earth tremors and tidal waves.

There are no recorded cases of a meteorite killing a human, though there have been some close encounters. In 1911, a dog was struck down in Egypt, and slices of the asteroid that did for a cow in Venezuela in 1972 are collectors' pieces. In 1954, a meteorite came through the roof of a house in Alabama and bruised a woman's arm, and the car that was hit by a small meteorite in New York in 1992 was sold to a museum. The chance of a collision on this scale in our lifetime is minute.

Dr Richard Binzel of the Massachusetts Institute of Technology has devised the Torino scale, similar to the Richter scale for earthquakes, by which the chaotic orbits of near Earth objects (NEOs) are tracked, and their danger to the planet assessed. He says that every 100,000 to one million years an NEO more than a mile across will hit the Earth, triggering a "global climactic catastrophe".

It's happened before. Sixty-five million years ago, a meteorite measuring between six and 12 miles across hit Chicxulub on Mexico's Yucat n peninsula. Huge methane deposits in the Earth's crust were released, setting off a global firestorm. The planet was in darkness for six months and temperatures plummeted. Half the species on Earth - including dinosaurs - were wiped out.

Although the number of NEOs is not increasing, our awareness of them is. More than 500 NEOs greater than one kilometre in diameter are currently being tracked, but another 1,000 are thought to be travelling undetected. Following a campaign headed by MP Lembit Opik, whose grandfather was a renowned meteorologist, the Government set up a task force, whose report last year concluded: "If ever there was an issue affecting the whole world, it is the threat from near Earth objects."

It may not arrive for a million years, but you've been warned.

Steve Farrar

Weblinks

Task force report on NEOs: www.nearearthobjects.co.ukTerrestrial impact crater pictures: www.solarviews.comengtercrate.htmNew York meteorite car http:nyrockman.compeekskill.htm

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