This week, an asteroid narrowly missed crashing into us. But could it really happen? JAMES CHAPMAN reports

It appeared without warning, a massive chunk of rock hurtling towards us at 20 miles a second, and missed by an astro-whisker.

The human race, meanwhile, went about its business, oblivious to the danger.

Earth survived a brush with catastrophe this week when a 1,000-foot wide asteroid capable of wiping out Leinster and plunging a large section of the globe into a nuclear winter flashed by us, just 370,000 miles away.

To the layman, that may seem like a comfortable enough distance. The moon is 238,857 miles away. But in space terms, be assured it is too close for comfort.

Astronomers, who spotted the asteroid less than a month ago, breathed a sigh of relief as it passed us by without incident.

But they said it served as a dramatic reminder of just how many potentially devastating fragments there are out in space and how ill-prepared we are to counter their threat.

If the rock, codenamed 2001 YB5, had been on a direct collision course with Earth, we would have had only about 25 days to initiate a defence. And so far no country has developed any form of asteroid shield.

Scientists have calculated that if 2001 YB5 had struck Dublin, everything within 100 miles would have been devastated and everything within a further 500 miles would have been severely damaged.

If it had landed in the sea, it would have generated a tidal wave strong enough to swamp coastal cities and submerge much of Ireland.

Clouds of dust and debris hurled into the atmosphere could have caused a drastic drop in temperature in the region of the impact.

But while planet Earth got away with it this time, Irish astrophysicist Professor David Fegan believes we have much to fear from a collision between our planet and a comet, an asteroid or a smaller meteor at some unknown point in the future.

"The 1908 impact in Siberia, which is thought to have been an asteroid exploding several miles overhead, caused phenomenal devastation over an area of hundreds of kilometres. It was similar to a nuclear attack," said the UCD scientist.

That is not surprising given the breathtaking scale of these space-travelling monsters.

"The Barringer Meteorite Crater at Winslow, California, is 1.2 kilometres wide and 0.2 kilometres in depth. It is thought that was caused 25,000 years ago by a meteorite 50 metres wide, which weighed 200,000 tons."

The last important collision in the Earth's solar system was recorded in 1994 by two astronomers Shoemaker and Levy who spotted a comet as it fragmented in the atmosphere of Jupiter, battering the planet with numerous huge fragments.

"It was incredibly spectacular. The comet got pulled into Jupiter's massive gravity," said David Fegan.

Scientists like Fegan and his UCD colleagues realise that like Jupiter, the Earth will literally drag debris of potential doom down on itself. Unfortunately, there is no defence for humankind should a large asteroid be attracted by the Earth's gravitational pull.

"It is very difficult to spot these things. They are not self-luminous. So you wouldn't really see them until they get really close.

"You can't get rid of them once they meet the Earth's gravity. If the trajectory is right, we're in real trouble. It would be very difficult for a nuclear bomb to move off course something of this size."

The asteroid 2001 YB5 was picked up by the NEAT (Near Earth Asteroid Tracking) survey telescope on Mount Palomar in California on December 12. It is following an 1,321-day elliptical orbit of the Sun, crossing the orbits of Mars, Earth, Venus and Mercury. And though it missed us this time, it has been deemed potentially hazardous and could crash into our planet at some point in the future.

The only known object that will come nearer to Earth is an asteroid called 1999 AN10, which will pass a few tens of thousands of miles closer to us on August 7, 2027. However, minor space debris is reaching Earth all the time and at least some is reckoned to have landed on Ireland. On any clear night, a "shooting star", the common misnomer for a meteorite, can be seen streaking across the Irish skies, incinerating in a trail of flame as the Earth's atmosphere provides a natural defence.

But Professor Fegan said that while "nothing major" is known to have landed here, pieces of asteroid ranging from the size of a marble to a tennis ball or bigger have probably got through the atmosphere shield, crashing on Ireland, but without inflicting any known damage.

Last year, a task force of scientists set up by the British government concluded that an asteroid strike on Earth, presented in chilling style by Hollywood in films like Armageddon and Deep Impact, is a possibility.

A £15 million super telescope to monitor the thousands of rocks hurtling through space would be just a first step towards protecting the future of mankind, they said.

On New Year's Day, British Science Minister Lord Sainsbury announced the setting up of a new nerve centre to mastermind the operation.

The centre will work with the Natural History Museum in London and a consortium involving the University of Leicester; Queens University in Belfast; Queen Mary College, London; and the Royal Observatory, Edinburgh.

There are believed to be more than 900 asteroids even larger than 2001 YB5 two-thirds of a mile or wider in diameter with orbits that bring them dangerously close to Earth.

All are large enough to wreak worldwide devastation in the event of impact. Scientists say there is a real chance that one of them could arrive with the power of millions of Hiroshima atomic bombs, which would be the most destructive since an asteroid measuring seven miles across wiped out the dinosaurs 65 million years ago.