In the line of ifre

Imagine you are standing against a wall, a lit cigarette gripped tightly between your teeth and a blindfold over your eyes. Then imagine that a bullet fired directly at you ruffles through your hair, a millimetre above your scalp, and imbeds itself in the wall.

A reprieve from imminent destruction can brighten anyone’s day. But on Friday, 14 June 2002, the British population was more concerned about Mick Jagger’s knighthood and the stock-market crash than savouring each precious lungful of air. The fact that the "bullet" was an asteroid the size of a football pitch and that it missed the earth by 75,000 miles, a whisker in the expanse of space, is no longer the point. The point is that we dodged one bullet but remain in the path of a potential galactic firing squad.

Hollywood movies such as Armageddon and Deep Impact relied upon the skills of Bruce Willis and Robert Duvall to save the planet from destruction by a big, dumb rock. In Armageddon, Willis, having landed on the rock with his elite band of deep drillers, stayed on alone to ensure the thermonuclear charge was placed at the correct depth, bravely blasting himself into oblivion and the big, dumb rock into the path of some other poor planet.

In Deep Impact, Duvall took his crew with him when they agreed to fly into a fissure in their rock before detonating their charges with the same result - Earth 1: Meteors 0.

In reality, the score should be roughly Meteors 650: Earth 0 and this score only includes the big ones that have struck since a lump of rock between five and ten miles in diameter wiped out the dinosaurs 65 million years ago. Their extinction was not, as Gary Larson the cartoonist suggested, as a result of smoking. Scientists estimate that a "big one" hits Earth once every 100,000 years. Really big ones, like the sucker that took out the Tyrannosaurus Rex, are believed to be separated out by tens of millions of years, but even small ones, such as the football pitch that narrowly missed us a fortnight ago, would cause catastrophic damage.

Had the meteorite landed on London on Friday the 14th, it would have been travelling at more than 23,000 miles per hour, more than 10 million people would have been killed and 3,000 square miles would have been devastated. If it had landed in the Atlantic, a giant tidal wave would have swamped the eastern seaboard of the United States.

The potential devastation of such an incident, coupled with the relatively high frequency of occurrence, would make you suspect that world governments would have the issue under control, that the flying football field was being carefully tracked for months with thermonuclear weapons locked on to its trajectory in the event that a direct collision proved likely.

The fact that no-one noticed this one’s presence until three days after it had zinged past is surely a cause for deep unease. As Jon Giorgini, a senior engineer at NASA/Caltech Jet Propulsion Laboratory in Pasadena explains, the number of people actively involved in tracking asteroids worldwide is "about the same as a McDonald’s store: a couple of dozen people, at most".

In April, Giorgini discovered a nightmare scenario with which our descendants will have to deal. A giant asteroid, a kilometre in diameter and named "1950 DA", appears to be on a collision course with Earth. The good news is that the point of impact remains 878 years in the future. The bad news is that, should it strike, a large proportion of the world’s population could perish. As Giorgini explains: "1950 DA would carry about 100,000 megatons of energy if it hit. It would make a crater about ten to 15 miles across and devastate hundreds of thousands of miles around it, kick up dust and steam into the atmosphere. Some of it would even orbit the Earth for a while. It would be a global problem."

Asteroids or meteorites are the rubble discarded during the creation of the solar system 4,560 million years ago. The planets were born from the formation of trillions of lumps; the spare parts that now compose our galaxy’s principal asteroid belt, lying between Mars and Jupiter, were prevented from coalescing into a planet by the pull of Jupiter’s gravity. Ever since, pieces of varying sizes have been knocked out of the main belt and into the paths of other planets such as Earth.

The pounding of the Earth’s surface by meteorites has dramatically shaped the planet’s history, with each giant twist in evolution caused by what is known within the field as "near-Earth objects". The "Great Dying" that wiped out 90 per cent of all living species 250 million years ago has been laid at their door, and mammals were given a leg-up by a meteorite at the expense of reptiles 65 million years ago.

Even within the last 5,000 years, however, human history has been altered by these killers from space. Forget the Chariots of the Gods, the discredited theory that space aliens bequeathed our ancestors with the knowledge to construct the pyramids. The only greeting they received from the heavens was a fiery missile with an impact akin to the film Independence Day. Evidence is emerging that the earliest human civilisations were devastated by a meteor storm 4,000 years ago. A giant, two-mile-wide crater has been discovered in southern Iraq, tying in with the discovery of impact points as distant as Sweden and Argentina, all dated to around 2200BC. It was into this post-apocalyptic world that the biblical patriarch Abraham was born.

Great collisions between humanity and lumps of space rock have occurred ever since. Meteorite showers are thought to have occurred in ninth-century France and 12th-century New Zealand. In the 15th century a meteorite is believed to have killed more than 10,000 people in China. In 1908, a meteor half the size of the flying football pitch destroyed 800 square miles of forest in Siberia.

The citing of celestial bodies as harbingers of bad times began with the Egyptians. Shakespeare, in Julius Caesar, wrote: "When beggars die, there are no comets seen/The heavens themselves blaze forth the death of princes." It was only in France during the 18th century, when a scientific committee was formed to dispel the idea, that the facts about rocks falling to Earth were established.

The odds on any individual being wiped out in an asteroid attack are actually ridiculously long. Two scientists in America, Clark Chapman and David Morrison, have computed there was probably the same likelihood of this happening as dying in a plane crash. Other scientists, such as Britain’s Duncan Steel, believe the odds are slightly higher. Their calculations are based on balancing the great infrequency of an asteroid impact against the huge potential destructiveness if one should occur. Steel, an expert on near-Earth objects at Salford University, argued in an essay in Prospect magazine that if the government spent money on tackling the issue it would have a better balance of cost versus benefit than any other public spending project. "We have seen what happens to a planet in the firing line. In 1994, 20 comet fragments slammed into Jupiter, causing damage over an area much greater than that of Earth. We don’t have to get a bloody nose ourselves before we realise that it will hurt," Steel wrote.

So who is actually looking out for these giant fists of rock? The answer, as Jon Giorgini explained, is not that many. Until the late 1980s asteroids that sailed a little too close to Earth were discovered by chance in the course of a variety of other astronomical tasks. Today, the Lincoln Laboratory Near-Earth Asteroid Research Project based in New Mexico dedicates its time to tracking space rubble. Its main concern is larger asteroids and the most recent near-miss, codenamed 2002MN, was discovered by chance. NASA, meanwhile, also concentrates on the larger asteroids. "NASA has a goal of discovering orbits for all the near-Earth objects with diameters larger than one kilometre. Asteroids of this size could potentially destroy civilisation as we know it," says Thomas Morgan, a scientist at NASA’s headquarters in Washington. NASA is spending tens of millions of dollars each year on a series of satellites designed to track asteroids and comets. But such is the blizzard of material hurtling towards the Earth that scientists around the globe discover on average one asteroid per night that is approaching our planet.

It seems we will all have to take our chances with the smaller meteors, but what about 1950 DA, that huge hunk of rock speeding towards us at 23,000 miles per hour with a docking date scheduled for 2880AD? Or, worse, the other ones we don’t yet know about? Will we one day have to send the great, great, great, great, great-grandson or daughter of Bruce Willis and Robert Duvall into space to do deadly battle with a monstrosity the size of Glasgow City Centre and the IQ of a table top?

Perhaps, but given enough time, there are other less violent options. "In hundreds of years, it’s hard to imagine what ways we’ll have to deal with it," says Giorgini, the rock’s proud discoverer. "It’s sort of like guys 900 years ago trying to plan the interstate highway system. It would probably be more sensible to leave it to future generations.

"If you have centuries of warning like this, you can just change the way it absorbs and reflects light and heat. Sunlight shines on it and heats one side, and it rotates around to the back and the heat radiates off into space and pushes on it like a weak rocket. Over centuries, that’s enough to push it out of the way. If you have hundreds of years of warning, you could spread chalk or charcoal over the surface, which would change the way it reflects light and its velocity, or you could send a solar sail - a big sheet of mylar-like plastic - and shrink-wrap it. The sunlight over centuries would push it away."

If the lead-time shrank to a matter of weeks or months, then it would be time to dust down the nuclear missiles, according to Steel, who explains that the targeting would have to be perfect in order to push the meteor off-course rather than shattering it into a number of rocks all heading for Earth.

A discovery only a few days before impact would be of little use. Professor Jim Emerson of Queen Mary, University of London - the leader of the Vista project, which is building a giant telescope in the Chilean Andes in order to track near-Earth objects - explains his position: "Then it might be better not to know. If I’m going to have a heart attack next week, I’d rather not be warned about it." If the warning did come, there would be just enough time to slip on a blindfold and light up a fag. This time the firing squad would be unlikely to miss.