SCIENCE / Beware bolts from the blue: Your chances of being killed by a meteorite may be higher than you think, reports Philip Ball

METEORITES, long a source of fascination in their guise of shooting stars, are now causing scientists to look to the skies more in concern than in wonder. According to estimates published recently, we each have as much chance of being killed in a meteorite impact on Earth as in an aeroplane crash.

These claims are made by Dr Clark Chapman, of the Planetary Science Institute in Tucson, Arizona, and Dr David Morrison, of Nasa's Ames Research Centre in Moffett Field, California. In a paper in Nature, they argue that 'the impact threat is much higher than from certain carcinogens, wild animals such as grizzly bears, fireworks accidents, terrorist bombs and airline hijacking'. The troubling statistics emerge from recent searches for large extraterrestrial objects, such as asteroids, that drift across Earth's orbit.

The cosmic debris that falls to Earth has two origins: asteroids and comets. Both are made up of loose rubble left over from the formation of the planets about 4.5bn years ago. When the gaseous material of the early solar system turned into planet-sized lumps, some material was left scattered in forms ranging from fine dust to bodies a few kilometres across. Many of these objects became arrayed in a belt concentrated between Mars and Jupiter - the asteroid belt. Others are loosely held by the Sun's gravitational field beyond Pluto, and are occasionally sent plunging back into the inner solar system by a gravitational nudge from nearby stars. These are the comets.

Comets fly by only rarely, and pose a less persistent threat than asteroids. The asteroids are cosmic wanderers: most confine their travels to the region beyond Mars, but some may stray into the Earth's path. Just how many of these Earth-crossing boulders there are is not clear, but Nasa has set up a dedicated telescope, the Spacewatch Telescope on Kitt Peak in Arizona, to spot them. So far, 163 Earth-crossing asteroids have been catalogued.

Even though objects larger than a few kilometres across are relatively easy to see with ground-based telescopes, there may be 20 times as many as have been identified so far. If an object this large struck the Earth's surface, say Chapman and Morrison, millions of people would surely perish.

Smaller objects, of which Spacewatch has found only a tiny fraction so far, may also have dramatic consequences. But we needn't start digging shelters just yet. Most meteorites break up high in the atmosphere, where they do no damage beyond showering small fragments earthward. (Strictly, objects like this that do not strike the ground are called meteoroids.) It is estimated that, on average, a meteoroid explodes once a year with the force of the Hiroshima atom bomb - but at such a distance that shock waves do not reach the ground.

Occasionally, a meteoroid will penetrate further into the atmosphere before detonating. This happened in 1908 above Lake Tunguska in Siberia, where an asteroid perhaps 30m in diameter broke up with enough energy to flatten trees over an area of about 1,000 square kilometres. There were no recorded casualties, but the consequences of a similar event over a city would be terrible to contemplate.

We know of no events like Tunguska in more recent times, although showers of small meteoroid fragments fall regularly. As these may reach the size of bowling balls, they could prove fatal in themselves; but the chances of an individual being hit are rather slight. All the same, Michelle Knapp got a shock in October 1992 when her Chevrolet Malibu was struck by a 12kg lump of asteroid while it sat in her front drive in Peekskill, New York State. The car was written off but, with meteorite included, its value soared to dollars 69,000. But there is not a single authenticated record of a human fatality caused by a meteorite impact. So why the scary figures from Chapman and Morrison?

The answer is that most of the killing power of meteorites lies at the high end of the range - in very large impacts. But the chance of an impact gets rapidly smaller as the impactor gets larger. So the statistics are largely the result of hugely destructive events happening once in a blue moon. Chapman and Morrison estimate that an impact that kills 100 people is likely to happen once every 100 years. A really dramatic event - say, one that kills 1.5 billion people, a quarter of the world' s population - happens only once every half a million years.

The juxtaposition of the tiny probability and the awesome consequences make catastrophic impacts 'very difficult to think about', say Chapman and Morrison. But catastrophes of this kind do happen. The most celebrated was 65 million years ago, marking the end of the cretaceous era and the demise of the dinosaurs. It is widely believed that the impact wiped out at least half of the living creatures on the planet. This impact was first proposed by Walter Alvarez and colleagues of the University of California, who suggested that it could explain the layer of dust rich in the rare metal iridium, found worldwide in sediments of the cretaceous age: iridium is abundant in comets. It now seens likely that the impactor, perhaps 10 kilometres across, left its awful scar in the form of a crater at least 180 kilometres wide at Chicxulub in Mexico. It is estimated that the collision released an energy of 100 million megatons, equivalent to 10,000 million Hiroshimas. The dust thrown into the atmosphere may have plunged the planet into a long and frigid night.

This year we will witness further evidence that impacts of unimaginable ferocity take place in the solar system. In July, the comet Shoemaker-Levy 9 is expected to plunge into Jupiter, releasing energy in quantities that render Earthly comparisons meaningless. Sadly, the fireworks will be largely hidden from direct view on the far side of Jupiter, but some scientists believe that they will be bright enough to be seen reflected in Jupiter's moons.

Is there anything we can do to protect ourselves from this fury from the skies? Dr Thomas Ahrens, of the California Institute of Technology, believes that there is. He has suggested that, if detected far enough in advance, large asteroids approaching the Earth might be deflected by blasts from nuclear weapons. This assumes, however, both that we can spot the threat well in advance and that we have a suitable weapons system on stand-by. Even then, more drastic measures would be needed for very large objects.

Some scientists, Chapman among them, feel uneasy about allowing these remedies to justify weapons research. Already, proponents of Ronald Reagan's controversial Star Wars programme, such as the veteran nuclear physicist Edward Teller, have seized on such ideas as an argument for keeping the project alive. But Dr Jay Melosh, a planetary scientist from the University of Arizona, has a greener alternative: using huge space-based mirrors to focus sunlight on to approaching asteroids, frying off some of their surface and thus altering their course. The process is gentle, clean and reusable, says Melosh. Is there a green antidote to Armageddon after all?-