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STILL HERE? The celestial snooker in progress around Jupiter has awoken fears that life on our own planet might one day, if not this week, be potted into extinction. It's not a risk worth worrying about, say astronomers.

Meanwhile, over-protective parents are stifling the emotional development of their children by constant parental supervision because they fear their children will be kidnapped or assaulted. It's not a risk worth worrying about, say social scientists.

So when is a risk worth worrying about? The writer, John Allen Paulos, has proposed a Death Risk Index in which any activity may have its danger described by a number, defined as the logarithm of the average number of participants per death per year. The lower the number, the higher the risk.

So lightning, which kills around a dozen people each year in Britain, scores around 6.7 (a dozen deaths is about 1 in 5 million, and the logarithm of 5 million is 6.7). Bee-stings are 6.8, driving 3.7 and Russian Roulette (one game a year) at 0.8.

If one cataclysmic asteroid hits the earth every million years, that's a risk of 6.0. One child a year in Britain is killed by a stranger, which is less of a risk than bee- stings, lightning or asteroids.

Yet there is another aspect of anxiety: our ability to take preventative measures. Unable to influence bees and asteroids, we ignore them and take our chances. It would be neurotic to do otherwise. But we still hold our children's hands on the way to and from school. It's not a risk worth taking.