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Movies have been devoted to it, scientists make a fuss about it and, in the shape of Lembit Opik MP, it has a vocal parliamentary lobby. But the possible extermination of Homo sapiens by asteroid impact is a test of society's ability to cope with complex facts and choices.

Mr Opik's claim that the asteroid danger equals that of Chernobyl may be true in a trivial sense. It would kill far more people than Chernobyl, but is far less likely to happen, so any individual is at about as much risk from each. But this tells us nothing about the effort we should devote to countering the space menace.

Our awareness of the asteroid threat has grown in recent years as knowledge of our corner of space has increased. And a detailed search for possible impactors is a useful wheeze for getting more cash into solar system studies. But beyond this, judgements get more tricky.

If nuclear weapons are the only way of diverting an incoming asteroid, is it worth keeping them in existence for the purpose, or would it be safer on balance to disarm and risk the impact? What about the industry needed to support asteroid diversion? Would the project merely support high-technology arms-makers whose business plans have never recovered from the end of the cold war, or are there potential new technologies, such as ion thrusters for pushing astroids gently aside, that might make benign space policing feasible?

One thing that nobody doubts - last week's report from the National Audit Office makes the point yet again - is the weapons industry's ability to turn a plan into a hard-to-cancel project where budgets are merely the framework for later overruns. A global space defence would be even more difficult to manage than previous projects and needs much greater thought before it becomes commonly accepted as a necessity.