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Your treatment of the cosmic impact threat ("Threats from deep space require deep thinking", THES, July 16) confuses two aspects of the problem. One is the detection and cataloguing of potential impactors, the other concerns what to do about the next large impact. The former project is straightforward and relatively cheap to implement. The latter remains largely science fiction.

If it turns out that the Earth will be hit by a 1km asteroid in 1,000 years, we can leave it to future generations to worry about. But if such a calamity is predicted in, say, 30 years, then an intensive effort to divert the impactor would become an urgent priority.

Either way, money used for detection would be well spent.

Paul Davies South Australia