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According to the Duckworth Riskometer ("Odds in the game of life", THES, July 9), the risk factor due to asteroid impacts is more than five times that of taking a 100-mile rail journey and only slightly less than that of driving a car. It is also striking, if you will excuse the pun, that asteroid impacts are listed as an "activity".

In fact, if asteroids and comets were a business, they would have been closed down years ago - until the impact hazard had been solved.

The contrast between risks that we choose to accept, such as rock climbing or driving a car, and those imposed externally is especially acute in the case of asteroid impact. The potential loss is unbounded, yet the impacts are in principle accurately predictable and avoidable. There is an urgent need to discover, assess and monitor the most threatening comets and asteroids on Earth-crossing orbits, in effect to regulate the "cowboy" operator and reduce the risk.

Mark E. Bailey Director, Armagh Observatory