"Space is big," the British humourist Douglas Adams once observed litotically. "I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." And yet, in all this vastness, there are now some regions so crowded that it is possible for a pair of satellites no bigger than compact cars to collide by purest accident, with no malice aforethought, as happened 800 kilometres above Siberia on 10 February (see page 940).rnThe fact that humans have managed to spread traffic hazards beyond the confines of their planet is humbling. It is also a serious problem - one that could severely hamper scientific research, weather forecasting, commerce and the national defence of various nations. It needs to be sorted out.
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