If war ever breaks out in space it's not the loss of individual satellites that will do the damage, but the debris this produces. It will stay in orbit and go on harming satellites for decades, according to two studies presented at the American Physical Society meeting in St Louis, Missouri, last week.
A commission chartered by the US Congress in 2000 warned that the US military's dependence on satellites would mean a space attack could be crippling. Last year, China heightened fears of a space war by testing an anti-satellite weapon, while earlier this year the US destroyed one of its own defunct satellites using a missile.
Now it seems that the immediate impact of a space war, at least on the US military, would be limited. "We have built up such high redundancy to space assets that we're almost invulnerable," says Geoff Forden of the Massachusetts Institute of Technology, who assessed the risk posed by China to the US. He found that only a few of the US's low-Eart-orbit satellites are over China at any one time, and that higher-orbiting satellites used for GPS, communications and surveillance could only be destroyed by multistage missiles, for which China has only three launch pads.
Crucially, any space attack would increase debris, which can have a long-lasting effect on satellites. David Wright of the Union of Concerned Scientists in Washington DC reports that destruction of one 10-tonne spy satellite in low-Earth orbit "would double or triple the debris" in that zone. Every new collision produces even more debris, triggering a cascade of satellite break-ups with time.