AT ORBITAL SPEEDS a tennis-ball-sized piece of space junk packs enough energy to obliterate a satellite. It makes good sense, then, to track orbiting debris, the better to steer spacecraft away from danger. That this is hard was underscored on April 23rd, as a SpaceX capsule sped toward the International Space Station (ISS). The crew were preparing to sleep when ground control hastily announced they had just 20 minutes to complete a safety procedure before a potential impact. The object, probably a piece of defunct spacecraft, later whizzed past harmlessly.

At the moment, space-going junk is mapped mostly by radar. But of an estimated 34,000 orbiting objects ten or more centimetres across, only about 29,000 are being tracked with reasonable accuracy. Smaller pieces are more numerous, and harder to follow. Those between one and ten centimetres across number more than 900,000; those at least a millimetre across, perhaps 128m. Even tiny bits of debris can do damage. In May the Canadian Space Agency said an untracked piece of junk had punched a hole 5mm across in Canadarm2, a robotic limb attached to the ISS.