The trash that can take out a satellite

Millions of pieces of space debris pose a danger to hardware and astronauts. What can be done about all the rubbish left out there?

- Ian Sample

The five astronauts and two cosmonauts on the International Space Station were ordered to suit up and take refuge in their capsules for fear their spaceship might be struck by flying debris. Russia had deliberately destroyed one of its own satellites with a missile, producing a cloud of wreckage that threatened the orbiting outpost. “It's a crazy way to start a mission,” Nasa told its sheltering crew, who had arrived only days beforehand.

The incident in November last year revealed how hairy Earth's orbit has become, and it wasn't a one-off. Two weeks later, mission controllers received another alert that the ISS might be hit by more debris. This time, Nasa delayed a planned spacewalk amid concerns that astronauts could be in danger if they went outside. Before the week was out, yet another warning came in, this one forcing the space station to dodge a US rocket body that has been barrelling around Earth since the 90s.

"It's a particular problem in low Earth orbit because that's where everybody wants to be, and it's where collisions have happened in the past," says Holger Krag, the head of the space debris office for the European Space Agency (Esa). Low Earth orbit is any altitude up to 2,000km. While many satellites are far higher, those orbits are much less cluttered.

Ever since the early days of the space age, there has been more junk in orbit than active satellites. No one worried that much at first: space is a big place, after all. But the amount of tumbling detritus has risen steadily over the past six decades. Space agencies, the military, and private operators have launched thousands of satellites for spying and navigation, scientific missions, communications, and more. Earth's orbit is not the void it once was.