It’s supposed to be a tranquil place; a peaceful vantage point on a troubled world. But these days the astronauts aboard the International Space Station (ISS) spend as much time looking nervously over their shoulders as they do gazing at Earth, fearful of the growing amounts of man-made detritus hurtling erratically through orbit. Several times over the past few years the orbiting platform has been forced to use its boosters to avoid a possible collision. Most dramatically, in March 2009, its crew had to take refuge in the attached Soyuz return craft when a chunk of debris from a US launcher passed within a mile of the fragile structure.