Scientists are increasingly worried about the amount of debris orbiting the Earth

FEBRUARY 10th 2009 began like every other day in Iridium 33’s 11-year life. One of a constellation of 66 small satellites in orbit around the Earth, it spent its time whizzing through space, diligently shuttling signals to and from satellite phones. At 3pm a report suggested it might see some excitement: two hours later it would pass less than 600 metres from a defunct communications satellite called Cosmos 2251. It did. A lot less. The two craft collided and the result was hundreds of pieces of shrapnel more than 10cm across, and thus large enough to track by radar—and goodness knows how many that were not. This accident came two years after the deliberate destruction by the Chinese of their Fengyun-1C spacecraft in the test of an anti-satellite weapon. That created over 2,000 pieces of junk bigger than 10cm, and an estimated 35,000 pieces more than 1cm across. Together, these incidents increased the number of objects in orbit at an altitude of 700-1,000km by a third (see chart).

Such low-Earth orbits, or LEOs, are among the most desirable for artificial satellites. They are easy for launch rockets to get to, they allow the planet’s surface to be scanned in great detail for both military and civilian purposes, and they are close enough that even the weak signals of equipment such as satellite phones can be detected. Losing the ability to place satellites safely into LEOs would thus be a bad thing. And that is exactly what these two incidents threatened. At orbital velocity, some eight kilometres a second, even an object a centimetre across could knock a satellite out. The more bits of junk there are out there, the more likely this is to happen. And junk begets junk, as each collision creates more fragments—a phenomenon known as the Kessler syndrome, after Donald Kessler, an American physicist who postulated it in the 1970s. …