ORBITAL DEBRIS offers a classic “tragedy of the commons” problem. Six decades of space launches have littered Earth orbits with derelict satellites, rocket parts and other scrap. More than 750,000 pieces are at least a centimetre wide—enough to shatter a satellite. Even flecks of paint, of which there are millions, can damage kit, including the International Space Station. Over time, this junk breaks up into smaller pieces that become harder or even impossible to track with radar, and therefore to dodge. Mid-space collisions can be catastrophic. In 2009, for example, a defunct Russian satellite smashed into and obliterated one operated by Iridium, an American firm. As William Shelton, a former head of the US Air Force’s Space Command, puts it, “debris begets debris”.

Valuable orbits could eventually become unusable. Friction from molecules in space near Earth brings unpowered objects closer to Earth (“decays” their orbit) but it can take many years, even centuries, before a given piece of scrap “de-orbits” by descending far enough to meet a fiery end in the atmosphere. If, over time, the pieces of dangerous debris that de-orbit are outnumbered by those newly jettisoned in space or multiplied by impacts, the frequency of collisions will accelerate. In 1978 Donald Kessler, a scientist at NASA, warned that this chain reaction could render whole orbits impracticable. The Kessler Syndrome, as this “collisional cascade” has become known, has probably already begun, though it is likely to unfold over at least several decades, not mere hours as depicted in the 2013 hit film “Gravity”. Some reckon the process could be reversed if at least five big dead satellites were removed from orbit every year. (Satellites may “die” when they run out of the fuel needed to keep their proper place in orbit, or lose the ability to generate power with solar panels, or suffer an electronic failure.)

An object can be de-orbited by slowing its movement, ensuring it heads rather more quickly towards the atmosphere and arrives within weeks or months. Reducing velocity by just 100-odd metres per second is often enough, though this is no easy task. In September a European Union spacecraft in an orbit close to Earth (“low Earth orbit”) used computer vision and laser rangefinding to eject a net that snatched a small “cubesat”, a sort of miniature satellite (eg, pictured), that it had pushed into space. In February the spacecraft, named RemoveDEBRIS, will attempt another first by shooting a harpoon to snag a piece of panelling brought along for the test and held at the end of a boom. A European Space Agency mission to de-orbit a big defunct satellite in 2023 has decided against nabbing it with a harpoon, and will probably opt for a net or robotic arm. The latter approach has been tested by China.

Japan’s space agency, JAXA, started developing a grappling system that would separate from a mothership and clamp onto a big piece of scrap. Interaction between Earth’s magnetic field and a 700-metre steel-and-aluminium tail dangling from the grappling mechanism was meant to generate braking power, with the debris and grappling system eventually falling into the atmosphere together. But the system failed a test in 2017. Fuel economy would probably limit missions that seek to grab debris to removing just a few pieces of junk. This is why BBN Technologies, a subsidiary of Raytheon, an American firm, proposes using hot-air balloons to loft explosives into the upper atmosphere. A large blast there would shoot up a column of air that might slow a cloud of debris. Numerous outfits propose zapping debris with a laser mounted on a mountaintop or spacecraft. Vapourising a bit of the object in this way would generate thrust that could slow it down. All of these approaches promise to be astonishingly expensive, but there is an additional drawback. Any system that de-orbits debris could also be used to attack an adversary’s satellites.