On Monday 2 July, the CryoSat-2 spacecraft was orbiting as usual, just over 700 kilometres above Earth’s surface. But that day, mission controllers at the European Space Agency (ESA) realized they had a problem: a piece of space debris was hurtling uncontrollably towards the €140-million (US$162-million) satellite, which monitors ice on the planet.
As engineers tracked the paths of both objects, the chances of a collision slowly increased — forcing mission controllers to take action. On 9 July, ESA fired the thrusters on CryoSat-2 to boost it into a higher orbit. Just 50 minutes later, the debris rocketed past at 4.1 kilometres a second.
This kind of manoeuvre is becoming much more common each year, as space around Earth grows increasingly congested. In 2017, commercial companies, military and civil departments and amateurs lofted more than 400 satellites into orbit, over 4 times the yearly average for 2000–2010. Numbers could rise even more sharply if companies such as Boeing, OneWeb and SpaceX follow through on plans to deploy hundreds to thousands of communications satellites into space in the next few years. If all these proposed ‘megaconstellations’ go up, they will roughly equal the number of satellites that humanity has launched in the history of spaceflight.
All that traffic can lead to disaster. In 2009, a US commercial Iridium satellite smashed into an inactive Russian communications satellite called Cosmos-2251, creating thousands of new pieces of space shrapnel that now threaten other satellites in low Earth orbit — the zone stretching up to 2,000 kilometres in altitude. Altogether, there are roughly 20,000 human-made objects in orbit, from working satellites to small shards of solar panels and rocket pieces. And satellite operators can’t steer away from all potential collisions, because each move consumes time and fuel that could otherwise be used for the spacecraft’s main job.
Concern about space junk goes back to the beginning of the satellite era, but the number of objects in orbit is rising so rapidly that researchers are investigating new ways of attacking the problem. Several teams are trying to improve methods for assessing what is in orbit, so that satellite operators can work more efficiently in ever-more-crowded space. Some researchers are now starting to compile a massive data set that includes the best possible information on where everything is in orbit. Others are developing taxonomies of space junk — working out how to measure properties such as the shape and size of an object, so that satellite operators know how much to worry about what’s coming their way. And several investigators are identifying special orbits that satellites could be moved into after they finish their missions so they burn up in the atmosphere quickly, helping to clean up space.
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