In November last year, the five astronauts and two cosmonauts on the International Space Station (ISS) 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 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 the 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 was all worryingly reminiscent of the 2013 movie Gravity, in which debris from a shot-down satellite damages not just the ISS but the Hubble space telescope and a visiting space shuttle.
“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, head of the European Space Agency (Esa) space debris office in Darmstadt, Germany. Low Earth orbit (LEO) is any altitude up to 1,200 miles. While many satellites are far higher up, those orbits are much less cluttered.
Even 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 and its combined mass has risen steadily over the past six decades. The military, space agencies and private operators launched hundreds and then thousands of satellites for spying and navigation, scientific missions, communications, the internet and more. Earth’s orbit is not the void it once was.
More than 8,000 tonnes of space junk now circle the planet, a mix of spent rocket parts, dead satellites and fragments of hardware that are doing their best to defy the rule that what goes up must come down. Far above the drag of the atmosphere, old space kit can stay aloft for centuries, millennia even, where it can smash into other objects. According to the UN Office for Outer Space Affairs, had the Romans launched a satellite into a 750-mile-high orbit, it would only fall back to Earth about now.
“I think it’s fair to say that space is becoming congested,” says Wing Commander Thom Colledge, the station commander at RAF Fylingdales in the North York Moors. The base (motto “Vigilamus” for “We are watching”) is first and foremost an early warning system for intercontinental ballistic missiles. But the same radar that keeps watch for nukes also looks out for collisions in space. And, as more and more satellites are launched, collisions are an ever-growing danger.
Fylingdales is part of the US Space Surveillance Network, which monitors about 30,000 items of space junk larger than 10cm. The network issues warnings when objects might collide, hopefully giving operators time to assess the risk and move if need be. But there are more than 100,000,000 pieces that are too small to track and eminently capable of causing damage. Travelling at more than 15,000mph, micrometre-sized particles can chip windows and dent solar cells, while millimetre-sized flakes can destroy satellite cameras or puncture astronauts’ space suits. At 1cm and above, a speeding fragment can take out an entire satellite. Operators may never know what happened.
Space debris tracking at Fylingdales is performed by a military-private partnership, with the RAF operating the radar and analysts from Serco interpreting the data. They watch the heavens from the pyramid, a nine-storey building that’s been a striking landmark on the surrounding moors for the past 30 years. The three square sides of the building form the solid state phased array radar, or SSPAR, which provides a 360-degree view of space up to 3,000 miles high. The radar tracks orbiting objects by focusing a beam of energy on a region of space and analysing the reflections that bounce back. “We can pick up an object the size of a can of Coke,” Colledge says.
I’ve arrived to see how the tracking is done, but to get inside the pyramid – the radar station doubles as a workplace – visitors must first ditch their laptops, phones and smartwatches. On the way to the training room, where teams learn the ropes on a simulation before taking shifts on the live system, we pass the operations centre where the real work takes place. It has a thick metal door complete with a hand wheel that wouldn’t look out of place on a submarine.
Warnings of potential collisions are issued all the time. The UK Space Agency (UKSA) receives nearly 3,000 alerts a month from the US Space Surveillance Network. These are triaged and risk-assessed by drawing on data from Fylingdales, other sites around the world and sensors across the UK. The UKSA already provides alerts to some UK satellite operators and will roll the service out to the rest this year, says Jacob Geer, head of space surveillance and tracking at the agency.
Many operators move their satellites if the risk of collision is more than one in 1,000, but there is a cost to taking evasive action. Dodging debris consumes precious fuel, which can shorten a satellite’s lifetime. Ideally, avoidance manoeuvres are performed days in advance, and can sometimes be incorporated into pre-planned station-keeping burns. “What you don’t want is a completely unscheduled burn,” says Charlotte Newton, a Serco spacetrack analyst at Fylingdales.
Moving a satellite to avoid space debris can have other downsides, too. If a satellite has to dodge a stream of debris, it might end up in a less useful orbit. In the case of spy satellites, this could hamper what can be seen and when. The fact isn’t lost on space-savvy nations: there’s potential for space junk to be weaponised, to disrupt and degrade what others can do, and render entire orbits unusable.
There are times when crashes cannot be avoided. When two dead satellites are on course to collide, all observers can do is watch and wince. When one of the satellites is operational, it will usually have a chance to swerve. But the dance becomes more complex when two active satellites are involved. There is no highway code in space, no accepted right of way. So it’s often those with most to lose who ensure disaster is averted.
One of the biggest new challenges for satellite operators comes from “mega-constellations”. Companies such as SpaceX and OneWeb are launching hundreds and potentially thousands of small satellites into low Earth orbit to provide services such as global broadband access. The satellites will undoubtedly bring benefits, but they add to the congestion. In 2019, Esa took – for the first time – evasive action to avoid an operational satellite. The Aeolus Earth observation satellite fired its thrusters to dodge a Starlink satellite after SpaceX declined to move. “At Esa our assets are so expensive that we’d rather move and take the effort than have a situation where we don’t know what’s going to happen,” says Krag. Others had similar close shaves. In December, China criticised the US for threatening the lives of astronauts on board its Tiangong space station after it was forced to dodge one Starlink in July and another in October. The US said it would have let China know if there was “a significant probability of collision”.
Collisions in space do happen. The most spectacular was in February 2009 when an operational Iridium communications satellite slammed into a defunct Russian military satellite at 26,000mph. The impact, 480 miles above Siberia, created thousands of fragments that continue to threaten satellites today. The incident made headlines, but many more suspected collisions are never confirmed, often because satellites are taken out by fragments too small to track.
Among the debris that has accumulated in orbit are some particularly risky objects. Old Russian rocket bodies known as SL-12s are prone to spontaneous explosion, leaving their orbital paths strewn with fresh clouds of debris. Meanwhile, large drifting satellites such as Esa’s doubledecker bus-sized Envisat are sitting ducks in congested orbits. It’s this big stuff that space agencies worry about most: a major collision with Envisat could add a profound amount of debris to low Earth orbit. One theoretical concern for the future is called Kessler syndrome, where there is so much junk in a particular orbit that the debris from one collision sets off a chain reaction of more collisions, and more debris, until the orbit is off limits for years.
The risk that space debris poses to satellites and humans in orbit has not stopped countries adding to the problem when they want to flex their muscles. In 2007, China destroyed a defunct weather satellite with a missile, increasing the amount of space debris in low Earth orbit by 30%. The US and India followed suit in 2008 and 2019 respectively, cluttering the environment even more. Russia’s own act of orbital vandalism last year created more than 1,500 fragments that will circle the planet for years. This week, the US announced a national ban on further so-called direct-ascent anti-satellite (DA-ASAT) missile testing and urged other countries to follow suit, claiming it wanted to establish “a new international norm” for responsible behaviour in space.
Faced with a growing problem, space agencies and others are taking action. One option is to build better instruments to track debris and satellites more accurately. Modern satellites often carry reflectors that allow operators to track their trajectories precisely from laser ranging stations on Earth. Space junk is harder to track this way, but it can be done with more powerful lasers, a feat Esa hopes to achieve in the next two years from a laser ranging station in Tenerife. Having more precise information on the positions of satellites and wandering junk should slash the number of collision alerts that turn out to be false alarms. Further efforts are focused on automating avoidance manoeuvres so teams of engineers are not needed to execute every sidestep and swerve.
Guidelines for space debris mitigation and, more recently, the long-term sustainability of outer space activities, published by the UN Office for Outer Space Affairs, lay out how nations and companies should behave in space. Operators are meant to remove satellites from orbit within 25 years of completing their missions. For satellites in geostationary orbits, which are more than 20,000 miles high, this means nudging them up to a “graveyard orbit” well out of harm’s way. Satellites in low Earth orbit should be steered into the atmosphere where they burn up on re-entry. For large satellites that may partly survive re-entry, operators are advised to time their re-entries so plummeting remnants land in the Pacific.
But not everyone follows the guidelines. While most new rockets are now “de-orbited” once their work is done, the vast majority of satellites in low Earth orbit are simply abandoned to their fate.
Esa’s approach to the problem is to come up with cost-effective ways to remove dead satellites, rocket parts and large chunks of debris, and hope regulators endorse those that work. Krag favours a similar approach to national park protection, where people must leave with whatever they bring in: whenever a satellite reaches the end of its life, it must remove itself from orbit, or be disposed of by a space junk removal service.
How well that works will soon become clear. Astroscale, a Japanese tech firm with a facility in Oxfordshire, hopes to offer a satellite removal service in the next few years. Meanwhile, Esa’s ClearSpace-1 mission aims to become the first to remove an item of space debris from orbit in 2025. The same mission will test a propellant kit developed by D-orbit, an Italian firm also with offices in Oxfordshire, that will steer the mission’s spent rocket body back towards Earth where it will burn up in the atmosphere.
“It becomes almost corporate good practice to show that you are sustainable, to make your satellites able to move out of orbit when they reach the end of their life, or to make it possible for a company to come and retrieve or service your satellite,” says Geer. By developing the knowhow to retrieve dead satellites, the UK can build expertise for the coming “orbital economy”, he adds, where satellites are serviced and assembled in orbit.
“Every year, dozens of objects are left behind that could have been disposed with their own means,” says Krag. “But that’s not happening because of the lack of technology and the lack of policy. It’s like plastic in the oceans. Would you start cleaning up the old stuff while, every year, people add more? No. You concentrate on stopping them adding more. We need to do the same with debris in space.”