Robots, harpoons and nets: how we’ll clear our orbital rubbish
Image credit: Getty Images
As the space around Earth becomes increasingly cluttered, the world’s space agencies are looking at ways to clean up the mess.
On 4 April 2018, SpaceX’s Dragon capsule berthed with the International Space Station (ISS) carrying a load of supplies and scientific experiments. Among the three tonnes of kit that astronauts transferred into the bowels of the orbital outpost was a 100kg spacecraft, approximately the dimensions of a washing machine. The spacecraft, called RemoveDebris, has since become the largest satellite to date to be released to space from the ISS. It is also the first satellite platform for testing active space debris removal technologies in orbit through a series of four experiments.
Active space debris removal technologies will play a similar role in preventing a full-blown environmental crisis in space as carbon capture and storage is expected to play in slowing down global warming. Harpoons, nets, robotic arms and other devices are being developed that will enable a garbage-removal spacecraft to grab a defunct satellite and pull it down towards the Earth’s atmosphere, where it would burn up.
Experts estimate that five large satellites will have to be removed from low-Earth orbit – the altitude up to 2,000km – every year, if the orbital environment is to remain stable. Otherwise the so-called Kessler syndrome, an unstoppable cascade of collisions in which every new space crash creates a large quantity of fragments that further destroy other spacecraft, might become a reality.
In fact, according to Holger Krag, who heads the Space Debris Office of the European Space Agency (ESA), the first indications of the phenomenon, originally predicted by American astrophysicist Donald Kessler in 1978, can already be observed.
In August 2016, ESA nearly lost the €280m satellite Sentinel-1A, part of the EU-spearheaded Copernicus Earth observation programme, due to a collision with a bullet-sized piece of space debris.
The agency’s controllers first detected that the 2,170kg spacecraft, at that time only a little over two years into its planned seven-year mission, was inexplicably slipping from its 693km orbit. Data from the satellite’s on-board computer indicated one of its 10m solar wings was producing less electrical power. The team managed to shed light on the situation by reactivating an on-board camera, which had originally been used to monitor deployment of the solar panels when the spacecraft first reached the orbit. Footage revealed a 40cm hole in one of the solar wings, which, according to Krag, must have been created by a piece of space junk about 1cm in size.
“We were lucky because we could continue the mission, but if the object had hit the main body of the satellite the outcome could have been much more severe,” says Krag.
“Seven fragments have been produced during the collision that are now tracked by the space debris surveillance system,” he continues. “One of them generated a conjunction alert with sister spacecraft Sentinel-1B, which is flying in the same orbit but 180 degrees apart.”
The scenario somewhat resembled the Oscar-winning film ‘Gravity’. According to Kessler, 10 per cent of all satellites in low-Earth orbit are experiencing similar collisions. As these satellites don’t carry cameras, the exact cause of their problems may never be determined.
With the high cost of space technology a lot is at stake, and experts are calling on the interested parties to start treating the environmental crisis in space at least as seriously as climate change.
The RemoveDebris spacecraft is one of the first steps that could lead to such efforts becoming a reality.
It carries three types of technology that in the future might be used to remove defunct satellites from the low-Earth orbit faster in order to prevent devastating collisions – a harpoon, a net and a drag sail. It also carries a novel vision-based navigation system consisting of a lidar laser system and an optical camera for tracking a floating object to enable smooth capturing.
The satellite was released from the ISS on 20 June via NanoRacks’ Kaber microsat deployer and is being commissioned before the programme of experiments begins in October.
Guglielmo Aglietti, professor of space engineering at the University of Surrey, director of Surrey Space Centre and principal investigator for the RemoveDebris mission, explains: “Although the experiments themselves are relatively short, there might be long gaps between them. Overall we plan to finish the whole mission by the end of the year.”
For the purpose of this demonstration, the spacecraft carries its own pieces of space debris – two 2U cubesats 10x10x20cm in size. The first DebrisSAT will be used for the net capture experiment, the second to validate the vision-based navigation system.
“We will eject a small cubesat to begin with – a door opens and some springs will push it out,” says Jason Forshaw, former RemoveDebris project manager at the University of Surrey, which has overseen the €15.2m EU-funded development. “At a set time later we will deploy the net and that captures the little cubesat. It sounds quite simple. You can do a lot of testing on Earth, but no one has ever demonstrated this in the harsh and dynamic space environment.”
In a proper mission, Forshaw said the net would be connected to the chaser spacecraft with a tether. After catching the debris, the chaser spacecraft would use its thrusters to deorbit it. For the purposes of this demonstration, engineers decided to leave the tether out.
“There are a lot of problems that could occur with a tether,” explains Forshaw. “The cubesat could bounce back and hit your main satellite, for example.”
Researchers will film how the net captures the spacecraft, beam the footage down to Earth and use it to analyse the experiment. The cubesat in the net will naturally deorbit within one year, according to Forshaw.
Later, the second cubesat will be released and as it floats away from the main spacecraft, the researchers will use it to train the vision-based navigation system. In a real mission, a chaser spacecraft would use such a system to assess its target before attempting the capture. In this case, the researchers will just gather data to further fine-tune the algorithms behind the rendezvous technology.
Finally, a pen-sized titanium harpoon will be shot across a 1.5m distance into a 10x10cm fixed plate that will extend from the main spacecraft on a boom.
The net and sail have been built by a RemoveDebris project partner, Airbus. In its facilities in Stevenage, the company’s engineers are already testing a much larger harpoon that could one day be used to remove the Earth observation satellite Envisat – one of the largest pieces of space junk in low-Earth orbit.
About the size of a double-decker bus, Envisat stopped responding to ESA controllers in April 2012 after 10 years of service. Since controllers lost contact with the satellite unexpectedly, they didn’t manage to conduct any manoeuvres to send the 8-tonne juggernaut to a lower altitude, so it would deorbit within the 25-year period recommended by space safety guidelines.
Since then, the craft has been slowly slipping from its 770km orbit, but according to Luisa Innocenti, who leads ESA’s Clean Space Initiative, it would take 200 years for Envisat to deorbit naturally.
“There are estimates that there is a risk of between 15 and 20 per cent that Envisat could collide during that period with another piece of debris,” said Innocenti. “Yet it could be also an order of a magnitude higher, if we start launching more satellites” – which is exactly what is expected.
If Envisat were to collide with another piece of debris or hit an operational spacecraft, the event would generate a massive quantity of fragments that would remain zooming through space, threatening everything in their way.
It wouldn’t be the first collision. In February 2009, a defunct Russian spacecraft Cosmos-2251 hit and destroyed the then-functional telecommunication spacecraft Iridium 33. The crash generated thousands of fragments, many of which still hurtle through the orbital environment.
ESA is therefore working on plans to send a garbage collector spacecraft to remove Envisat in the mid-2020s.
In early March this year, Airbus tested a harpoon almost as large as it would need to capture Envisat. The 1m-long 2.2kg device made of titanium, steel and aluminium was shot across a 1.5m distance into a breadboard, hitting it at the speed of 25m/s.
“We are trying to demonstrate we can successfully capture a piece of spacecraft with our harpoon design,” says Alistair Wayman, advanced projects engineer at Airbus. “To capture Envisat, the harpoon would have to be about a metre and a half long and weigh 2.5kg. In comparison to Envisat, it’s pretty small.”
‘We are designing the harpoon around Envisat because it’s the largest piece of debris. If you can capture Envisat, you can capture everything.’
By the end of this year, Wayman and his team would like to conduct a more realistic test, shooting the harpoon across a 25m distance, as if it was capturing a real satellite in space.
The harpoon development has been funded by ESA’s Clean Space Initiative as a contender for the Envisat-removal mission. However, the agency has decided to opt for a robotic arm, as it can also be used for in-orbit servicing – repairs and refuelling of operational satellites – and is therefore more likely to find support among governments, according to Innocenti.
However, Wayman believes there will be commercial demand for the harpoon technology as the intensity of the space debris problem increases.
“We are designing the harpoon around Envisat because it’s the largest piece of debris,” said Wayman. “If you can capture Envisat, you can capture everything. Active space debris removal is getting more and more important. Every year we don’t achieve the target of deorbiting five large pieces of space debris, the situation gets worse and we are more likely to have more collisions.”
The RemoveDebris team say their mission won’t leave any debris behind. All satellites used in the experiment will burn in the atmosphere within one year. The two small cubesats will deorbit naturally. The 100kg main spacecraft will, after completing all scheduled experiments, deploy a 3m drag sail that will speed up the orbital decay process from months to weeks, according to Professor Aglietti. The sail, made of heat-resistant polyimide Kapton, will slow down the satellite by increasing drag caused by the residual atmosphere at the relatively low altitude of the ISS.
“The system has an inflatable boom to move the sail a bit away from the satellite,” says Aglietti. “The sail itself is deployed via four booms made in carbon fibre, which pull the four quadrants of the sail.”
Unlike the harpoon and the net, passive debris removal sail technology is almost ready for commercial deployment. Aglietti says the University of Surrey, which developed the device, is already talking to commercial partners who might be interested in deploying such systems on their spacecraft in the not-so-distant future.
According to Aglietti, although it will work most efficiently at low altitudes with “a reasonable amount of residual atmosphere”, the solar sail would also work at higher altitudes, taking advantage of solar wind to slow down the spacecraft.
“It significantly speeds up the deorbiting process,” says Aglietti. “What would take years becomes months or weeks and what would take 100 years to deorbit, goes down within a few years.” This would help operators to meet the requirement to take down satellites within 25 years from the mission end. Only 60 per cent of satellites are deorbited within the 25-year period, which contributes to the increasingly cluttered space environment.
“The situation is not beyond dramatic, but we have to prevent getting there,” says Aglietti. “The risk of collisions is a real risk, it’s not imaginary. We want to make sure that when we put in orbit new satellites, the probability of damage because of debris impact is as low as possible.”
At the end of the day, Aglietti says, cost is likely to be the decisive factor and engineers designing deorbiting devices – active and passive – would have to take it into account.
“It’s a little bit like the issue of cleaning the oceans,” Aglietti says. “Who is going to pay for it? If it’s going to be not too expensive, then people are going to do it, but if it costs a fortune, it is more difficult to persuade people.”
Cool ideas for tackling space junk
A team from the University of Cape Town in South Africa is developing a tentacle made of shape memory alloy that could gently grasp a small piece of debris tumbling in the orbit. The tentacle hand, dubbed Medusa, is made of nitinol, a material that can repeatedly switch between solid and molten state. This provides the spacecraft catcher with the opportunity to attempt to capture debris repeatedly – unlike one-shot solutions such as harpoons and nets.
Engineers from Stanford University and Nasa’s Jet Propulsion Laboratory developed grippers inspired by gecko feet that could gently catch pieces of space junk. The surface of the grippers is covered with microscopic flaps that stick to a surface when in full contact with it. The so-called Van der Waals forces – weak, intermolecular interactions that result from subtle differences in the positions of electrons on the outsides of molecules – are behind the effect.
Like gecko feet, the grippers are only sticky when pushed in a certain direction. To make them stick requires only a gentle push, which could be an advantage when catching space debris compared to harpoons or complex robotic arms.
British firm Hempsell Astronautics proposes a system dubbed Necropolis that would collect defunct satellites left in geostationary orbit – the highly sought orbit at the altitude of 36,000km, where speed of the orbiting satellite matches velocity of Earth’s rotation, allowing the satellite to virtually hang above a single spot on Earth.
The Necropolis system would consist of two spacecraft: a Hunter, which collects other spacecraft and pushes them up away from the geostationary ring, and a Terminus, the final parking spot for the no-longer-needed satellites. Hempsell Astronautics said the strategy would be more sustainable than the current – and rather chaotic – use of the graveyard orbit, which is an area a few hundred kilometres above the geostationary ring, where operators push their spacecraft at the end of life.
In June 2017, Italian start-up D-Orbit launched a cubesat designed to test a new deorbiting thruster. The intelligent thruster was equipped with a power supply and communication systems independent from the satellite so it could propel the cubesat, dubbed D-Sat, towards the atmosphere when the satellite was no longer functional. An attempt to execute the manoeuvre, the first of its kind, was conducted after a three-month mission. However, the company said the technology didn’t achieve the intended goal.