HUMAN BEINGS are messy. They tend to leave rubbish behind them wherever they go—and to expect someone else to clear that rubbish up. This is true even in outer space. The problem of orbiting debris, and the concomitant risk of it colliding with and damaging an active and probably expensive satellite, has been around for a while. But it is rapidly getting worse. In the past three years, the number of times such bits of junk have almost hit operating satellites has roughly doubled.

That, at least, is the calculation made by Daniel Oltrogge, an expert whose conclusion is drawn from his two jobs. Mr Oltrogge is an adviser to the Space Data Association, an industry body that feeds orbital and manoeuvring information from many satellite operators into a computer model which forecasts likely collisions so that spacecraft, or at least those with appropriate thrusters, can be moved out of harm’s way. Mr Oltrogge is also the director of the centre for space standards and innovation at AGI, an American firm that develops orbital-mechanics software which also helps satellite operators sidestep collisions.

Part of the problem is the growing number of launches taking place. On January 13th, for example, Virgin Orbit, a firm in Richard Branson’s Virgin Group that is another new entrant to the market, is due to loft ten satellites into orbit using a rocket released from a modified Boeing 747-400. Another part, though, is that, every year, a dozen or so sizeable chunks of debris orbiting Earth break up. Around half of these explosions are caused by things like the ignition of leftover rocket fuel and the bursting of old batteries and pressurised tanks. The rest are the result of collisions.

The upshot is a chain-reaction of impacts in orbit. Unlike the fictional version of such a chain-reaction, which inconvenienced Sandra Bullock’s character in “Gravity”, a film released in 2013, this real one is accelerating only slowly, so there is still time to curtail it. But if action is not taken soon, insurance premiums for satellites will rise, spending on tracking and collision-avoidance systems will have to increase, and certain orbits risk becoming unusable. If things get really bad, the authorities may even have to step in to restrict the number of launches.

Drop shots
Stopping this orbital-junk-generating chain reaction means casting part of the superfluous tonnage in space down into Earth’s atmosphere, where the frictional heat of re-entry will burn it up. A clean sweep is not necessary. Removing a handful of the larger derelicts every year would be enough. Exactly how many is debated. Yamamoto Toru of Japan’s space agency, JAXA, estimates somewhere between three and seven. Ted Muelhaupt of America’s Aerospace Corporation, a taxpayer-funded research centre, reckons a dozen. But even that sounds doable. Except that no one knows how to do it.

People are, though, planning to practice. One practice mission, scheduled for lift-off in March, is led by Astroscale, a firm based in Tokyo. Astroscale proposes to launch, from Baikonur Cosmodrome in Kazakhstan, a mission dubbed ELSA-d. This consists of a 175kg mother ship called a servicer, and a 17kg pod fitted with a ferrous docking plate that will act as a dummy target. If all goes well, the servicer will eject and recover the pod three times, in successively harder trial runs, before thrusters push the whole kaboodle to fiery doom in the atmosphere below.

In the first test, the servicer will use springs to push the pod out and then, once it is ten metres away, will approach it again, lock onto the docking plate using an arm fitted with a magnetic head, retract the arm and pull it back to the servicer. For the second test, it will push the pod at least 100 metres away before its starts approaching it. A reaction wheel and a set of magnetic torque-generators will then put the pod into a tumble involving all three axes of motion, at a speed of half a degree a second.

This is, as it were, an important twist—for chunks of orbiting debris typically spin in this fashion. A real deorbiting mission will therefore have to deal with such spinning objects. Markings on the pod will help the servicer work out its prey’s motion. Using eight thrusters, it will manoeuvre itself until those markings appear, to its sensors, to be stationary. This will mean its motion exactly matches that of the tumbling pod, and that the magnetic head can therefore be extended to do its job.

For the third capture test, the servicer will first use its thrusters to back off several kilometres from the pod, putting the pod beyond sensor range. Then it will search for it, as would need to be the case if it were hunting for a real derelict spacecraft.

For all the technological prowess these tests will require, however, real derelicts pose a greater challenge than dummy ones. For one thing, unlike Astroscale’s pod, few spacecraft have been designed to expedite their own removal. Also, those objects which most need removing are dangerously heavy. A spacecraft that miscalculates while attempting to capture such a piece of tumbling debris could be smashed to smithereens, thus contributing to the problem it was supposed to be solving.

Grasping the matter
The Commercial Removal of Debris Demonstration, a plan by JAXA to deorbit a discarded Japanese rocket stage, highlights these difficulties. Before a spacecraft can be designed to capture whichever derelict Japan’s space agency selects as the experiment’s target, a reconnaissance mission must first be launched to study it up close. JAXA has awarded the contract for this part of the demonstration to Astroscale, which plans to do it using a craft called ADRAS-J, which will be launched in two years’ time. To measure the motion and features of a rocket part that might weigh tonnes, ADRAS-J will approach within mere metres. Once it collects the necessary data, another spacecraft can be designed to seize the junk on a subsequent mission.

In this case, magnets will be not be used to grapple with the target, for normal spacecraft have no iron in them. Using a harpoon to capture such an object might, however, be feasible. In a test conducted in 2019, Airbus, a European aerospace giant, successfully shot a harpoon from a satellite into a piece of panelling 1½ metres away. That panelling was, however, attached to a boom extending from the satellite, so this was but the most preliminary of experiments. Also, a harpoon can miss, ricochet or—worse—break off parts of the target which will then contribute yet further objects to the celestial junkyard.

Another option is to shoot a net. Airbus tested this idea in 2018. That test successfully enveloped a small “cubesat” which had been pushed seven metres away from the net-throwing craft—though this net was not tethered to the mother ship, which would therefore have been unable to deorbit its target. Tethers, indeed, are hard to manage in the weightlessness of orbit, which is why Airbus chose not to use one in this preliminary net-tossing experiment. And some doubt that such cosmic retiarii are a sensible idea. Chris Blackerby, Astroscale’s chief operations officer, expects the best approach will be to design robotic arms to clench the target vehicle’s fairing ring (the shallow cylinder that connected it to the jettisoned launching stage that lifted it from Earth), if this is still intact.

If all that works, JAXA’s debris-removal demonstration will face a final challenge. This is to execute a safe re-entry. Many pieces of the re-entering complex of captor and captive will survive frictional melting and slam, at speed, into Earth’s surface. Were re-entry to occur at a random spot, the probability of a human casualty would now exceed the threshold of one in 10,000 that NASA, America’s space agency, set as an acceptable level of risk in 1995, and which was adopted by Japan and other countries thereafter. The complex will therefore need to be put into a steep descent aimed at an uninhabited area—probably part of the Pacific Ocean.

As to the first clearance of actual orbiting debris, that is likely to be a European affair, for, in 2019, the European Space Agency awarded a contract to ClearSpace, a Swiss firm, to grab a 100kg piece of rocket debris that has been looping Earth since 2013. This mission is scheduled for 2025.

ClearSpace plans to use a capture craft fitted with four robotic arms. Unlike harpoons or net tosses, this strategy permits repeated attempts at recovery to be made. Even so, Luc Piguet, ClearSpace’s boss, expects his spacecraft will spend at least nine months in trials near the target before it secures the derelict and decelerates sufficiently to descend.

Pay up!
An era of serious cleanup in space is still some way off. Besides the technological obstacles, removing junk will be expensive. In addition to the costs of lobbing something into orbit, controlled re-entry of an object requires fuel, big thrusters and close attention from a ground controller. These things can tack millions of dollars—perhaps more than $20m—onto a deorbiting operation’s price tag. ClearSpace’s mission, for example, may cost as much as €100m ($122m), though Mr Piguet hopes subsequent jobs will be cheaper.

Cheaper or not, though, the question remains, “who will pay”? The littering of space is a textbook example of the tragedy of the commons, in which it is in everyone’s interest for a problem to be solved, but no one’s to be the lone individual who takes on the burden of solving it.

The solutions to tragedies of the commons usually, therefore, have to be imposed from outside, often by governments. One idea is a special launch tax, with the proceeds hypothecated to pay for clean-up operations. A more creative proposal is what Mr Muelhaupt calls “a bottle-deposit system”. Spacefarers would pay a deposit for each craft they lofted into orbit. If owners then failed to deorbit their equipment after its mission was over, the job could be done by someone else, who would then collect the deposit. That would encourage people to build deorbiting capabilities into satellites from the start, so the celestial dustmen would eventually no longer be needed. A third suggestion, proposed by Akhil Rao of Middlebury College, in Vermont, is to charge rent, known as orbital-use fees, for every commercial satellite in orbit. That would have the same effect.

Support for such schemes is growing, though they would require both international agreements between countries with launch facilities and an enforcement mechanism to stop outsiders with laxer rules from undercutting the arrangement.

There is also one other point. As Jean-Daniel Testé, once head of the French air force’s joint space command, notes, equipment developed for orbital cleanup could also be used to disable satellites. Mr Testé says advances in orbital robotics made by France’s adversaries, not to mention the lack of any international “space gendarmerie”, are leading his country to plan spacecraft to defend its military and intelligence satellites.

Mr Testé is coy on specifics. But France’s armed-forces minister, Florence Parly, has revealed more about her country’s plans than have her equivalents in other powers, America included. She foresees France launching special “lookout” and “active defence” spacecraft, to protect its assets in space. The latter are likely to be armed with powerful lasers. As Ms Parly has put it, “we intend to blind” threatening spacecraft. Preferably without disintegrating them.

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