The increasing preponderance of space junk – bolstered by new satellites being thrown into orbit – means we need to think more about how to bring them down again.
“We’ve known for quite some time that debris mitigation efforts will not be sufficient to bring the growth of the population of orbital objects under control,” says Hugh Lewis, professor of astronautics at the University of Birmingham. As a result, there’s an increasing need to try and bring down orbiting objects when they’ve expended their useful life in orbit, to make space for the next clutch of devices that can keep us connected.
One way to do so is to try and snatch it from its orbit using a specialist device that is launched into space in order to clean up the environment, then – once under control – either shift it away from the busiest points in space or to bring it back down into Earth’s gravitational pull, at which point it should burn up in our atmosphere. A handful of other proposals seek to shield the items from the worst of reorbiting’s constraints, and allow items to be reused multiple times.
That's something the UK government is increasingly interested in. In October, the UK Space Agency awarded a £61 million contract to ClearSpace to develop a spacecraft equipped with robotic claws that will capture and remove two defunct British satellites. The mission, which is scheduled to launch from Saxavord Spaceport in Scotland in 2029, aims to dispose of both satellites by 2032. The spacecraft itself will be designed to be refuelled and reused, with the hope that the technology could eventually make cleaning up space a more routine operation.
It’s a bold approach to tackle a real problem – but it’s also easier said than done. “To dock or to grapple an object is really hard, especially when that other object has not been designed for that and it's not under active control,” explains Lewis. Until recently, when the number of extraterrestrial items got out of control, there was little consideration for what would happen at the end of a space-borne object’s life. It was just assumed that it would stay up in orbit, regardless of the consequences. That means unlike current generations of items like those satellite internet firm OneWeb intends to equip with docking plates and grapple fixtures, older satellites and other items don’t have handles for grabbers to latch on to.
That’s a miniscule engineering issue in comparison to the broader problem. For one thing, in space the norms we’re used to on Earth go out the window, with items floating freely and able to be knocked and disturbed with even the slightest touch, meaning it’s exceptionally difficult to grab an item without problems. Add in the lack of friction or gravity and some of the tricks that engineers use closer to home to get good purchase on items isn’t possible.
Even just getting close to a satellite requires deft use of thrusters, alongside a range of different sensors, to try and understand where the object is. Items at the end of their life might be less orbiting a planet and more tumbling around in open space, so anything needing to grab it has to catch up and match its movements.
That gets hairy the bigger an object is. Nabbing a 4,000kg satellite would need a similarly large spacecraft , Lewis reckons.
There are other risks, too. Items have spent decades in space exposed to extreme ultraviolet radiation. Space also blows hot and cold, all of which can have an impact on the brittleness of materials. Some old rocket stages also contain residual propellant that can become dangerous if disturbed. “If you stop that thing from tumbling before you grab it, then there's a chance that any residual propellant might mix, and then it explodes,” says Lewis.
It's those unexpected interactions that Praveen Elavazhagan, a PhD researcher in space robotics at Cranfield University, is trying to understand. His work on multi-arm robots looks at how robotic systems can move around spacecraft and manipulate objects without inadvertently destabilising themselves or whatever they're working on. “The multi-arm robot should know what is each arm doing, and then it has to adjust itself to reduce the reaction forces,” says Elavazhagan. The spacecraft also needs to understand how the movement of its robotic arms changes its centre of mass, and compensate accordingly.
Elavazhagan’s research is contributing to a recognition that things have to change in order to work. And that means – for the first time in a while – the concept of regularly grabbing and deorbiting items from space isn’t just a hypothetical. It also has a timescale, albeit a potential one. “What we feel is we will be getting there in like six to 10 years, because that's rate at which we are progressing regarding each individual aspect of this,” says Elavazhagan.
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