杏吧原创

Earth鈥檚 orbit is crammed with junk. Since the launch of Soviet satellite Sputnik 1 in 1957, the skies over our heads have become increasingly crowded with the objects and debris we have placed or dumped up there. The more of this space refuse there is, the more likely it is there will be potentially catastrophic collisions with working satellites or spacecraft. Here is our guide to what鈥檚 circling our world and the best ways to deal with it.

Reduce, reuse, recycle

The pollution of outer space is rapidly approaching a crisis point. Preventing orbits from becoming unusable will require the same approaches we have been using on Earth

鈥淭he situation is critical,鈥� says Luisa Innocenti, head of the European Space Agency鈥檚 Clean Space Programme. If we keep throwing junk into outer space, she says, whole orbits could be rendered unusable for satellites. The nightmare scenario is the onset of the Kessler effect, in which debris from one collision cascades into further impacts, creating a self-sustaining effect that endangers satellites and threatens the future of space flight. Getting rid of what is up there already is a start (see 鈥淐osmic clean-up鈥�), but to keep the environment above our heads in a usable condition, we need to adopt some good Earth-based wisdom: reduce, reuse, recycle.

Reducing the number of satellites will be tricky. More than 390 objects were launched into orbit in 2018, and the number is likely to keep on rising. Elon Musk鈥檚 SpaceX has recently received permission to launch 12,000 satellites into orbit to provide internet coverage from space, and the OneWeb satellite constellation hopes to do the same thing with about 900 more.

For Innocenti, that makes debris mitigation vital. 鈥淭here is no point in cleaning up if the day after someone launches a satellite and abandons it there,鈥� she says. She advocates 鈥渄esigning for demise鈥�, programming satellites to self-destruct within 25 years of their missions ending. This can mean either blasting a craft to a nearby 鈥済raveyard orbit鈥� or ensuring it safely burns up in the atmosphere.

鈥淚f we keep throwing junk into outer space, whole orbits could be unusable鈥�

The Clean Space Programme is investigating foldable satellites that open up on re-entry to ensure they fully burn up, and sails for lower-orbit satellites that could produce enough drag to send them gliding down. RemoveDEBRIS, the team behind a number of recent debris-removal experiments, has developed its own version, a dragsail, to bring its test spacecraft out of orbit.

Finding ways to empty fuel tanks and disconnect batteries when decommissioning satellites would also reduce the risk of explosions, but more experiments are needed before these technologies can be trusted not to destroy spacecraft prematurely.

Reuse might be a more appealing option for craft owners. SpaceX and Blue Origin, founded by Amazon鈥檚 Jeff Bezos, have already pioneered reusable rockets, promising a shift to green space flight. But they probably won鈥檛 be eco-friendly in the long run: by lowering the cost of lift-off, they are also likely to increase the amount of stuff we send into space.

Another option for reuse may come from robotic capture arms of the kind proposed for debris disposal. These could be used to service satellites or bring dead ones back to life. Altius Space Machines, a company based in Colorado, is working on a magnetic connector called a MagTag that could be attached to satellites, allowing a service craft to latch on and transfer data, power or even provide software updates. This could all mean that spacecraft might soon be getting breakdown cover.

Fully fledged recycling, however, is some way off. US defence agency DARPA has considered employing robotic systems to grab old bits of debris and turn them into new satellites, but has abandoned this in favour of developing in-orbit servicing for spacecraft. Some firms have even contemplated building craft from space junk for missions further afield, such as to colonise Mars. But with most capture mechanisms still in development, and the difficulty of building something while in orbit, the odds of Frankensteining a satellite together are still pretty low.

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Low Earth orbit

Medium Earth orbit

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Cosmic clean-up

With a sea of debris in orbit posing an increasingly serious problem for space-faring nations, engineers are busy coming up with solutions. Here are some of the most creative

Tethers
In space: 2017

Think of it as space-fishing. Get close to a piece of debris, hook it on a line and haul it back into the atmosphere. Sounds simple, but plenty can go wrong. In 2017, the Japanese Aerospace Exploration Agency tested its Kounotori Integrated Tether Experiment (KITE), which was supposed to deploy a 720-metre-long electrified tether with a grabbing mechanism on the end. The interaction between the tether and Earth鈥檚 magnetic field would then generate a force large enough to drag any hooked debris into a doomed orbit. Unfortunately, the tether never unfurled and KITE was sent to its own fiery death not long after.

Nets
In space: 2018

If a rod won鈥檛 do it, try another way to fish. Remove DEBRIS, a consortium of space companies and research institutes funded by the European Commission, plans to catch debris in a net. Performing a test in orbit earlier this year, the team slung its 5脳5-metre Kevlar web at a small CubeSat released for the experiment. Masses in the corners helped the net wrap around the dummy junk while a motorised winch acted as a drawstring to pull it tight. In the future, nets would be tethered to spacecraft, which could then reel in the debris or tow it out of orbit.

Harpoons
In space: 2019

Piercing a defunct satellite is a risky business 鈥� you could hit a fuel tank or battery, causing an explosion that would send even more debris hurtling through space, or break off a weaker section such as a solar panel. But Guglielmo Aglietti, director of Surrey Space Centre in the UK, has reason to be confident. He has overseen pioneering harpoon tests on Earth and in orbit over the past few months that successfully demonstrate the idea鈥檚 potential. 鈥淲e haven鈥檛 really detected fragments and so on flying away,鈥� he says.

Gecko Grippers
In space: 2023

For finer control than nets and harpoons, scientists at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, have been designing sticky robotic arms capable of grasping and manipulating debris, inspired by hair-like structures on the underside of geckos鈥� feet. The structures stick with electrostatic forces, and luckily for the lizards, the effect doesn鈥檛 wear off with use. Tests on the International Space Station have shown that the grippers can capture and release different objects, but a trial in outer space has yet to come.

Particle beams
In space: 2028

Grabbing onto debris can be downright dangerous, making removing junk from afar an appealing prospect. Thankfully, in the vacuum of space, it doesn鈥檛 take much force to get something moving. Beams of charged particles shot from a distance, known as ion beam shepherds, could decelerate debris and send it tumbling into the atmosphere to safely burn up. Unfortunately, the imparted momentum would also push the clean-up spacecraft in the opposite direction. That has led scientists at Tohoku University in Japan to devise a system capable of ejecting beams in opposite directions. Known as a helicon thruster, this would allow a spacecraft to blast junk while staying a constant distance away, out of danger.

Lasers
In space: ~2038

鈥榃ith recent talk of a US Space Force, putting lasers into orbit might seem a terrible idea. But what if they stayed safely in scientists鈥� hands? 鈥淪pace-based lasers already exist,鈥� says Massimiliano Vasile at the University of Strathclyde, UK, though not terribly powerful ones. Much stronger lasers would be needed to get rid of debris, vaporising small pieces or pushing larger objects into doomed orbits. For Vasile, the risk is that we end up creating even more junk or send something into an unpredictable orbit. 鈥淭here are all these what ifs, and very few answers,鈥� he says. So, there is still some way to go before we can play space invaders for real.

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Getting crowded

The number of tracked objects in orbits continues to grow

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Junk worth saving

Among all the junk and debris in orbit, is there anything worth saving? Instead of seeing our planet surrounded by a celestial scrapheap, some people think of it more like a museum of floating artefacts. 鈥淭hese celestial resources have a lot of cultural, socio-economic, geopolitical, and other significance attached to them,鈥� says Paul Quast, trustee of the Beyond the Earth Foundation, an organisation examining celestial heritage. He believes that 60听years on from the start of the space race, icons of humanity鈥檚 first extraterrestrial adventures deserve preservation.

鈥淰anguard 1 is an obvious choice,鈥� says Alice Gorman, a space archaeologist at Flinders University in Australia. Launched in May 1958, the US satellite is the oldest surviving artificial object in space. Gorman is fond of its distinctive look, resembling a grapefruit with six long antennas jauntily sticking out. 鈥淚t鈥檚 very retro, very space-agey,鈥� she says.

Then there鈥檚 the US TRAAC radiation detection satellite, launched in 1961, which put the first poem in space, inscribed on one of the instruments. Gorman particularly appreciates the final line: 鈥淭o warm with human love the chill of space.鈥� 鈥淚 think that鈥檚 wonderfully beautiful,鈥� she says.

On Quast鈥檚 list of favourites is the observation satellite LAGEOS-1A, a brass sphere with the appearance of a luxurious golf ball, launched in 1976. A time-capsule developed by astronomer and writer Carl Sagan听sits inside, which听鈥� when it re-enters Earth鈥檚 atmosphere about eight million years from now听鈥� is designed to explain the spacecraft鈥檚 function to a futuristic population. The Hubble Space Telescope听ranks pretty high too. For decades, Hubble has provided astonishing images of the cosmos, but is headed for doom in the 2020s.

Then of course, says Quast, 鈥渢here are the Westford Needles鈥�. To secure US communications at the height of the cold war, thousands of small copper needles were blasted into orbit to create an artificial ionosphere around Earth. Many of them remain floating above our heads now.

Should we be hoarding old satellites and bits of junk when they pose a threat to the future of space flight? 鈥淚t鈥檚 always important to maintain planetary stewardship鈥� says Quast. By working in collaboration with clean-up operations, he and Gorman believe a balance can be struck between looking after the space environment and protecting cultural heritage so that future generations might journey out of the atmosphere and see some of humanity鈥檚 early steps off Earth first-hand.