The six astronauts were awoken early and scrambled into escape capsules. Nasa ground control had spotted a piece of space debris hurtling towards their temporary home aboard the International Space Station 244 miles (390 km) above the Earth. The fast-moving junk was spotted just one day before its potential impact, making it too late to manoeuvre the station to a safer orbit. The only course of action for the three Russians, two Americans and one Dutch astronaut crew was to take shelter and prepare to evacuate if required.

In the event, the debris, a chunk of a defunct military communications satellite, missed by the ISS by approximately 7.5 miles (12 km). The incident, in March, was the latest close shave for the space station. Six other crew members had to take shelter in June last year when another piece of junk whistled just 1,100 feet (335 metres) past the station. Again, it was a precautionary measure, however Nasa estimates that anything larger than a baseball poses a potentially catastrophic threat to the ISS. It's a problem that is getting rapidly worse. US Air Force Space Command is tracking around 22,000 pieces of man-made space debris, mostly debris bigger than 10 cm across, and there are estimated to be hundreds of thousands more smaller bits.

"It would totally wipe out the space station if it were hit by a catalogued object," says Donald Kessler, a former Nasa scientist, who is widely regarded as the leading expert on the issue.

Snowball effect

Kessler first started thinking about space debris in 1970 when there was concern about pieces of asteroids and comets, known as meteoroids, and the risk of them striking an Apollo spacecraft. Kessler was studying asteroid collisions, which are responsible for much of the meteoroid population, when it suddenly occurred to him that sooner or later satellites in Earth’s orbit would also collide, producing growing numbers of fragments. “I was just curious as to how long it would be before that would happen,” recalls Kessler, who is now retired and living in North Carolina.

Before he could come up with solid estimates, Nasa ended the Apollo program and put him to work in another area. But Kessler continued to think about satellite collisions, and he got another shot at pursuing the problem in the late-1970s when Nasa created an office to look at the environmental impacts of the space shuttle program. Kessler was asked to look at solar power stations in Earth’s orbit. “I used that as an excuse to then work on orbital debris and did the calculations,” he says.

At the time Nasa scientists generally believed that North American Aerospace Defense Command (Norad) was cataloguing all man-made objects in space, but Kessler suspected there might be small debris that was slipping past the military’s radar network. And if the debris wasn’t there yet, it would be eventually, he reasoned. In a landmark paper published in 1978, he calculated that the number of trackable man-made objects orbiting Earth had increased by 13% per year from 1966 to 1976. He predicted that by about the year 2000 fragments from satellite collisions would pose a serious threat to other satellites, creating a "debris belt". “I was shocked at the answer,” he says.

That’s not all, however. Because each collision in space would result in hundreds of pieces of new debris, the threat of future collisions would grow exponentially, until low Earth orbit, the region between 100-1,240 miles (160-2,000 km) above our planet, would become a no-go zone for spacecraft. “That ended up getting a lot of people’s attention," he said.

Following the publication of his report, Kessler became the seer of space junk, and the snowball effect he described was dubbed the “Kessler syndrome.” For many years, however, his work remained mainly of interest to scientists and space agencies.

That quickly changed five years ago. In 2007, China conducted an anti-satellite weapon test, which destroyed a decommissioned weather satellite, turning it into 150,000 pieces larger than 1 cm across. And two years later a defunct Russian communications satellite called Cosmos 2251 struck an Iridium communications satellite some 500 miles (800 km) above the Earth. Just as Kessler predicted, the catastrophic collisions spawned tens of thousands of new pieces of debris, some of which were large enough to take out other satellites, and large numbers of smaller pieces big enough to cause significant damage. It was a piece from the Cosmos satellite that struck the ISS in March. The Kessler syndrome has long since ceased to be a merely theoretical doomsday scenario.

Collision course

Today, low Earth orbit is something of a space junkyard: out-of-commission satellites, discarded bits of rocket stages, and fragments of spacecraft circle the planet like zombies roaming a shopping centre. And though large collisions of the sort that destroyed the Iridium satellite are rare, the growing debris cloud poses an increasingly real threat to spacecraft.

Last year a US National Research Council study carried out by a group led by Kessler, reported that when a number of reasonable assumptions are fed into computer models, "the current orbital debris environment has already reached a ‘tipping point’." According to this scenario the numbers of objects, as well as their mass, had reached a level at which frequent collisions and spacecraft failures were now increasingly likely.

There is no shortage of good, or at least novel, ideas for removing space junk, ranging from Death Star-style lasers that would vaporize debris, to space-borne sweeping contraptions reminiscent of an illustration by W. Heath Robinson, the English cartoonist best known for his drawings of eccentric, ingenious and unnecessarily complex machines. Other concepts, no less creative, have included space nets, giant lassos, adhesive blankets, and low-power lasers to gently nudge debris away from collision paths.

One recent proposal that attracted some attention, though has no funding as of yet, is for a design that works in a way reminiscent of sticky flypaper. It would involve sending room-sized, adhesive-covered foam balls into space. Proposed by Sean Shepherd, a librarian from New Mexico, the balls would collect around 30 items of rubbish, before being guided by remote control towards the atmosphere to burn up.

In 2009, the Defense Advanced Research Projects Agency (Darpa), the Pentagon’s far-out research arm, took a serious look at the various proposals. The agency looked at, among other ideas, spacecraft equipped with magnets that would gather up debris, umbrella-shaped “sweepers” and a space tow truck that would drag junk into Earth’s orbit so that it would be incinerated.

Its report, called Catcher’s Mitt, concluded that many of the proposals were unrealistic with existing technologies. For example, the lasers required for most of the laser-based schemes are decades away from development. And "tens to hundreds" of ground-based lasers would need to be operated over a year to remove a single object, the report concluded.

It also found the greatest threat to operational spacecraft comes from medium-sized debris because there is so much of it and it often can't be detected from the ground. For these reasons the report recommended governments should concentrate on “pre-emptive removal” of large objects. Three years after its publication, none of the proposals have been funded.

Junk collectors

The first commitment to go beyond evaluating theoretical approaches has come from the Swiss Space Center (SSC), in Lausanne. Last year its scientists came up with the idea for CleanSpace One, a one-off spacecraft they intend to test by guiding it to recover one of the centre's own satellites. The design is inspired by the way sea anemones wrap their tentacles around their prey. It will have up to eight arms that can be unfurled and wrapped around pieces of debris.

“We thought that this debris issue is getting really important, so let’s launch the project to clean up in front of our own doorstep,” says Volker Gass, director of the SSC. “Once we pull it and hold it to our bosom we’re going to give the order to de-orbit the whole system. So actually we’re going to burn up on re-entry the catcher satellite and the catchee, the debris.”

Funding is a major issue for the project. Switzerland’s space budget is only a fraction of that spent by Nasa. If Gass can raise the required $11 million, he hopes to launch the experimental satellite within the next three to five years.

The United States, though not actively funding a debris removal program, is working on the problem. The US Air Force, for example, is developing a $3.5 billion “Space Fence”, a powerful new S-band radar system which uses a higher wavelength frequency than the current space surveillance radar to enable it to spot smaller chunks of debris. Defence companies Raytheon and Lockheed Martin are building prototypes.

“I like to describe the current system as tracking debris that is basketball size and the new system will see things softball size and even potentially smaller than that,” says Steve Bruce, vice president for Space Surveillance Systems at Lockheed Martin.

Darpa, in the meantime, is also pursuing a recycling approach. It proposes the launch of a robotic repair spacecraft called Phoenix to salvage and reuse components in order to rebuild decommissioned satellites. Rather than cleaning up the debris, the aim is to reduce the need for new satellites to be placed into orbit. “If this program is successful, space debris becomes space resource,” said Regina Dugan, the former Darpa director, when she announced the new project in 2010.

Of course, rebuilding a satellite with a robotic arm in space is easier said than done. The technology does not exist to do this sort of tricky rebuilding work autonomously, so a human would be required to guide the repair work. That means contending with the 10-second communications delay between the ground station and the spacecraft.

Glen Henshaw, a senior engineer at the Naval Research Laboratory in Washington, DC, which is working on the Darpa project, says: “There’s got to be a lot of smarts on board the spacecraft because, can you imagine driving your car down the Beltway, and there’s a ten second lag between when you turn the steering wheel and when the car turns? You’re probably going to crash."

Taking responsibility

But technology is only part of the issue. Another key obstacle is the lack of an underlying legal groundwork, according to Brian Weeden, of the Secure World Foundation, a Washington, DC-based think tank for space sustainability. Of the 22,000 pieces of debris being tracked by Norad today, the launch state and owner have been identified for only 16,000. Figuring out who owns space junk is essential for cleanup, because no country can unilaterally remove rubbish that may belong to someone else, and there are liability issues, according to Weeden. “Let’s say that you have Bob’s Debris Removal service and you go up there and your mission is to grapple with this rocket body, grab a hold of it and deal with it somehow, and in the process of doing so it explodes because it’s got leftover fuel inside,” he says.

In this scenario, who is liable for the destruction? “Is it the launching state or is it a country that licensed the removal service?” asks Weeden. The Outer Space Treaty, which governs international space law, states that satellites are sovereign objects that belong to the countries that launched them. It includes legal procedures for dealing with damage claims. "The important point is that it’s never actually been invoked," says Weeden.

Until the legal issues are sorted out, all proposed solutions will remain notional, or at best, limited to a small number of debris pieces. In the meantime, the threat continues to grow. As to what it might take to get policymakers to finally move forward with a solution, a lesson could perhaps be drawn from Kessler’s own experience.

After the seventh space shuttle flight returned in 1983 with a 4mm-deep crater in the front window, a committee was formed to investigate. It found the damage was the result of a collision with a paint chip less than 0.2mm in length travelling at just 5km/s (3 miles per second).

When Kessler, a member of the committee, met with then undersecretary of the Air Force, Edward “Pete” Aldridge, to convey the seriousness of the incident, he could see his message wasn’t getting across. That is until Kessler finally pulled the piece of chipped window out of his pocket and showed it to Aldridge, who suddenly seemed to get it.

As Kessler recalled: “He looked at it and said, ‘You know, one person’s data point is worth a thousand people’s speculation'.”

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