Japan's huge magnetic net will trawl for space junk

Gravity wasn't all fiction: tiny pieces of high-speed orbiting debris endanger our satellites. Now Japan is set to launch an electromagnetic net to catch them

SOMEWHERE in Earth's orbit, a satellite explodes into a terrifying cloud of debris. Moments later, Sandra Bullock and George Clooney are left scrambling to dodge the deadly space junk. This problem isn't confined to the Oscar-nominated space thriller Gravity – scientists are struggling with it in real life. Now a rather unusual solution is being tested: a really big net.

Next month, the Japanese space agency, JAXA, will pilot its "electrodynamic tether" for the first time. It is one of many possible solutions that have been proposed to deal with space debris (see "Catch 'em, drag 'em, blast 'em").

Hundreds of thousands of pieces of spacecraft, satellites and other equipment from human spaceflight zip around our planet, some travelling faster than the speed of sound. According to a report released by the US Congressional Research Service this month, running into even a small piece of junk can be disastrous. An object 10 centimetres across could "catastrophically damage a typical satellite", it says. One just 1 centimetre across could disable a spacecraft. The worst-case scenario is the Kessler syndrome, proposed by astrophysicist Donald Kessler in the 1970s. Too much trash, he warned, and the pieces would collide with each other, resulting in more and more debris.

To build its debris-catching net, JAXA brought in Nitto Seimo, a company that specialises in fishing equipment. Unlike a net you would use in the ocean, this one is a 700-metre-long mesh of aluminium and steel wires that hangs from an uncrewed spacecraft. The net is fitted with sensors that look for light reflecting from small pieces of debris and automatically aligns itself so that it can attract the material. The tether changes its orbit thanks to an electrical current flowing through the wires, which creates an electromagnetic field that attracts the debris and pushes the net away from Earth's geomagnetic field. Once the net has grabbed enough debris it is ordered to slow down and de-orbit, allowing the debris, spacecraft and net to burn up as they enter Earth's atmosphere.

JAXA thinks the net's main advantage is its simplicity – it's lightweight and doesn't require any propellant to move. If next month's test launch goes well, it plans to build a 10-kilometre-long version to capture satellites that have reached the end of their lives.

However, the test will also explore some possible drawbacks. One concern is that the net will work very slowly, taking several months or even a year to de-orbit. Then there is the risk that the net will run into operational satellites. The engineers also worry that the debris they are fighting could fight back. "There is a possibility of the tether being severed by impacts of small debris objects or micrometeoroids," says a JAXA spokesperson.

Not everyone is convinced of the idea. A net isn't necessarily the best option to collect debris, says Hugh Lewis, an aerospace engineer at the University of Southampton, UK. He has reservations about the net's ability to deal with space junk. In particular, he believes it could actually generate debris if it collides with a large satellite.

"There is a growing trend for organisations to put forward a concept for a debris removal device without considering fully the potential risks involved in deploying and operating the device," says Lewis. "I believe that these ideas should be subjected to international scrutiny before they are deployed."

This article will appear in print under the headline "Gone junk fishing"

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