How to Wipe Out Space Junk

image frrom http://jemeuso.riken.jp/en/

Half a century of rocket launches has turned the space into a junkyard. Around 3,000 tonnes of empty rocket stages, defunct satellites, astronauts’ toothbrushes and flecks of paint are thought to be in orbit.

Besides being messy, such debris can be dangerous. Anything circling Earth is moving pretty quickly, so collisions between space junk and satellites can happen at closing velocities of 10km a second or more. Large bits of junk are routinely tracked by radar. The International Space Station (ISS), for instance, regularly tweaks its orbit to avoid a particularly menacing piece of litter. But at such high speeds, even a small, hard-to-follow object can do tremendous damage.

Rocket scientists have been pondering how to deal with this problem for years. But a paper just published in Acta Astronautica by Toshikazu Ebisuzaki and his colleagues at RIKEN, a big Japanese research institute, has gone further and proposed actually building a test device.

Dr Ebisuzaki’s plan involves zapping things with lasers. He proposes to point these lasers in the right direction using a telescope intended for a different job entirely. This is the Extreme Universe Space Observatory (EUSO). It is designed to be bolted on to the ISS. From that vantage point it will monitor Earth’s atmosphere, looking for showers of radiation caused by cosmic rays hitting air molecules. Dr Ebisuzaki, however, realised that the characteristics of a telescope designed for this job—namely a wide field of view and the ability to register even fleeting flashes of light—would also be well-suited for spotting small bits of debris as they whizz past the ISS.

Having identified something, the next step is to get it out of orbit—and that is where the zapping comes in… Fire a laser head-on at a piece of space debris for long enough, then, and you can slow it down to the point where its orbit will decay and it will burn up in Earth’s atmosphere.  This idea is not new. But putting lasers into orbit is tricky. Those powerful enough to do the job need lots of electricity and this is hard to deliver with the solar panels from which satellites typically draw their power. Dr Ebisuzaki proposes instead to employ a new, more efficient laser called a coherent-amplification network device, which was developed for use in high-energy physics.

He and his colleagues suggest a three-stage test. The first, with a smaller version of the EUSO and a fairly weedy laser, would serve as a proof of concept. The second would use the actual EUSO telescope and a much more potent laser. Finally, he says, the equipment could be mounted on a purpose-built satellite, from which it would be able to shoot down tens of thousands of bits of space junk every year, thus gradually sweeping the skies clean . 

Orbiting debris: Char wars, Economist Apr. 25, 2015, at 75

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