Spacecraft, by nature, are delicate machines; they are designed to be lightweight and, typically, to fold elaborately to fit inside the fairing of their launch rocket. Unlike cars or aeroplanes, they are not designed to withstand rough treatmentduring operation – and they are certainly not designed to cope with being hit by an exploding hand grenade.
Indeed, that is exactly how the European Space Agency describes the impact of a 1cm piece of man-made space debris – and there are an estimated 700,000 or so pieces that size whizzing around Earth, many in the orbits useful to satellites. About 22,000 bigger pieces of orbiting junk are tracked by ground-based radar, and satellites or the International Space Station are regularly manoeuvred to avoid them, consuming precious propellant. But those small pieces – from fuel-tank explosions, in-orbit collisions and even an ill-advised 2007 Chinese anti-satellite missile test – are an invisible threat so serious thatESA, NASA and the US State Department have all warnedthat unless the problem of orbiting debris is tackled, valuable orbits could become unusable and, at an extreme, human spaceflight become too risky to undertake.
An elegant solution, however, may be in reach. By orbiting a super-wide field-of-view telescope and a laser originally designed to power particle accelerators, an international team of researchers thinks it can find those flying bullets and shoot them down.
The key is the EUSO (Extreme Universe Space Observatory) telescope designed by Japan’s RIKEN research institute to be mounted on the ISS, where its exceptionally wide view and sensitive optics would detect the faint light showers produced by high-energy cosmic rays entering the atmosphere at night. That mission was delayed but, says Toshikazu Ebisuzaki, who led the effort, the team realised that such a telescope could be put to another use: “During twilight, thanks to EUSO’s wide field of view and powerful optics, we could adapt it to the new mission of detecting high-velocity debris in orbit near the ISS.”
Twilight conditions, where the Earth is in shadow but the ISS and local debris are in sunlight, prevail for about 5min during every 90min orbit of the station. As a proof-of-concept, Ebisuzaki and his colleagues in Japan, California, Paris and Turin want to install on the ISS a miniature version of EUSO with a 20cm mirror (the full-size telescope planned for mounting to the ISS would have a 2.5m mirror) for passive detection of debris targets. A narrow field-of-view telescope allied with a pulsing laser would then track targets and illuminate them.
A subsequent mission featuring the full-size EUSO telescope and a more powerful laser, again mounted on the ISS, would then attempt to de-orbit debris particles. The trick will be to focus on them a laser beam intense enough to eject some of a debris particle’s material as ablation plasma – in effect, a jet that would slow it down to drop out of orbit and burn up harmlessly on re-entry. That demonstrator should be able to detect and target 1cm debris as far as 100km from the ISS.
In addition to the EUSO telescope, the concept relies on a so-called CAN laser. Built of a bundle of optical fibres, these Coherent Amplifying Network lasers are capable of pulsing at more than 10kHz and can be scaled up to sufficient power for the ablation. And, critically, CAN lasers are very energy efficient – necessary given the limited solar electric power available on the ISS.
But where Ebisuzaki’s team hopes to have an arguably profound impact on the future of spaceflight is in a subsequent free-flyer mission. They calculate that a EUSO-CAN system put into a 1,000km-high polar orbit that is gradually reduced, by 10km per month, would in 50 months – about four years – have mostly swept clean all of the useful low-Earth orbits.
Such a system could have some effect against debris larger than 1cm – it is intended to work on objects in the 5mm to 10cm range, which is about the limit of radar tracking – although it would become less effective as the size of the debris grows, according to RIKEN. However, a EUSO-CAN mission could also play a significant part in attempts to clear space of even the largest targets. As Ebisuzaki et al write in the journal Acta Astronautica: “Laser impulse control can also be used to modify the rotation rate of large debris objects such as derelict satellites or spent rockets.
“Prior to their capture by dedicated removal spacecraft it is necessary to stop their rotation.”
No small amount of engineering effort is being expended to devise such dedicated removal spacecraft, with concepts includecapturing large pieces with netsorharpooning them to attach decelerating thrusters. What all such concepts have in common, though, is that they would be expensive missions dedicated to the de-orbiting of a small number of debris objects.
De-orbiting large debris pieces is an important part of any drive to clean up space. Some of them have fuel on board, which can explode, and collisions between large pieces create small pieces, which may evade detection by ground-based radar.
The RIKEN concept stands out not just for targeting that smallest – and possibly most dangerous – of debris, but also because, as Ebisuzaki puts it, this space-based approach is “accurate, fast and cheap”.