As the cloud of orbiting junk shrouding the Earth grows ever denser, the most sophisticated garbage collectors of all time are taking shape
IN SEPTEMBER 2009 a giant robotic arm beneath the International Space Station plucked an uncrewed Japanese cargo ship from the void of space. It was the first time this spectacular capture mechanism had been tried, but this robotic grab was no one-off. On 27 January this year, the Japanese space agency, JAXA, was involved again with HTV2, its second cargo craft (pictured). The feats show that “robotic capture” can be a reliable option in orbit.
Their success was critical for engineers developing technologies designed to clear space debris, because they need related orbital snatch-and-grab technology to drag defunct satellites to a lower orbit to burn up on re-entry. This matters because there are now 22,000 human-made objects larger than 10 centimetres across in orbit and half a million larger than 1 centimetre – and all pose a grave risk to space missions.
Advertisement
More debris is on its way. Hugh Lewis, a space scientist at the University of Southampton in the UK, has calculated that the debris population in low Earth orbit will increase by at least 33 per cent over the next two centuries. Even if space agencies never launched another rocket, the cloud of debris will continue to grow as pieces of space junk crash into one another.
There are a number of ideas about how best to go about clearing up this mess. At Star Technology and Research (STR) in Mount Pleasant, South Carolina, Jerome Pearson proposes a scheme in which a spacecraft comprising a conducting-cable tether would orbit Earth, grabbing debris and casting it into lower orbits (see diagram).
Studded with solar arrays that generate electric current in the cable, STR’s Electro Dynamic Debris Eliminator (EDDE) slowly rotates and uses the current’s interaction with Earth’s magnetic field to change its orbit. EDDE is manoeuvred until it matches orbits with the target, and rotates so it either robotically grabs the junk or ensnares it in a net. The debris can then be slung into a lower, re-entry orbit or EDDE can descend and then release it.
That’s all very well if the target object is intact. But if sharp shards are protruding from a tumbling, disintegrating dead satellite, it could damage any robot arm that tries to capture it. So Shin-Ichiro Nishida and Satomi Kawamoto of JAXA’s Kanagawa lab are researching how to capture such “non-cooperative” targets (Acta Astronautica, ). Their trick is to place long polymer brushes on the tips of the robot arm’s graspers. The deflection of the brushes as it closes in are sensed so that the safest moment to grab hold of the craft can be calculated.
Once a craft is captured, de-orbiting it can be hastened in many ways (see opposite). JAXA says a tether can be added to create drag in the rarefied upper atmosphere, for example. Christophe Bonnal at the French space agency CNES, in Evry, says a debris de-orbiting robot could fix a small rocket motor to a captured craft to send it down.
Re-entry is not the only option: dead satellites in geostationary orbit, at an altitude of about 36,000 kilometres, are normally placed in a “graveyard” orbit 300 kilometres above the ring of active satellites. But if a satellite runs out of fuel before it can be sent there it’s stuck. Shoji Kitamura at JAXA believes an ion-engine-powered service craft could fly close to a dead satellite and use a beam of ions from a second ion engine on the opposite side to slowly nudge the dead craft into the graveyard zone. It has drawbacks – backscattered ions might damage the service craft’s solar arrays for instance – but simulations suggest that it could work.
We are not going to find out a ny time soon which of these ideas will fly: legal issues over who owns space junk, combined with the expense of these technologies, means spacefaring nations first need broad agreement on debris management.
“The eventual remediation of the near-Earth space environment is a global issue and it will likely be undertaken via an international effort,” says Nicholas Johnson, NASA’s chief scientist for orbital debris.
System 1 The tether
The Electro Dynamic Debris Eliminator (EDDE, above) is an 11-kilometre-long aluminium tether that, once in orbit, needs no conventional propellant to carry out its mission. When launched, one end of it is given a kick by a small thruster to set it slowly spinning and to keep it under tension. The Earth’s magnetic field interacts with the current flowing in the straight tether to create a net force which controls its orbit – change the current and you change the orbit. The current is produced from solar panels positioned along the tether.
To help EDDE snare a defunct spacecraft, the current can be changed across different segments of the tether to induce small torques that allow its rate of rotation to be changed. Once it has reached its target, EDDE can then deploy either a net or a robotic arm to grab the object and de-orbit it.
System 2 Smart grab
The robotic arm on the space station proved that grabbing objects in space is possible. But when a satellite is retired it starts tumbling out of control as its gyroscopes fail. Combined with disintegration or collisions with other spacecraft, this can make it hazardous to grasp. This means that sensors are needed to ensure the robotic grasper system is not damaged as it grabs its target (see main story).
System 3 Socket rocket
Space junk can have a small solid-fuel rocket motor attached to it by a de-orbiting craft – requiring only a short burn in the right direction to take it down to a re-entry orbit. The craft could carry many such motors and be resupplied when necessary. The best place to attach the motor would depend on the target. Or a tether may be attached to create atmospheric drag, making the junk fall and burn up.
System 4 Ion engine
Spacecraft that fail in geostationary orbit, 36,000 kilometres from Earth, put their neighbouring TV and telecoms satellites in harm’s way. The Japanese space agency (JAXA) says a “re-orbiter” with one ion engine providing its own thrust and another firing an ion beam at a geostationary debris object could lift it into a graveyard orbit 300 kilometres higher – keeping it safe for a century.
