Near-Earth space is anything but pristine. More than 18,000 chunks of junk larger than 10 centimetres across are hurtling around the planet at thousands of kilometres an hour.
The fragments of failed spacecraft, redundant satellites and spent booster rockets are accompanied by hundreds of thousands of smaller scraps of man-made space shrapnel, some with the potential to penetrate or destroy a functioning vessel.
Unless the world's space agencies – which between them launch roughly 120 spacecraft a year – find ways to stop the pollution and start a methodical clean-up, manned missions to the Moon and Mars, and the robotic exploration of outer space, may be at risk of colliding with space junk.
"Even the smallest paint fleck can cause huge damage at orbital velocities," says Jason Held, the director of Saber Astronautics.
His Sydney-based space technology company has built an instrument, known as a tether deployer, or DragEN for short, to prevent space junk from building up by safely de-orbiting decommissioned satellites.
The yo-yo-shaped device works by unrolling 200 metres of special conductive string, which interacts with the Earth's magnetic field to create a force that gently pulls a redundant satellite back to Earth, clearing the way for new instruments.
So excited is Nasa about DragEN that the US space agency's Flight Opportunities Program has selected Saber Astronautics, from a range of top universities and space companies, to participate in this year's parabolic test flight aboard a modified Boeing aircraft.
"Being chosen for the flight is a great opportunity for us to test the conditions that DragEN will experience in space," says Held.
"Of the 21 experiments selected by Nasa, our device is one of only two from outside the United States."
The Boeing 727-200, flown by the Zero-G Corporation, will follow the stomach-churning path of a roller-coaster, providing repeated stints of weightlessness.
Part of the aircraft will test DragEN's ability to deploy and unroll the tether.
"Measuring the speed at which the tether deploys is an important check before going into space," Held explains.
"We'll get about 20 parabolas, each 30 seconds in duration."
Three phases will be tested.
"First, we'll ensure the ballast releases properly and that unrolling can start without tangling," he says.
"Then we'll simulate a rollout phase to make sure the unrolling path does not deviate too much during deployment. Finally, we'll measure DragEN's deceleration during breaking at the end of the line."
One person will release DragEN, another will a record video and a third will monitor safety at the unrolling end.
"Two other people will reroll the tether and provide other support tasks during the flight," Held says.
HOW IT WORKS
Once a satellite reaches the end of its mission, a command from the ground will tell it to release DragEN.
The device consists of two parts: A base, secured to the underside of the spacecraft, and a ballast, held in place with a small burnable fibre. These two parts are spring-loaded, enabling DragEN to deploy by burning through the fibre.
The tether is wound around the ballast like a yo-yo, enabling it to unroll the same way once the springs have pushed it away.
A few metres of adhesive on the tether decelerates the device at the end of the line to keep it from bouncing back as a yo-yo normally does.
In essence, DragEN is an electrodynamic tether, explains its inventor, Daniel Bunker. This means the tether material is conductive.
"A conductive tether moving through the magnetosphere – the Earth's magnetic bubble – will generate a potential along it that is proportional to its length," Bunker says.
"It's like moving a wire through a magnetic field."
The potential drop in voltage along the tether generates a current.
"The current can be augmented through the emission of electrons at one end – or the tether can float free, with one part collecting ions and another part collecting electrons from the ionosphere," he says.
"The current in the wire moving through the magnetic field then generates a force which assists with deorbit. There are also increased deorbital forces from increased drag."
The first DragEN was selected as the science payload for a student satellite from Manipal Institute of Technology in India.
"India is one of the world's top space producers," Held says.
"Australia has potential as a space exporter and it augurs well for further international collaboration."
The Australian government, through the Australian-India Council, recently awarded the company a grant to foster new relationships between Australian and Indian academics and students interested in space.
US authorities, concerned about the fate of their military, remote-sensing and communications satellites, are planning novel ways to remove space debris by collaborating with private ventures.
For example, a South Carolina-based company, Star Inc, proposes launching a fleet of lightweight, reusable and propellant-free spacecraft, using Earth's magnetic field for propulsion.
The idea would be for the craft, dubbed the ElectroDynamic Debris Eliminator, to capture redundant satellites and other cosmic flotsam and jetsam using 200 lightweight nets. The detritus would be positioned in a safer Earth orbit, or sent hurtling into an ocean.
"If Star Inc can pull it off, they will have a game-changing technology for small and highly mobile spacecraft," Held says.
Also under consideration are ground-based lasers for bombarding space junk, causing it to deorbit and re-enter the Earth's atmosphere, where it should burn up.
"The project is in its early stages," Held says.
"If successful, it will be able to remove any size of debris that can be imaged from Earth."
Still other possibilities on the table include sending a giant umbrella into space to collect debris, or launching robotic spacecraft dedicated to chasing and destroying spent satellites or individual pieces of junk – or perhaps docking with, and then deorbiting, them.
Yet another suggestion would be for a new generation of satellites with folded-up helium balloons. When each satellite has done its work, its balloon would inflate, slowly dragging the redundant instrument to a lower altitude where it could safely burn up.
"Some of these technologies, while based on largely unproven technology, are scheduled for flight in the next few years," Held says.
HOW TRASH GETS THERE
Launching a vehicle into space requires two, three or even four stages, stacked on top of each other like building blocks. The last stage places the payload, or space cargo, in the right position at the right time. When a rocket's final stage has accomplished this task it, too, is in orbit.
"Each launch creates many bits of space junk, big and small," Held says.
"Most drop back to Earth, burning up in the Earth's atmosphere, but the rest remain in orbit for some time."
Some space junk successfully runs the fiery gauntlet through the atmosphere, ending up like the Skylab space station, which came down in Western Australia in 1979.
International agreements have raised standards for engineering a space launch, he says, to reduce debris.
"But no launch is completely clean – there is no such thing," Held says.
Nasa's Orbital Space Observatory keeps tabs on the distribution of space debris. Ground-based radar systems, meanwhile, have recorded about 21,000 individual scraps of junk over the years, much of it still up there.
Roughly 6 per cent of the objects circling the Earth are working payloads, with 21 per cent no longer in use. Another 16 per cent are final stages that placed the payloads in space.
About 12 per cent of the detritus comprises hardware such as clamps or nuts and bolts. The disintegration in orbit of the cargo or final stage accounts for the remaining 45 per cent.
Sometimes the garbage collides with a spacecraft. This happened when a French spy satellite was damaged by a fragment the size of a briefcase, travelling 20 times faster than a rifle bullet.
Another incident forced Nasa to consider making a critical avoidance manoeuvre when space debris threatened to disrupt the former shuttle Discovery's mission to the International Space Station. Although the sizeable section of an Ariane 5 rocket proved to be less of a hazard than feared, it could have wrecked the shuttle.
"All the more reason for space agencies in the US, Europe, China, Russia, India and Japan to reduce the junk they send into orbit," Held says.
- Sydney Morning Herald
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