THINKING about space junk in an entirely new way may have provided the crucial insight needed to tackle the problem. Researchers at the University of Southampton in the UK claim that removing a few key pieces of space junk from orbit should dramatically reduce the amount of debris that is produced in future. The idea comes from thinking about space junk as an internet-like network.
Space debris is a catch-all term for spent rocket stages and dead satellites, and the fragments they create as they collide, disintegrate or are deliberately destroyed – like the defunct Chinese weather satellite shot down by the Chinese army in January 2007 (see illustration). NASA estimates that there are 13,000 objects larger than 10 centimetres wide orbiting the Earth, only one-third of which are functioning spacecraft.
The amount of space junk is set to increase as Earth orbits become littered with derelict spacecraft. Some eventually fall apart due to the stresses and strains of being in space. In others, leftover fuel may explode, creating more debris, or the residual power in their batteries can unexpectedly bring gyros back to life and cause them to move into the path of active spacecraft.
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Now the (ISO) in Geneva, Switzerland, is developing guidelines designed to minimise these hazards. Future space vehicles must be able to survive the continual heating and cooling as they move in and out of Earth’s shadow, and at the end of their useful lives their excess fuel must be vented and their batteries drained of power. The ISO standard has the backing of the (IADC), an umbrella group of the world’s 11 major space agencies, and is expected to come into force next year. NASA, Ariane builder CNES of France and SpaceX, the nascent space-tourism firm, have all taken on board these recommendations in the design of their rockets.
But alarmingly, and colleagues at Southampton have found that the amount of space debris will keep growing if nothing is done. Lewis’s team created a computer simulation of the way objects collide in orbit that calculates how the amount of debris would change over the next two centuries. “The number of objects in orbit continues to rise even if we don’t have any more launches,” Lewis says.
That means that space agencies are going to have to start removing objects from orbit – but where should they start? To study how a population of orbiting objects interacts, Lewis and his colleagues turned to network theory.
Using maps of known fragments of debris, they worked backwards to determine which satellites and rocket stages had given rise to them. What they ended up with, says Lewis, was a network in which some objects have very few connections to others, while some have a great many connections.
That suggested a novel approach to reducing the amount of space junk. Removing junk with a low degree of connectedness would have little effect on collisions, but removing junk with high connectedness has a crucial impact on the production of debris.
The Southampton team liken this to the way the internet can survive most attacks but is vulnerable to others. “A targeted attack on certain highly connected nodes could bring it all down. That would be the aim here,” Lewis says. “If we take out the crucial pieces of junk, the network will effectively be destroyed.”
“If we take out the crucial pieces of junk, the debris network will be destroyed”
The next stage for the team is to model future impacts so that key pieces of junk can be targeted for removal. They presented their work at last week’s in Glasgow, UK.
Other ways to reduce space debris were also discussed at the IAC. Anatoly Alpatov of the in Kiev revealed his idea for a probe powered by an ion thruster that would use a giant sieve to capture millimetre-size debris as the craft spirals in towards Earth.
Another option is to find ways to remove spacecraft from orbit safely. Alex Fitié of , a space technology company in Delft, the Netherlands, proposed that low-Earth-orbit satellites should be fitted with an aerobrake – a device like an inflatable sail that would maximise drag from the meagre atmosphere to hasten their descent into the burn-up zone. His simulations show that decommissioned satellites fitted with aerobrakes would be removed from orbit within five years rather than the 25 years it normally takes.
A more down-to-earth option is to give operators an incentive to limit junk, by refusing to insure craft that are launched into debris-infested orbits. “The insurance market can and should impose conditions that force insurance seekers to decrease orbital debris production,” says Maria Buzdugan, a space-industry lawyer with in New York City.
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