If we don't deal with orbital debris, Earth will one day have rings of refuse – and we'll be cut off from space
EARTH'S rings have never looked so beautiful, you think as you look up at the pallid sliver of light arcing through the night sky. Yet unlike Saturn's magnificent bands of dust and rubble, Earth's halo is one of our own making. It is nothing but space junk, smashed-up debris from thousands of satellites that once monitored our climate, beamed down TV programmes and helped us find our way around.
This scenario is every space engineer's nightmare. It is known as the Kessler syndrome after Donald Kessler, formerly at NASA's Johnson Space Center in Houston, Texas. Back in 1978, he and colleague Burton Cour-Palais proposed that as the number of satellites rose, so would the risk of accidental collisions. Such disasters would create large clouds of shrapnel, making further collisions with other satellites more likely and sparking a chain reaction that would swiftly surround the Earth with belts of debris. Orbits would become so clogged as to be unusable and eventually our access to space would be completely blocked.
On 10 February 2009 it started to happen. In the first collision between two intact satellites, the defunct Russian craft Kosmos-2251 struck communications satellite Iridium 33 at a speed of 42,100 kilometres per hour. The impact shattered one of Iridium 33's solar panels and sent the satellite into a helpless tumble. Kosmos-2251 was utterly destroyed. The two orbits are now home to clouds of debris that, according to the US military's Space Surveillance Network (SSN), contain more than 2000 fragments larger than 10 centimetres. The collision may also have produced hundreds of thousands of smaller fragments, which cannot currently be tracked from Earth.
Such debris is a serious worry. With satellites travelling at tens of thousands of kilometres per hour, any encounter with debris could be lethal. "Being hit by a 1-centimetre object at orbital velocity is the equivalent of exploding a hand grenade next to a satellite," says Heiner Klinkrad, head of the space debris office at the European Space Agency in Darmstadt, Germany. "Iridium and Kosmos was an early indication of the Kessler syndrome."
Space junk isn't just made up of dead satellites. It also includes spent upper-stage rockets, used to loft the satellites into orbit, and items that have escaped the grasp of butterfingered astronauts, such as the glove Ed White dropped in 1965 as he became the first American to walk in space, and the tool kit that slipped from Heide Stefanyshyn-Piper's hand during a 2008 space walk. Protective covers and the explosive bolts used to separate them from uncrewed spacecraft have also been left to float away, along with a few lens caps for good measure. Some of these objects re-enter the atmosphere and burn up, but most are still up there.
The SSN has catalogued 12,000 objects in Earth orbit that are at least 10 centimetres in size, about three-quarters of which are space junk. For objects bigger than 1 centimetre, the estimates are frightening: there are anything from hundreds of thousands to millions of them, mostly in unknown orbits and each capable of smashing a satellite to smithereens. Every rocket launch creates yet more space debris, edging us ever closer to the Kessler syndrome becoming a reality.
Graveyards and zombies
So what can be done? For a start, we can try not to make the problem worse. This can be as simple as ensuring that protective covers are tethered to spacecraft rather than jettisoned. It also includes sticking to international guidelines intended to minimise new debris, drawn up by the Inter-Agency Space Debris Coordination Committee (IADC), which represents all the world's major space agencies. These require, for example, that spacecraft in low Earth orbit must be made to re-enter the atmosphere and burn up within 25 years of finishing their missions.
Communications satellites in the high-altitude geostationary orbit cannot be brought down practically. Instead, the guidelines say operators should use the last of their satellites' fuel to boost them into a "graveyard orbit" 300 kilometres higher up (see diagram) . Yet even with these guidelines in place, Klinkrad says, "It is pretty common to leave your spacecraft stranded."
Twelve satellites in geosynchronous orbit failed in 2008, but only seven were boosted in accordance with the guidelines. And more than 800 of the 1200 trackable objects near the geostationary corridor are not active satellites. The most recent drama there involved the communications satellite Galaxy 15, which became widely known as the "zombie satellite" (see "March of the zombie").
Even if the guidelines were followed to the letter, the number of debris fragments would still go up. "We could even stop launching and the amount of debris would still rise," says Hugh Lewis of the University of Southampton in the UK. That's because accidental collisions would still happen.
