A few weeks ago locals on the Isles of Scilly off south-west England were amazed when a huge piece of metal flotsam 10 metres by 4 metres was found floating in the sea, encrusted with shellfish. It's believed to be a piece of debris from an American space rocket, which has been brought ashore by ocean currents, far from where it fell from the sky.
It's a reminder of one of the downsides of space exploration, the issue of 'space junk'. This rocket piece fell back to earth, into the sea - maybe not an ideal way to dispose of it, but at least now it can be recovered and recycled. Many other smaller man-made bits also fall back towards earth and are burned up in the atmosphere before ever reaching the surface.
But millions of pieces of debris from damaged or discarded spacecraft and satellites remain in space, ranging in size from flecks of paint to whole satellites, the vast majority orbiting close to the earth. There are around half a million pieces of this orbital debris larger than a marble, and over 20,000 larger than 10cm in diameter. So why should we care about all these orbiting bits if they are not falling into the sea and polluting the environment?
The problem is that even tiny pieces in low orbit (less than 2000 km up) can reach speeds of 25,000 km per hour or more, and any impact with functioning spacecraft that also orbit at this altitude can cause significant damage. The International Space Station orbits in this zone, at 400 km, and has to move to dodge larger debris around once a year. The surfaces of spacecraft can become badly damaged from bombardment from small debris pieces – like this window from a returned space shuttle.
Even more serious, its possible that collisions between existing pieces of junk could create a domino effect known as the Kessler Syndrome, causing the number of bits of debris to multiply continuously, eventually making it impossible to use satellites in certain orbits. In our modern world that relies so much on satellites for collecting and transmitting information, this would be a major problem.
So what can be done about this issue? Debris can remain in orbit for many years before falling back to to earth, from a few years for items at an altitude of 400 km, up to hundreds of years for items over 1000 km high. Just waiting for it to fall to earth and burn up in the atmosphere is not a solution
Projects are under way to try and develop new technology for removing large pieces and for servicing and re-using old satellites. Bur for now all that can be done about existing debris it to track the larger bits so that collisions can be avoided, and to design spacecraft that can withstand damage from the smaller bits that can't be tracked.
Efforts are also being made to avoid creating new debris. Satellites in high orbits are often programmed to move out to an even higher orbit, nearly 36000 km up, once their life has ended - it's known as the 'graveyard orbit', well out of the way of any functioning spacecraft. Those in low orbit can be manoeuvred so that they drop out of orbit and fall back to Earth in an unpopulated area, often the middle of the ocean.
But there is still so much existing debris that, just like waste disposal on Earth, it's an issue that is unlikely to go away any time soon. This graphic, with each dot representing an artificial object in orbit, gives an idea of just how much man-made stuff is out there.
http://orbitaldebris.jsc.nasa.gov/ NASA Orbital Debris Program Office
http://www.iadc-online.org/index.cgi?item=links Inter Agency Space Debris Co-ordination Committee
http://www.space.com/topics/space-junk-orbital-debris-news/