Space junk refers to the growing collection of debris, waste, and other artificially produced detritus in Earth orbit - essentially, the refuse and by-products of fifty years of space flight. The growing problem of space junk is particularly serious because, as the number of satellites in Earth orbit gradually increases, the risks of collision between space junk and functional satellites also increases.
There are believed to be tens of millions of individual pieces of space junk floating in Earth orbit, the vast majority of them tiny and more or less irrelevant pieces of matter, such as flakes of paint from space vehicles or slag from rocket fuel. Like tiny micrometeorites, these small pieces of debris do not pose a serious threat to spacecraft, although the long-term cumulative effect is to wear away the outer surfaces of spacecraft and satellites. Long-term spacecraft, including the International Space Station, are now built with minimal shielding to protect them against such tiny flecks and pebbles.
It is the larger space junk, however, which is considerably more worrying. Larger space junk ranges from gloves and cameras accidentally lost in orbit by spacewalking astronauts, all the way up to upper rocket stages and dead satellites. The fourth spacecraft ever launched, America's Vanguard 1, is still floating in medium Earth orbit today, is high enough that it will likely remain there for centuries. Objects in low Earth orbit tend to re-enter the atmosphere relatively quickly, but those in higher, more stable orbits will remain there for extremely long periods of time.
- Risks of Space Junk -
The obvious threat, both to manned and unmanned spacecraft, is that sufficiently large pieces of space junk will eventually collide with an operational satellite or manned ship, puncturing the hull and causing serious or fatal damage both to the spacecraft and to its human occupants, if any.
So far no manned spacecraft have been involved in such serious collisions - and both the American and Russian space agencies take precautions to prevent this from happening by coordinating mission planning with those agencies responsible for tracking space junk in orbit. Still, the Space Shuttle's windows have been struck by paint flecks at least twice; once, the fleck made it halfway through the Endeavour's front window. Had the object been much larger, the astronauts aboard would have been imperilled.
However, from the occasional experiences of unmanned spacecraft encountering space junk, the threat posed by growing space junk is obvious. Several Russian satellites have gone dead after suspected collisions, and the French Cerise collided in 1996 with fragments from an Ariane rocket that exploded a decade before. Most recently, in 2009, the Iridium 33 communications satellite was totally destroyed when it collided with a dead Russian satellite, Kosmos 2251.
The real fear to many, however, is not that such collisions will sometimes happen - but of what happens afterward. At the extreme velocities necessary to maintain orbital flight, even relatively small objects can have devastating impacts on larger spacecraft. One destroyed spacecraft will scatter hundreds or thousands of pieces of debris into orbit. If these go on to collide with other satellites, destroying them in turn, then still more debris will be scattered into orbit. This cascading effect, known as the Kessler effect or Kessler syndrome, could theoretically reduce Earth orbit to a lethal and impassable cloud of debris within a matter of years, or even months.
- Monitoring and Cleanup -
The United States has maintained several programs to track the growing catalogue of space junk. NASA once maintained a telescope for this purpose, at the Orbital Debris Observatory. The U.S. Strategic Command now maintains the national list of space junk objects, and now tracks about 19,000 separate objects large enough to be of interest and potentially risky to other craft.
Eventually, if space junk continues to accumulate at current rates, it will become necessary to actually do something about it. How to clean up Earth's orbit is not actually difficult to imagine. Objects in heavily used sectors, like geosynchronous orbits, are now required to be lifted up into a higher "graveyard" or disposal orbit at the end of their operational lifetimes.
Eventually we could build specially designed clean-up satellites to "sweep" Earth's orbital paths, using either lasers or some mechanical means to capture debris and either collect it in orbit or direct it down into the atmosphere. However, the volume of space to be covered is vast, and the budget available for such measures minimal. For now, space junk simply accumulates, awaiting the political and technical will to solve the growing problem.