According to estimates by NASA and the ESA, there are about half a million pieces of space debris between 1 centimeter and 10 centimeters in size in low-earth orbit. More than 21,000 pieces are larger than 10 centimeters long. As for objects smaller than 1 centimeter, there are a whopping total of more than 100 million of them. Although such tiny objects may seem to pose little danger, the fact is that they're speeding around the world at speeds of up to 17,500 miles per hour, so collisions pack a lot of momentum and can be quite destructive. Only 7 percent of this hardware serves any function whatsoever; the rest of it is completely nonfunctional.

This debris makes it difficult for useful machinery to operate in space unhindered, and seem destined to eventually threaten the orbital instruments that human society depends on for weather monitoring, satellite internet, and the global positioning system. Now that there are many low-cost satellites available and many private companies are actively pursuing their use, it's expected that even more objects will be sent into orbit, worsening the congestion.

Also threatened by space junk is the International Space Station - as humanity’s only permanent colony away from the Earth’s surface, it is perhaps the most important installation in space. Constructed over a period of more than a decade at a cost of perhaps $150 billion, it houses crew members whose well-being may be in jeopardy from objects hurtling into it. Several times per year, the whole station has to be moved slightly in order to avoid collisions, and in other cases, there's not enough warning time to execute maneuvers, so the astronauts must “shelter in place.”

The hazards posed by high-speed junk in space have captured the imagination of fiction writers and filmmakers. In the 2013 film Gravity, pieces of a destroyed satellite hit a space shuttle, rendering it inoperative and killing most of the crew. In the Japanese manga “Planetes,” later turned into an anime TV series, the story revolves around the crew of a spaceship whose mission is to dispose of space debris before it can cause any damage.

The dangers of space debris aren't just confined to movies and television; there have already been serious incidents. In 2012, the functioning satellite Iridium 33 smashed into the derelict Cosmos 2251 above Siberia. This caused the formation of thousands of new pieces of debris, flying off in unpredictable directions.

This collision highlighted the possibility of what is called the “Kessler Syndrome.” First proposed in 1978 by the astrophysicist Donald Kessler, this model begins with the uncontroversial assessment that the more objects there are circling the earth, the more likely they are to collide. The more collisions there are, the more debris is generated, continuing the cycle. At a certain point, Kessler argued, there will be so much uncontrollable detritus moving around that it will be virtually impossible for humans to accomplish anything in space.

Thus far, there has been little done to clear out space debris either by NASA or other agencies around the world. Most of the efforts to mitigate this problem have focused on tracking pieces of debris and avoiding them rather than removing them from orbit. There have been occasional efforts to down unwanted satellites, such as in 2008 when the United States fired a missile from the cruiser USS Lake Erie to take out the misbehaving spy satellite USA-193.

A Swiss project plans to use a “Pac-Man” vehicle in 2018 to capture a small satellite. Officially named CleanSpaceOne, this craft will identify its quarry through the use of video cameras, chase it down by executing precise maneuvers and then catch it within a net. CleanSpaceOne will then fall back into the earth's atmosphere, burning itself and its target up. The RIKEN research institution in Japan, meanwhile, is drawing up plans to mount a laser on the International Space Station and use it to shoot down troublesome pieces of flying debris. Japan is also working on various strategies and technologies for de-orbiting obsolete or broken machinery although there are concerns that some of its efforts may violate the pacifist clauses of its constitution.

Because space belongs to no single country and therefore isn't the exclusive domain of any particular agency, proposed ways of dealing with the space junk situation must be multilateral in nature. The world has only made a few halting steps towards ameliorating the mess it has made in space, but this will have to change if we want to keep exploring space and using it to house some of our critical telecommunications and other infrastructure. Now that people are aware of the problem, we ought to ramp up efforts to devise clean, efficient methods of countering the threat of space debris before it's too late.