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Space debris, sometimes known less politely as space junk, is debris of human origin which is in orbit around the Earth. Thousands of objects have been launched into orbit since the 1950s, and most of these objects have been left in place, rather than being brought back down. The result has been a huge accumulation of material orbiting the Earth, and in fact the space junk problem has become so severe that many operational spacecraft and satellites are at serious risk of damage from collisions with pieces of space debris.
Several things have contributed to the accumulation of space debris. The first is abandoned satellites and other objects launched into space. The second is intentional release of various components of spacecraft, such as the stages used in in rockets. Space debris is also caused by accidental releases, ranging from tools dropped by spacewalking astronauts to sections which have fallen off satellites and other equipment in space. Explosions of old and unstable craft such as satellites with unstable batteries are another contributing factor.
Each of these objects orbits the Earth at incredibly high speeds, and when it collides with another object, it can cause serious damage. A piece of debris the size of a pea has the potential to disable a satellite if it happens to hit at the right angle, and collisions create even more space junk by causing craft to break, explode, or lose pieces. The more collisions, the more debris will be created, and the more debris, the greater the risk of collision. The result is a cascading series of events which can quickly fill the sky with objects ranging in size from grains of sand to entire satellites.
Obviously, space debris poses a significant navigational hazard, and in fact several satellites and manned spacecraft have been forced to adjust their course to avoid crashes. Space debris is also an issue because it can fall out of orbit and enter the Earth's atmosphere. At a minimum, this will cause a distinctive meteor-like effect in the sky, but it can also result in the introduction of toxins such as heavy metals to the Earth's environment, and in injury to people and property.
Several international agencies track space debris, out of concern for collisions, and to make sure that space debris is not accidentally identified as an approaching missile or other weapon. Several astounding images plotting all of the known space junk have been published to highlight the issue. In these images, the Earth appears almost entirely obscured by hatch marks, each of which represents a single piece of space debris.
Several nations have also promoted plans to reduce the amount of debris in space, ranging from designing craft which can be brought back to Earth safely to actively reclaiming and containing some of the debris which surrounds the Earth.