Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion and collision fragments. As the orbits of these objects often overlap the trajectories of newer objects, debris is a potential collision risk to operational spacecraft.
The vast majority of the estimated tens of millions of pieces of space debris are small particles, less than 1 centimetre (0.39 in). These include dust from solid rocket motors, surface degradation products such as paint flakes, and coolant released by RORSAT nuclear powered satellites. Impacts of these particles cause erosive damage, similar to sandblasting. This damage can be partly mitigated through the use of the "meteor bumper", which is widely used on spacecraft such as the International Space Station. However, not all parts of a spacecraft may be protected in this manner, e.g. solar panels and optical devices (such as telescopes, or star trackers), and these components are subject to constant wear by debris (and to a much lesser extent, micrometeoroids).
A much smaller number of the debris objects are larger, over 10 centimetres (3.9 in). Against larger debris, the only protection is to maneuver the spacecraft in order to avoid a collision. If a collision with larger debris does occur, many of the resulting fragments from the damaged spacecraft will be in the 1 kilogram (2.2 lb) mass range, and these objects become an additional collision risk. As the chance of collision is a function of the number of objects in space, there is a critical density where the creation of new debris occurs faster than the various natural forces remove these objects from orbit. Beyond this point a runaway chain reaction can occur that reduces all objects in orbit to debris in a period of years or months. This possibility is known as the "Kessler syndrome", and there is debate as to whether or not this critical density has already been reached in certain orbital bands.
A runaway Kessler syndrome would render the useful polar-orbiting bands difficult to use, and greatly increase the cost of space launches and missions. Measurement, growth mitigation and active removal of space debris are major activities within the space industry today.