Space junk is a growing problem, and one that experts now believe could put future space exploration at risk. 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. (click on image to enlarge)
These objects consist of everything from spent rocket stages and defunct satellites to explosion and collision fragments. The debris includes slag and dust from solid rocket motors, surface degradation products such as paint flakes, coolant released by RORSAT nuclear powered satellites, clusters of small needles, and objects released due to the impact of micrometeoroids or fairly small debris onto spacecraft. As the orbits of these objects often overlap the trajectories of spacecraft, debris is a potential collision risk.
The vast majority of the estimated tens of millions of pieces of space debris are small particles, like paint flakes and solid rocket fuel slag. Impacts of these particles cause erosive damage, similar to sandblasting. The majority of this damage can be mitigated through the use of a technique originally developed to protect spacecraft from micrometeorites, by adding a thin layer of metal foil outside of the main spacecraft body. Impacts take place at such high velocities that the debris is vaporized when it collides with the foil, and the resulting plasma spreads out quickly enough that it does not cause serious damage to the inner wall. 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 micrometeorites.
The present means for spacecraft shielding, such as those used for the manned modules of the International Space Station, are only capable of protecting against debris with diameters below about 1 centimetre (0.39 in). The only remaining means of protection would be to maneuver the spacecraft in order to avoid a collision. This, however, requires that the orbit of the respective object be precisely known. The current equipment used to gather such information is only capable of tracking objects down to about 5 centimetres (2.0 in) diameter in low Earth orbit, and about 50 centimetres (20 in) in geosynchronous orbit. Out of the estimated 600,000 objects above 1 centimetre (0.39 in) diameter, only 19,000 can be tracked as of today. This leads to wide uncertainties in the estimated quantities of debris, and the predicted path of their orbits.
If a collision with larger debris does occur, many of the resulting fragments from the damaged spacecraft will also 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 that remove these objects from orbit. Beyond this point a runaway chain reaction can occur that quickly 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.