Kessler Syndrome

Kessler Syndrome is a type of orbital phenomenon that results from an overabundance of orbital debris. It is named for NASA Scientist Donald J. Kessler who first proposed the scenario. In a Kessler Syndrome event, the density of orbital debris in Low Earth Orbit (LEO) is so high that collisions between objects are frequent. Collisions would generate additional debris, which would in turn produce other collisions, resulting in a cascade of increasingly frequent collisions. This situation would be disastrous to orbital activities and spacefaring operations for the planet in question.
A Kessler Syndrome would result in damage or destruction of most satellites in orbit in a very short period. Likewise manned spacecraft would be susceptible to fast moving debris, and even microdebris travelling at orbital speeds could significantly damage ships without adequate armor and shielding. At its most extreme, a Kessler Syndrome situation would make space travel in orbit prohibitively dangerous for many years.
Such a situation became reality in the opening years of World War 3. The destruction of International Space Station 2 and widespread combat in orbit resulted in a massive influx of orbital debris which triggered the opening stages of a Kessler Syndrome as early as 2116. The debris from this event reached its peak by 2124, rendering human spaceflight completely unfeasible by that time. Without intervention, the bulk of the debris may have remained in orbit for centuries.
After World War 3, surviving nations banded together to clear a swath of debris from orbit. This mission was partially motivated by a desire to return to space, but also by news of survivors trapped on Luna. The endeavor to clear enough orbital debris to end the Kessler Syndrome came to be known as the Pathfinder Project. The Pathfinders suffered significant casualties as a result of the Kessler Syndrome debris, but through their hard work and bravery mankind established a safe sector of orbit, dubbed Corridor One, through which future missions could be launched.
Conflicts in space since World War 3 have had to take the possibility of triggering a second Kessler Syndrome into account. Containing large debris clouds has been a major priority after battles or accidental collisions. An entire industry has been built around keeping orbits relatively clean and preventing future Kessler Syndromes, using specialized ships known as Orbital Trawlers to collect and contain debris (which can be salvaged for raw materials or parts). Orbital Trawlers have been given the informal nicknames "Kessler Breakers" and "Kessler Runners" for preventing another such event.

Localization

Kessler Syndrome is not unique to Earth. It can occur around any celestial body with a significant amount of space debris in orbit. Earth is, however, the only planet where the conditions were right for a Kessler Syndrome to occur so far. During World War 3, large amounts of debris were created in Lunar Orbit, but it was not enough for a cascading series of collisions indicative of a Kessler Syndrome.
Kessler Syndrome is most likely to occur in Low Earth Orbit (LEO), but can occur in Geostationary Orbit (GEO) or Medium Earth Orbit (MEO) given an injection of enough debris. The Kessler Syndrome resulting from World War 3 was mostly centered around LEO but also triggered debris coverage in MEO and GEO with about 25-50% of the density of LEO debris.