Artist impression of space debris objects in orbit. Debris objects are not to scale. Courtesy of ESA.
Space debris encompasses both natural (meteoroid) and artificial (man-made) particles. Meteoroids are in orbit about the sun, while most artificial debris is in orbit about the Earth. Hence, the latter is more commonly referred to as orbital debris. Orbital debris is any man-made object in orbit about the Earth which no longer serves a useful function. Such debris includes nonfunctional spacecraft, abandoned launch vehicle stages, mission-related debris and fragmentation debris.
There are more than 20,000 pieces of debris larger than a softball orbiting the Earth. They travel at speeds up to 28,000 km/h, fast enough for a relatively small piece of orbital debris to damage a satellite or a spacecraft. There are 500,000 pieces of debris the size of a marble or larger. There are many millions of pieces of debris that are so small they can’t be tracked.
Trajectories of space debris objects in Earth orbit. Courtesy of ESA.
The number of space debris in LEO and GEO orbits is increasing at an alarming pace, which poses a serious threat for the continued exploitation of space as a strategic resource for many scientific and commercial missions. Debris impacts occur at an average relative velocity of 10 km/s, meaning that a regular 1-cm object can pierce any vehicle unless strongly shielded. Although catastrophic collision probabilities are still currently low, a plausible chain-reaction of collisions between debris could result in hundreds of thousands of pieces that would render such orbits unusable in practice for decades or centuries (scenario known as “Kessler Syndrome”). Worryingly, current studies predict that the number of debris in the orbits of interest will continue to increase unavoidably during the next years—even if no further launch were made, due to collisions and explosions of existing pieces—unless active removal actions are undertaken. Active Debris Removal (ADR), i.e. displacing space debris from crowded orbits using a dedicated orbiting and/or ground based facility, is however a difficult and risky operation.