# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 4 Space debris Mitigation
Satellite operators are being forced to undertake evasive manoeuvres on an increasingly frequent basis at the cost of scarce fuel--the defunct Telstar 401 and Galaxy 15 satellites have threatened operational satellites on several occasions. Although there is a natural atmospheric cleansing process operating in LEO below 800 km altitude, it is the LEO population at 800-1000 km sun-synchronous polar orbits around 86-110\({}^{\circ}\) inclination that are close to the Kessler limit, which is expected to be reached by 2055. The Kessler limit is the point beyond which the debris population becomes self-perpetuating and grows uncontrollably ([PERSON], 2007). The average growth rate of debris over the past 50 years has been around 300 objects/year but this is accelerating. Even if no further satellites were launched, the number of objects larger than 10 cm in the most populous ring (900-1000 km) will treble over the next 200 years due to debris-debris collision, increasing the collision rate by 10 times. The primary culprits for accelerating attainment of the Kessler limit were two specific events, one accidental and the other deliberate:  
1. In 2007, a Chinese anti-satellite test on its defunct meteorological satellite Fengyun FY-1C generated 2400 debris pieces larger than 10 cm and 35,000 pieces below the resolution limit at 860 km altitude--too high for natural atmospheric cleaning--increasing the space debris population by 30%. By contrast, a US ship-mounted Aegis ballistic missile was launched in 2008 in response, destroying one of its ailing spy satellites USA 193 at a low altitude of 250 km to prevent uncontrolled re-entry to Earth and to maximise the rate of atmospheric sweeping and so minimise debris. 2. In 2009, a 10 km/s collision occurred at inclination 72\({}^{\circ}\) N between the operational Iridium 33 satellite (at 86\({}^{\circ}\) inclination) and the decommissioned Cosmos 2251 (at 74\({}^{\circ}\) inclination). This generated two clouds of debris, (598 Iridium fragments and 1603 Cosmos fragments larger than 10 cm) at 790 km altitude. Russian debris also collided with and terminated the mission of Ecuador's first and only satellite, Pegasus, in 2013. Large objects can collide generating large amounts of debris--such breakups are estimated to occur at around 4-5 fragmentation events per year. If a 10 cm\({}^{3}\) debris fragment collides with a 1200 kg spacecraft, \(\sim\)10\({}^{6}\) fragments of \(\sim\)mm size can be produced. Similarly, discarded rocket stages with residual fuel can explode generating large numbers of debris fragments. The collisional cascading effect of the Kessler syndrome will soon cause entrapment of the Earth. Space debris has become a significant problem at all satellite-populated orbits, including GEO (Table 3). The primary source of debris resides in GTO (geostationary transfer orbit) within which some 239 objects intersect into the GEO ring. Within GEO itself which represents the greatest collision hazard, there are 1036 large tracked objects, only 340 of which are operational. However, a significant debris population of some 2000 small-scale objects greater than 10 cm size has been detected in GEO and GTO, in addition to the 1036 large objects presumably generated by fuel explosions of spent upper stages [25]. The most heavily spacecraft-populated longitudinal slots are around 75\({}^{\circ}\) East longitude (over India) and 105\({}^{\circ}\) West longitude (over the Pacific), primarily for telecommunications. The chief problem is that uncontrolled GEO platforms drift to these stable regions due to solar and lunar gravitational forces: between 1997-2003, 34 satellites were abandoned (mostly Russian), of which 22 librate over India and 10 over the Pacific.