# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 4 Space debris Mitigation  
The space debris problem has provided an urgent impetus to space manipulator-based missions that have traditionally focussed on on-orbit servicing of satellites. There have been around 5000 successful rocket launches from Earth into orbit and beyond since the dawn of the space age in 1957 (most launchers carry more than one spacecraft). There are currently 1382 active spacecraft in orbit (of which 340 reside in or near GEO) around Earth (446 American, 135 Russian, 132 Chinese and the rest others). Of the additional 15,888 (2015) pieces of orbital debris larger than 10 cm in size (dominantly in LEO), 2682 are defunct spacecraft, 1907 are spent upper stages, and 11,299 are other fragments [21]. This does not include the estimated 500,000 untrackable debris pieces in the range 1-10 cm (similar to or larger than the calibre of many infantry weapons) and a further 10\({}^{8}\) pieces down to 1 mm in size from paint chips to solid droplets, 98% of which is artificial space debris [22]. Most of the debris has been caused by the first spacefaring nations--predominantly the USA and USSR. In LEO alone, there are an estimated 150 million fragments of human-created junk less than 1 mm in size, all travelling at 8-12 km/s (for comparison, a rifle bullet travels at only 1.2 km/s). Smaller debris \(\sim\)1 mm in size can be protected against through Whipple shields though at the cost of mass suitable only for large and human-rated spacecraft. However, larger debris \(\sim\)2-3 mm in size or larger cannot be shielded against. The most debris-polluted orbits reside at 800-1000 km altitude beyond where natural clearing by atmospheric-induced orbital decay can operate. It is estimated that there are also 31 non-operational nuclear reactors which used NaK coolant between 700-1500 km in altitude from Russian radar ocean reconnaissance satellites (RORSAT) launched in 1967-1988. In 1977, the Cosmos-954 reconnaissance satellite, carrying an onboard nuclear reactor supplied with 50 kg of uranium-235, began behaving erratically and re-entered Earth's atmosphere without ejecting its reactor into a safe orbit as intended and scattered radioactive debris over a 600 km track in northern Canada. Although no injuries resulted, the cost of the subsequent clean-up operation was considerable. Although the probability of injury from re-entered debris is very small, in 1997, a piece of a spent Delta rocket struck a woman in Tulsa Oklahoma (though without injuring her) demonstrating that physical injury on Earth from space debris is plausible if remote. In LEO, the probability that a 10 m\({}^{2}\) satellite will be impacted by trackable debris is \(1\times 10^{-4}\)/y, which increases to \(2\times 10^{-3}\)/y for smaller untrackable debris. The estimated risk of a single debris collision in GEO between tracked objects is around 3% over a 30-year mission lifetime. As debris collides with satellites, they fragment into clouds of further debris. The orbital debris population is tracked and mapped by the US Space Surveillance Network--detectable objects are larger than 10 cm in LEO and larger than 1 m in GEO though new telescopes being constructed will reduce these limits significantly. It has become common operational practice to implement manoeuvres of active spacecraft to avoid collision with debris--for example, the ISS was manoeuvred into a 1 km higher orbit in 2001 to avoid collision with a Russian SL-8 upper stage (launched in 1971), and since then has had to be manoeuvred on average twice every year. In 1996, the boom of the French satellite Cerise was struck by debris from an exploded Ariane launcher shroud, although the mission was recovered through software workarounds. Indeed, NASA has developed a collision risk assessment process for its all high-value robotic spacecraft to prevent loss by collision with debris ([PERSON], 2007).