# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 4 Space debris Mitigation
An increase in cross section from 1 m\({}^{2}\) to 6 m\({}^{2}\) resulted in a much higher collision rate, but extension of satellite lifetime from 3 to 10 years reduced debris generation by 30%. If the strategy of satellite removal within 25 years were enacted in large constellations in LEO, it would obviate the mitigation effects of this strategy, and the prospects for extending it appear bleak [37]. Hence, self-disposal appears to be ineffective at LEO. Removal of debris is the only effective solution, but small-scale removal of debris will likely be ineffective [38]. Debris sweeper concepts are based on the use of large vanes rotating like a windmill, but such solutions require very large areas. The only effective solution is the active removal of large space debris by deorbiting, especially in critical sun-synchronous orbits, to prevent them from collisionally fragmenting [39]. It has been determined that it would suffice to remove 5-10 large pieces of debris per year to prevent debris population growth in LEO [40]--these could begin with the large 26 m long Envisat spacecraft and 20 Zenith upper stages, especially in polar orbits close to 1000 km altitude [41]. In addition, some 295 identical SL-8/Kosmos upper stages reside in near circular orbits clustered around 760 km/74\({}^{\circ}\), 970 km/83\({}^{\circ}\) and 1570 km/74\({}^{\circ}\). This strategy is sensitive to several conditions such as launch rates, compliance with debris mitigation measures, etc. [42]. A strategy that includes both post-mission disposal and active debris removal offers the best approach--a reduction of disposal timescale by one year reduces the annual number of collisions by 0.53 and the removal of one object reduces the annual number of collisions by 2.94 when employed synergistically, i.e., one active debris removal equates to a reduction in post-mission disposal of 5.5 years [43]. Issues that must be addressed during close proximity operations include minimisation of breakup and debris generation during debris removal. The robotic scenario comprises several well-defined phases: (i) approach--chaser acquires position around 10-25 m away aligned to the target with manipulators stowed; (ii) deployment--manipulators are deployed to capture the target by specific grapple points; (iii) grasp--manipulators engage the target at the grapple points; (iv) post-grasp--manipulators are locked and detumbling manoeuvres performed; (v) de-orbit--apogee engine kit is attached for de-orbit manoeuvre to drop perigee to \(<\)650 km altitude and engaged (for a 25 year decay by atmospheric drag). A representative scenario has been described in [44]. H-bar approach and approach along orbit motion is passively unsafe, favouring an approach against orbital motion [45]. Passive approaches to de-orbiting debris exploit perturbing forces such as aerodynamic surfaces, electrodynamic tether, etc. A passive strategy that exploits solar radiation pressure and Earth's oblateness could be adopted for MEO spacecraft such as global navigation satellites but requires high area-to-mass ratios except for satellites in Molniya orbits [46]. A chaser satellite mounting a robotic arm, robotic tentacles, a harpoon or a deployable net have been the commonest proposals for debris removal. The robotic arm and harpoon require precise targeting and the robotic arm and tentacles require close proximity to the target. ROGER (Robotic Geostationary Orbit Restorer) was a net-based concept for GEO satellite recovery to reduce the incidence of large space debris [47]. Two potential capture mechanisms were envisaged--a net capture or tether-based gripper with the former favoured--to stabilise and dampen its relative rotation to zero. The expandable net capture system is based on the deployment of four flying weights to pull out the net. The tether gripper system is a tether deployed free flyer system front end.