# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 3 Robotic On-Orbit Servicing
Geostationary equatorial orbit (GEO) is the most heavily populated orbit for commercial spacecraft--their orbital altitudes are identical and offering 24 h ground-spacecraft communications links. An example of a promising (subsequently cancelled) servicing capability was Europe's SMART-OLEV (orbital life extension vehicle) to implement life extension to GEO communications satellites [15]. It comprised a spacecraft based on the SMART-1 bus to mechanically latch onto a client satellite at the apogee engine nozzle to provide orbit transfer and/or station-keeping functions using electric propulsion--it required no interfacing with the client spacecraft which operates normally other than the mechanical dock with its apogee engine capture tool. It had heritage from the earlier cancelled ConeXpress [16] (formerly SLEs [17]), a service module to provide 12 years of extended life support to aging geostationary satellites. Using Hall thrusters, it would dock with the target satellite until the capture tool on a retractable boom was inserted into the thrust cone of the inert apogee kick engine without the use of robotic manipulators. The apogee engine nozzle--common to almost all GEO satellites for circularising into GEO orbit from elliptical geosynchronous transfer orbit--is used as a docking port by inserting and locking a DLR-type capture probe in the nozzle's throat and then retracting the probe until the two spacecraft are locked together. The DLR capture tool includes six sensors for feedback on the relationship between the capture tool and the apogee motor volume and locks when fully inserted using a set of spreading pins. An alternative is the launch adapter ring as the sturdiest part of most spacecraft with which to grapple the spacecraft with manipulators and tooling. The American experience has been similar to the European experience in cancelled robotic OOS missions, the most notable being the Ranger Robotic Satellite Servicer. The next robotic servicing mission is to be Robotic Servicer for Geosynchronous Satellites (RSGS), currently slated for launch in 2021 (private communication 2018). The chief hindrance to the adoption of the OOS philosophy in spaceflight has been the reluctance to implement widespread adoption of modular and serviceable spacecraft designs with standardised interfaces capable of sustaining kick loads imposed by servicing tasks and amenable to modular repair or simple disassembly. Design for servicing requires ORUs, which are modular boxes with standardised grapple fixtures and connectors that serve as containment packages for functional subsystems--wholesale module replacement is much cheaper than entire satellite replacement and easier than complex repair tasks. Typical servicing tasks involve the exchange of ORUs units through simple mechanical procedures afforded by the standardised interfaces. Indeed, SSRMS itself comprises ORUs of two booms joined by an elbow pitch joint. Each end has identical roll-pitch-yaw wrists mounting identical latching end effectors. Similarly, SPDM bolts onto the SSRMS using only four bolts. SPDM's standard end effector is an ORU/tool changeout mechanism (OTCM) which incorporates a retractable 7/16 th socket wrench. Pistol-grip eva power tools for astronauts were designed to transfer kick loads from the client to the servicer--Robonaut hands are also designed to handle the same eva power tools as the astronaut. However, for the final grapple, emphasis in space rendezvous and docking has been in latching. A lightweight quick-change mechanism with a novel locking mechanism with high repeatability, reliability and adaptability is described in [18]. The most representative example of a serviceable spacecraft design is the Multimission Modular Spacecraft (MMS) which was the basis of the Solar Maximum Mission spacecraft (1980), Landsat 4 (1982), Landsat 5 (1984), the Upper Atmosphere Research Satellite (1991) and the Extreme Ultraviolet Explorer (1992).