# Tutorial Review on Space Manipulators for Space Debris Mitigation
## 3 Robotic On-Orbit Servicing  
Although astronaut on-orbit servicing (OOS) has progressed in leaps and bounds since the Solar Maximum Repair Mission (1984), it nevertheless was foundational in illustrating the difference between the designed-for-servicing ORU exchange and the not-designed-for-servicing main electronics box (MEB) exchange. Robotic on-orbit servicing has not made such advances in comparison to astronaut EVA capabilities. Robotic OOS is a class of mission in which a robotic servicer (chaser) spacecraft intercepts and performs servicing tasks on a client (target) spacecraft. This potentially involves a range of complexity of servicing tasks from observing the state of the target to translating the target into a new orbital state to direct robotic manipulation of the target to repair or upgrade it. A particularly relevant aspect of OOS is its implications for military space situational awareness from ground telescopes [9; 10]. In all these cases that we consider, the targets have been cooperative, differentiating them from the targets from active debris removal which will be uncooperative. There have been several technology demonstrator missions beginning with the foundational ETS-VII mission. The Japanese Experimental Test Satellite, ETS-VII (1997), successfully tested a 6 DOF 2 m long robotic manipulator with a three-fingered hand teleoperated from the ground under a time delay of 5-7 s (introduced by a TDRSS--tracking and data relay satellite system--relay) to demonstrate r-bar and v-bar automated rendezvous and docking tasks of a 248 kg chaser (Hikoboshi) to a 380 kg target (Orihime) followed by taskboard experiments including general peg-in-hole and ORU exchange-type tasks [11]. The DLR GETEX (German Technology Experiment) software that formed the backbone of the teleoperation tasks was based on task-level programming to upload motion sequences for autonomous execution [12]. It supplemented its 2 Hz video frame rate cameras with proximity rangefinders and implemented force feedback control. ETS-VII successfully demonstrated several on-orbit tasks: (i) remote observation; (ii) autonomous target chase and capture; (iii) ORU exchange including bolt fastening and target berthing; (iv) ORU electrical and fluid interfacing; (v) peg-in-hole insertion task based on wrist force/torque sensing; (vi) model-based predictive teleoperation with a 5-7 s time delay: (vii) flexible object handling, mating/demating of electrical connectors, truss joint connections, tightening of latches, etc. In 2005, DART (demonstration of autonomous rendezvous technology) attempted to rendezvous and dock with another satellite, but its defective autonomous navigation system caused critical propellant consumption invoking an abort to the mission. This failure was followed by a success, Orbital Express (2007) demonstrated a suite of on-orbit tasks, including freeflying capture, ORU exchange and refuelling by an autonomous servicing satellite (ASTRO) that mounted a 6 degree-of-freedom revolute manipulator that performed tasks on a serviceable satellite (NextSat) [13,14]. While the manipulator arm was in motion, ASTRO operated in free drift with a free-floating base. It corrected its attitude once arm motion was complete, so attitude and manipulator control were implemented independently in time. This was similar to the approach adopted on the preceding ETS-VII servicing mission demonstrator. This is a rather cumbersome approach. Once the compliant grapple fixture on NextSat was captured by the manipulator, the join was rigidised and arm controller parameters adjusted to accommodate the new high inertia payload. Repeated demonstrations of autonomous rendezvous and docking, autonomous propellant transfer and changeout of batteries and onboard computers were performed. ASTRO used both visual and LIDAR tracking to compute relative position and velocity. It demonstrated the use of proximity station-keeping and the use of a robotic arm to grapple the target to minimise interaction forces that would be imposed by positive docking.