# Capturing a Space Target Using a Flexible Space Robot
## 1 Introduction  
With the development of science and technology, human explorations of space have been growing quickly. A number of space launches have been carried out in recent years to meet the demands of communication, weather monitoring, and the military. However, human space exploration activities have tremendously affected the space environment, leaving a large amount of space debris such as failure satellites, upper rocket stages, and collision derivatives, which bring a significant threat to human space activities. To ensure the security of spacecrafts, some active debris removal (ADR) schemes were proposed, such as lowering the orbit of near-earth target debris and burning them down in the atmosphere, [PERSON] the debris on the geostationary Earth orbit to a graveyard orbit. In all of these schemes, capturing space debris on-orbit is necessary. Since most space debris are non-cooperative and information about them, for instance, mass, centroid location, geometric shape, and motion state, is unknown, the capturing operation is a difficult task to realize. To date, some capture methods have already been proposed, such as manipulator, net, and hook. Among them, the research of manipulator-based capturing technology is a significant direction [1, 2]. Related works include those of McDonald Dettwiler Space and Advanced Robotics Ltd. (MDSAR) who developed a simulation tool to analyze the dynamic behavior between robot and target during contact [3][PERSON] [PERSON] and [PERSON] [4] studied the contact between a free-flying space robot and a non-cooperative target. They developed an algorithm to minimize the contact impulse by planning the robot trajectory tracking and impulse minimization. In [PERSON] and [PERSON]'s research [5, 6], post-capture dynamics of a flexible spacecraft manipulator were studied deeply. [PERSON] et al. [7, 8, 9, 10, 11, 12] carried out extensive studies on the operation of capturing a free-flying/free-floating object by a rigid space robot, including impact dynamics modeling, impulse minimizing, and impedance control. [PERSON] [13; 14] studied the dynamic modeling and control of the capture of a satellite by a rigid space robot. To reduce the difficulty of the capturing operation, [PERSON] et al. [15] proposed a contact/push-based control method for reducing the angular momentum of an uncontrolled target satellite. For the same reason, [PERSON] and [PERSON] [16; 17] utilized a controllable compliant wrist installed on a rigid robot to offset the impact of the target satellite. [PERSON] et al. [18] used a Magneto-Rheological (MR) damper to offset the impact from the target satellite too. [PERSON] et al. [19; 20] studied the captures of a non-cooperative object and a larger flexible spacecraft by a rigid space robot, respectively. [PERSON] et al. [21] and [PERSON] et al. [22] conducted a series of studies on the dynamics and control of capturing a floating rigid body by a rigid space robot. [PERSON] et al. [23; 24; 25; 26] studied the dynamics of capturing space debris by a rigid space robot, and some research results about capturing strategy were achieved. [PERSON] et al. [27], [PERSON] et al. [28; 29], and [PERSON] et al. [30; 31] studied the control problem of the capture of a larger tumbling object by multi rigid robot arms. [PERSON] et al. [32; 33] studied the control problem of cooperative object manipulation by a rigid-flexible arm. From the studies above, we can see that the capture of space objects using space robots has gained much attention, and there are many research results. However, there still are some problems remaining for further study. For example, in most existing studies, the capture missions are conducted by a rigid space robot. The flexible space robot has a longer manipulator, so the flexible space robot has a larger working space than the rigid space robot. Thus, a flexible space robot could finish the capture mission in a relatively safe position.