Grappling Spacecraft

This article provides a survey overview of the techniques, mechanisms, algorithms, and test and validation strategies required for the design of robotic grappling vehicles intended to approach and grapple free-flying client satellites. We concentrate on using a robotic arm to grapple a free-floating spacecraft, as distinct from spacecraft docking and berthing, where two spacecraft directly mate with each other. Robotic grappling of client spacecraft is a deceptively complex problem: It entails designing a robotic system that functions robustly in the visually stark, thermally extreme orbital environment, operating near massive and extremely expensive yet fragile client hardware, using relatively slow flight computers with limited and laggy communications. Spaceflight robotic systems are challenging to test and validate prior to deployment and extremely expensive to launch, which significantly limits opportunities to experiment with new techniques. These factors make the design and operation of orbital robotic systems significantly different from those of their terrestrial counterparts, and as a result, only a relative handful of systems have been demonstrated on orbit. Nevertheless, there is increasing interest in on-orbit robotic servicing and assembly missions, and grappling is the core requirement for these systems. Although existing systems such as the Space Station Remote Manipulator System have demonstrated extremely reliable operation, upcoming missions will attempt to expand the types of spacecraft that can be safely and dependably grappled and berthed.