The ReSWARM microgravity flight experiments: Planning, control, and model estimation for on-orbit close proximity operations

On-orbit close proximity operations involve robotic spacecraft maneuvering and making decisions for a growing number of mission scenarios demanding autonomy, including on-orbit assembly, repair, and astronaut assistance. Of these scenarios, on-orbit assembly is an enabling technology that will allow large space structures to be built in situ, using smaller building block modules. However, like many of these scenarios, robotic on-orbit assembly involves several technical hurdles, such as changing system models. For instance, grappled modules moved by a free-flying “assembler” robot can cause significant changes in the combined system inertia, which have cascading impacts on motion planning and control portions of the autonomy stack. Further, on-orbit assembly and other scenarios require collision-avoiding motion planning, particularly when operating in a “construction site” scenario of multiple assembler robots and structures. Multiple key technologies that address these complicating factors for autonomous microgravity close proximity operations are detailed in this work, in particular: (1) application of global long-horizon planning, accomplished using offline and online sampling-based planner options that consider the system dynamics; (2) adaptation of the recently proposed RATTLE information-aware planning framework for on-orbit reconfiguration model learning; and (3) connection with robust control tools to provide low-level control robustness using current system knowledge. These approaches were demonstrated for an autonomous on-orbit assembly use case by the RElative Satellite sWarming and Robotic Maneuvering (ReSWARM) experiments using NASA's Astrobee robots on the International Space Station. Results of the ReSWARM experiments are provided along with significant operational and implementation detail discussing the practicalities of hardware implementation and unique aspects of working with the Astrobee free-flyer robots in microgravity. ReSWARM provides a base set of planning and control tools for robotic close proximity operations, demonstrates them in microgravity, and outlines some of the important hardware aspects that future autonomous free-flyers will need to consider.