Before considering to develop larger launchers or to implement formation flying missions or in orbit assembly, the possibility of deploying structures once in orbit is an appealing solution.
The Lunar Space Tug represents an alternative to the SLS scenario, especially for what concerns all unmanned or logistic missions (e.g. cargo transfer, on orbit assembly, samples return), from Low Earth Orbit to Cislunar space.
Specifically, an enhanced version of the space tug that has been described in the paper could be used to support on orbit assembly of large spacecraft for distant and long exploration missions.
The need for on-orbit assembly also becomes apparent, as availability of a HLV alone does not provide sufficient payload capabilities for any manned mission targeting NEOs.
For in-orbit assembly of future large space structures, teams of robots will be needed to manipulate large flexible structural modules while mounted on a large flexible structure.
Key characteristics of 1) multiple small elements launched separately and 2) on-orbit assembly to form a larger instrument are included in the architecture.
It is an important tool in completing space tasks such as on-orbit assembly, maintenance, manipulation assistance, payload care and astronaut on-orbit support.
Such docking capabilities could also be used for the in-orbit assembly of large rockets, for example, in a manned Mars mission, which are too big to be launched from Earth in one piece.
However, on-orbit assembly brings its own challenges in terms of guidance navigation and control, robotics, docking mechanisms, system control and data handling, optical alignment and stability, and many other elements.
Next, it structures the space applications scenarios into internal and external robotics in Low Earth Orbit (essentially the Space Stations), robotics for servicing of geostationary satellites, for in-orbit assembly of large space structures, and mobile robots (rovers) and manipulator-type robotics for surface operations on the Moon, on Mars, on comets and on asteroids.
The presented review shows that the collision-free trajectory planning in space robotics is a very active field of studies, but there are still several open problems that need to be solved before the proposed methods could be applied during on-orbit servicing, active debris removal and on-orbit assembly of large structures.
