# Extensible Hook System for Rendezvous and Docking of a CubeSat Swarm +
## 1 Introduction  
CubeSats are miniaturized satellites that can reach a size between 10 cm \(\times\) 10 cm \(\times\) 10 cm (IU) and 22.63 cm \(\times\) 22.63 cm \(\times\) 45.4 cm (16U) and a mass lower than 25 kg for a 16U CubeSat. Based on standardization, their development is commonly fast and cheap. Initially, they were used for education and research, evolving toward a platform to demonstrate early technologies. However, they are increasingly used for commercial applications [1].  
It is common to launch a CubeSat alone or in constellations; however, the use of a CubeSat swarm arises as a novel method for operating as a single spacecraft with many advantages, i.e., scalability, robustness, etc. However, guidance, navigation, and control (GNC) of a CubeSat swarm is a very challenging task, requiring the use of novel methods and technologies. The field of guidance requires the use of complex motion planning methods that allow the coordination of the CubeSat swarm by generating trajectories to achieve a particular formation in space. Navigation requires knowing the 6 degrees of freedom (DoF) pose of each satellite by combining different proprioceptive and exteroceptive sensors. Finally, control must guarantee that the generated trajectory is followed by controlling the actuators.  
Currently, CubeSat swarms have been proposed for different applications. For example, in [2], the authors proposed the use of a distributed CubeSat swarm to perform low-frequencyradio astronomy, placing it orbiting the moon. The SWIMSat mission [3] proposed a CubeSat swarm placed in low-earth orbit (LEO) to monitor meteor impacts on the sky above North America. Finally, a CubeSat swarm has also been proposed in different missions related to Earth observation [4], highlighting the European Space Agency (ESA) Swarm mission to study the dynamics of the Earth's magnetic field [5].  
As stated before, CubeSats swarms have been investigated for different missions, taking advantage of the configurations to accomplish their objectives. However, emerging applications in space, such as active debris removal (ADR) [6] and space-based solar power (SBSP) [7] will require orbital rendezvous and docking (RVD) operations to capture debris or create space infrastructures. In the latter case, RVD can be achieved by attaching several satellites, e.g., the AAReST mission [8], with the objective of assembling a space telescope based on four 3U CubeSats that are docked. Other applications would require keeping the CubeSats connected to each other without requiring RVD. For example, building large structures in space, such as the one proposed for SBSP [9], could be made using a CubeSat swarm. In these cases, a hook system would be required to connect the CubeSat to each other, forming the required structure. Furthermore, an extensible hook system (EHS) provides an advantage related to avoiding the use of thrusters (propellant saving) in two scenarios: when RVD is required, the approaching stage can be completed by the EHS, and during a formation flight, where the EHS can be used to keep the CubeSat swarm aligned.  
In this sense, this paper proposes an EHS concept based on a scissor boom. This concept has previously been proposed to be used for different applications in CubeSats. For example, extending a set of sensors 13 cm from the CubeSat [10], with the objective of avoiding electronic interference with the sensors, or deploying solar array systems [11]. In this paper, we propose the use of a scissor boom mechanism to connect a CubeSat swarm that conforms to a hexagonal structure, as can be seen in Figure 1. In order to create this structure, not only the aforementioned mechanism is needed, but also the GNC system to link the CubeSats in the optimal way, which will be addressed in this paper by proposing some state-of-the-art methods that could solve the main issues.