REcommissioning and DEorbiting using Cubesat swarms with electrO spRAy ThrusTErs

The rapid growth of the space industry is increasing the number of satellites and uncontrolled objects in low Earth orbit. Collisions involving decommissioned satellites and other space debris can generate additional debris and threaten operational spacecraft used for communication, navigation, Earth observation, and scientific missions. New approaches are therefore required to make the use of space more sustainable.
REDECORATE investigated CubeSat swarms as a cost-effective approach to space-debris mitigation and satellite recommissioning. The concept uses small CubeSats equipped with staged ionic electro spray engines (iESEs). Their compact size, long operating lifetime, and controllable thrust make these propulsion systems particularly attractive for missions requiring several small spacecraft to operate together.
The envisioned CubeSats rendezvous with a decommissioned satellite and attach around it. Their propulsion systems can then act together as a distributed control system to stabilize the target, change its attitude and orbit, and ultimately support either deorbiting or, where possible, recommissioning of the satellite.
REDECORATE combined this application with the development of robust and safe-by-design control methods. The project aimed to develop the CubeSat and mission concept, design control methods for important mission phases such as rendezvous and control of the combined spacecraft, and provide robustness and safety guarantees in the presence of model uncertainty and nonlinear behavior.
The resulting methods are relevant not only to space-debris mitigation but also to other autonomous aerospace and robotic systems that must operate safely despite uncertainty in challenging and possibly varying environments.

REDECORATE developed mathematical and simulation models of iESE-equipped CubeSats and of the combined system formed when several CubeSats are attached to a decommissioned spacecraft. The models were implemented in MATLAB/Simulink, providing a digital environment for controller development and validation.
Robust attitude control was developed for an individual electrospray-propelled CubeSat. The project also advanced the concept of using several attached CubeSats as distributed actuators to control a larger decommissioned satellite.
A major part of the research focused on robust, safe control methods for such complex spacecraft operations. Robust model predictive control was developed for constrained spacecraft rendezvous under nonlinear uncertainty. Complementary work developed robust time-varying control barrier functions, which provide explicit conditions for maintaining safety despite uncertain nonlinear system behavior.
The project also produced new fundamental control methods that extend beyond the CubeSat application. These methods improve the ability to design and assess controllers for systems affected by uncertainty, nonlinear behavior and changing operating conditions. They include new approaches for robust controller design, finite-horizon robustness analysis, constrained predictive control and safety-critical control.
As the project progressed, research effort increasingly focused on the most challenging and transferable aspects of the mission: robustness to nonlinear uncertainty, constrained spacecraft operations and provable safety. This resulted in methodological advances that are applicable not only to the REDECORATE mission but also to a broader range of safety-critical aerospace and robotic systems.
The research generated a substantial portfolio of peer-reviewed publications, submitted manuscripts, and ongoing research. Results were presented at major international aerospace and control conferences and through invited research seminars.
The final integrated hardware-in-the-loop validation, originally planned for the last year of the fellowship, was not performed because the fellowship concluded after completion of the two-year outgoing phase. However, extensive analytical and simulation-based research had already established the main control concepts and developed the robustness methodology required for further validation.

REDECORATE advanced the state of the art at both the spacecraft-application and control-methodology levels.
For the spacecraft application, the project developed the concept of using several iESE-equipped CubeSats not only to support debris removal but also potentially to recommission decommissioned satellites. Once attached to the target, coordinated activation of their propulsion systems can provide a new distributed control capability for the combined spacecraft. This offers an alternative to relying on the target's original attitude-control or propulsion systems, which may no longer be operational.
For control methodology, REDECORATE developed new tools for systems that must operate safely and reliably despite nonlinearities, uncertain models, and uncertain trajectories. New results in integral quadratic constraints, robust model predictive control, and control barrier functions provide mathematical guarantees that complement conventional simulation and Monte Carlo testing.
These methods are transferable to a much broader class of safety-critical systems, including spacecraft, aircraft, autonomous vehicles, and robotics.
Further work continues beyond the fellowship. The developed MATLAB implementations are being prepared for public release, and selected methods are intended for integration into open research software. Ongoing collaborations are extending the robust-control, model-predictive-control, and safety-critical-control methods developed during REDECORATE.

Periodic Reporting for period 2 - REDECORATE (REcommissioning and DEorbiting using Cubesat swarms with electrO spRAy ThrusTErs)

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