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Space debris removal: promoting cooperation through game theory

Since the first space missions in the 1950s, our society and economy have become increasingly dependent on satellites. Since 1958 there have been more than 5250 space launches, and the density of objects in low Earth orbit (LEO) has become so high that collisions between objects could lead to a catastrophic sequence of collisions known as the Kessler syndrome, leaving LEO unusable for satellites.

Computer scientists from the University of Liverpool are now using game theory to understand how we can protect LEO for future generations.

The challenge

There are an estimated 42,000 objects in orbit, 23,000 of which are tracked by the US space surveillance network. But these are just the bigger ones (more than 5-10cm), and the higher estimated total (750,000 over 1cm) poses serious collision risks to operational spacecrafts and satellites.

Removing all this space debris is costly but beneficial for all governments and companies owning or operating space equipment. Unfortunately, this leads to a dilemma – everyone is waiting for someone else to start cleaning up in order to avoid huge costs.

Computer scientists at Liverpool are using their expertise in game theory to try to break the impasse. Game theory uses mathematical models to explore strategic interactions and can be used to develop scenarios that avoid conflicts and encourage cooperation.

Research action

The Liverpool team of Professors Rahul Savani and Karl Tuyls developed a game-theoretic analysis of the removal dilemma, working with experts from the European Space Agency (ESA) to understand the short and long term consequences of the debris accumulation and associated political dilemmas. The players are different space actors, their actions are debris removal strategies, and payoffs are quantified and compared based on removal costs, as well as potential damages and changes in collision risk.

The space debris removal game provides valuable insights both into single actor and multi-party decision-making and their dynamic behaviour. The study covered future test scenarios, which should allow the results to be used as a basis to form optimal debris removal strategies and steer the actions of actors into a desirable direction.

Working in partnerships

The project has been carried out in close co-operation with the  Advanced Concept Team at ESA. The technical expertise of space debris modelling from ESA ensured that the modelling is realistic. Scenarios and end-games were generated by extending a full-scale ESA simulator for satellite, launches, orbits, space debris and collision models.

The simulator is very computationally expensive to run, and so the team used the high-performance computing clusters at the Hartree Centre, funded by the Science and Technology Facilities Council, and a close partner of the University of Liverpool.

Outputs and outcomes

The model allows easy exploration of various different removal policies. Its results have been shared with ESA and are now ready to play a part in the future planning of active space debris removals, promoting cooperation between agencies that operate in space.

It has the potential to help formulate and implement fair international agreements with proportional contributions by different sized actors, assisting negotiations to convince disinclined parties to engage in multinational cleaning-up efforts.

Game theory analysis of space debris removal problems is promoting cooperation to ensure the world can benefit from orbiting satellites for generations to come.

Karl Tuyls, Professor of Computer Science

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