Preserving Security in Cislunar Space by Extending the ‘Exclusively Peaceful Purposes’ Principle

Abstract Activities in cislunar space are accelerating as spacefaring nations expand their lunar ambitions, raising urgent questions about how to ensure its peaceful and secure use. While the Outer Space Treaty provides a foundational legal framework grounded in the principle of ‘peaceful purposes’, it lacks provisions tailored to the emerging dynamics of cislunar space. This article argues for extending the ‘exclusively peaceful purposes’ principle, currently limited to the Moon and other celestial bodies, to this broader region. To support this extension, it proposes adopting an optional protocol to the Outer Space Treaty and developing soft law instruments to enable gradual, flexible implementation of enhanced security norms.