2.1. Arguments on defining the ‘peaceful purposes’ principle

Despite the normative importance of the principles, the OST does not define either ‘peaceful purposes’ or ‘exclusively peaceful purposes’. As a result, divergent interpretations have emerged. Scholars have relied on the interpretative methods set out in the Vienna Convention on the Law of Treaties, including textual analysis, travaux préparatoires and subsequent State practice, to clarify the meaning of the terms,Footnote ⁹ yet no consensus has been reached. In particular, two interpretations of the meaning of ‘peaceful purposes’ continue to coexist.

The first interpretation of ‘peaceful purposes’ equates it with non-military use of outer space, including the Moon and other celestial bodies. Proponents often point to the significant influence of the Antarctic Treaty during the drafting of the OST, particularly Article I which declares that Antarctica shall be used ‘for peaceful purposes only’, thereby establishing a fully demilitarised legal regime.Footnote ¹⁰ Drawing a parallel, this interpretation contends that outer space should similarly be reserved for non-military activities.

Historical negotiating positions lend some support to this view. During the drafting process, the Soviet Union (USSR) consistently advocated a strict interpretation, arguing that all military activities in outer space should be prohibited, irrespective of whether they involved weapons systems.Footnote ¹¹ Similarly, in the negotiations on Article IV, several delegations questioned the propriety of excluding outer space from the scope of the second paragraph, which requires activities to be conducted for exclusively peaceful purposes, thereby implying that outer space might otherwise be permissibly used for non-peaceful purposes.Footnote ¹² The language used in early United Nations General Assembly (UNGA) resolutions further reflects this aspiration. UNGA Resolution 1348 (XIII) which established the ad hoc Committee on the Peaceful Uses of Outer Space, referred to the ‘common aim’ that outer space should be used ‘for peaceful purposes only’,Footnote ¹³ mirroring the wording of the Antarctic Treaty. Commentators supporting this interpretation also argue that military activities inherently serve the interests of individual States or alliances, rather than ‘the benefit and in the interest of all countries’, as required by Article I OST.Footnote ¹⁴

Notwithstanding these early positions, the final text of the OST notably omits qualifiers such as ‘exclusively’ or ‘only’ with respect to outer space as a whole. Moreover, Article IV(1) OST does not directly refer to the term ‘peaceful’; it only forbids the placing, installing or stationing of nuclear weapons or other WMD in Earth orbits, outer space and celestial bodies.Footnote ¹⁵ This formulation is widely regarded as having been inspired by Article I of the Partial Nuclear Test Ban Treaty.Footnote ¹⁶ The focus on prohibiting nuclear and other WMD clearly falls short of establishing outer space as a fully demilitarised zone. This omission reflects the political and technological realities of the space age. From its inception, space activities were deeply intertwined with military objectives, particularly through satellite reconnaissance and communications. It has been estimated that approximately 75 per cent of satellites launched during the Cold War served exclusively military purposes.Footnote ¹⁷

As military involvement in space became entrenched, a strict non-military interpretation proved untenable. The US advanced a more flexible reading, interpreting ‘peaceful purposes’ as ‘non-aggressive’ use.Footnote ¹⁸ Under this approach, military activities not constituting aggression are considered permissible. This interpretation draws conceptual support from the definition of aggression adopted in UNGA Resolution 3314 (XXIX).Footnote ¹⁹ Over time, this understanding has gained broad acceptance among Western space-faring States,Footnote ²⁰ and it largely accords with prevailing State practice. This includes not only Russia,Footnote ²¹ but also emerging space powers such as China and India.Footnote ²² It also aligns with Article III OST,Footnote ²³ which requires States to conduct space activities in accordance with international law, including the United Nations (UN) Charter.Footnote ²⁴

However, equating ‘peaceful purposes’ with the prohibition on the threat or use of force under Article 2(4) UN CharterFootnote ²⁵ substantially limits the independent normative value of the principle. Rather than imposing additional constraints tailored to the unique environment of outer space, the ‘non-aggressive’ interpretation effectively reproduces existing rules.Footnote ²⁶ As a result, the scope of permissible military activities in space remains ambiguous and expansive, contributing to increasing militarisation and strategic competition.

2.2. The definition of the ‘exclusively peaceful purposes’ principle as applied to the Moon and other celestial bodies

Article IV(2) introduces a markedly stricter standard by requiring that the Moon and other celestial bodies be used ‘exclusively for peaceful purposes’.Footnote ²⁷ In terms of its substantive meaning, the ‘exclusively peaceful purposes’ principle is more closely aligned with the formulation adopted in the Antarctic Treaty, which mandates that Antarctica be used ‘for peaceful purposes only’.Footnote ²⁸ The use of emphatic qualifiers such as ‘exclusively’ and ‘only’ serves to underscore the comprehensive nature of the obligation and signals an intention to exclude military activities as such. Accordingly, unlike the broader ‘peaceful purposes’ principle, the ‘exclusively peaceful purposes’ requirement governing the Moon and other celestial bodies is best understood as embodying a rule of non-military use. This interpretation is further supported by the negotiating history of Article IV(2). During the drafting process, the USSR strongly advocated for the inclusion of the term ‘installations’, motivated in part by concerns that the Russian translations of ‘bases’ and ‘fortifications’ might fail to capture all conceivable forms of military infrastructure on the Moon.Footnote ²⁹ Although the US objected on the ground that the term was overly vague,Footnote ³⁰ it was ultimately retained in the final text. The inclusion of this broad terminology indicates an intention to prevent not only overt military fortifications but also more subtle or functionally equivalent military presence on celestial bodies.

Notwithstanding these intentions, the practical application of the ‘exclusively peaceful purposes’ principle has historically attracted limited attention, largely due to the absence, until recently, of sustained exploration or operational activities on the Moon and other celestial bodies. As a result, Article IV(2) has remained relatively dormant in both State practice and scholarly debate. This situation is now changing. Renewed lunar exploration and expanding commercial and dual-use activities have prompted fresh scrutiny of the existing legal framework. In this context, scholars have increasingly questioned whether Article IV(2) as currently formulated, provides sufficient safeguards to preserve celestial bodies as demilitarised zones.

A common point of comparison remains the Antarctic Treaty. Article I not only declares that Antarctica shall be used ‘for peaceful purposes only’, but also expressly prohibits, ‘inter alia, any measures of a military nature, such as the establishment of military bases and fortifications, the carrying out of military manoeuvres, as well as the testing of any type of weapons’.Footnote ³¹ The inclusion of the phrase ‘inter alia’ makes clear that the listed prohibitions are illustrative rather than exhaustive, thereby allowing the regime to adapt to new forms of military activity as they emerge. By contrast, Article IV(2) OST lacks a comparable open-ended qualifier. This omission has led some commentators to argue that the prohibitions enumerated therein, namely, military bases, fortifications, weapons testing and military manoeuvres, are exhaustive rather than indicative.Footnote ³² Under such a reading, certain military-related activities that do not clearly fall within these categories may escape prohibition. This textual rigidity significantly weakens the capacity of Article IV(2) to respond to the increasingly blurred distinction between civilian and military activities in space.

The Moon AgreementFootnote ³³ also incorporates the ‘exclusively peaceful purposes’ principle, but its contribution to the development of a more robust lunar security regime remains limited.Footnote ³⁴ Article 3(2) prohibits ‘any threat or use of force or any other hostile act or threat of hostile act on the Moon’,Footnote ³⁵ language that closely mirrors the prohibition on the use of force under the UN Charter.Footnote ³⁶ While this provision reinforces the applicability of general international law to lunar activities, it does little to advance the stricter, non-military understanding of ‘exclusively peaceful purposes’ embodied in Article IV(2) OST.

In sum, although the ‘exclusively peaceful purposes’ principle constitutes the strongest normative expression of demilitarisation in international space law, its current articulation suffers from significant textual and interpretative limitations. As lunar activities enter a new phase characterised by sustained presence and potential strategic competition, the shortcomings of Article IV(2) become increasingly apparent. Clarifying and strengthening the interpretation of this principle is therefore essential, not only to preserve the Moon and other celestial bodies as zones of restraint, but also to provide a conceptual and normative foundation for more differentiated and context-sensitive regulation within the broader cislunar space environment.

3.1. Cislunar space as a functionally integrated but legally undifferentiated domain

Cislunar space is commonly understood as the spatial region extending from GEO outward to encompass the Moon’s orbit, including the Earth–Moon Lagrange points.Footnote ³⁷ Although legally classified as part of outer space and therefore subject to the general legal framework established by the OST, cislunar space has only recently begun to acquire distinct operational, strategic and economic significance.Footnote ³⁸ For much of the space age, human activities were concentrated in low Earth orbit (LEO) and GEO, primarily to support Earth-based functions. Beyond these regions, space activities remained episodic and exploratory, and the broader cislunar environment was largely neglected in both practice and legal analysis. This situation has changed markedly with the renewed global focus on sustained lunar exploration and utilisation. Contemporary lunar programs increasingly rely on cislunar infrastructure to support surface operations, including logistics, navigation, communication relay and crewed and robotic transit.Footnote ³⁹ Planned installations such as the Lunar Gateway, developed by the US under the Artemis program,Footnote ⁴⁰ as well as comparable cislunar platforms envisaged within the China–Russia International Lunar Research Station (ILRS) framework,Footnote ⁴¹ demonstrate that cislunar space is no longer merely a transit corridor but has become an operational environment functionally integrated with lunar activities. In this sense, cislunar space increasingly serves as an extension of the lunar operational domain rather than as part of the traditional, Earth-oriented orbital environment.

Despite this functional transformation, cislunar space remains legally undifferentiated from outer space as a whole. Under the existing treaty framework, it is governed by the general ‘peaceful purposes’ principle applicable to outer space, rather than the more restrictive ‘exclusively peaceful purposes’ standard. This legal treatment reflects historical assumptions about the marginal role of cislunar space—assumptions that are becoming increasingly detached from contemporary and anticipated patterns of use. At present, military activity in cislunar space remains limited. Aside from the US, which has begun to articulate a security posture extending into the cislunar domain,Footnote ⁴² most spacefaring States have refrained from explicitly framing their cislunar activities in military terms. This relative restraint suggests that cislunar space remains at an early stage of normative development, offering a window of opportunity to shape its legal character before entrenched security dynamics emerge.

3.2. Normative tension under the OST: limits of the dual-track application of ‘peaceful purposes’ principles

As discussed in Section 2, the OST adopts a dual-track framework for regulating military activities in space, one that presupposes a clear and durable distinction between spatial zones governed by different security standards. In practice, however, that distinction is increasingly difficult to maintain. Whereas the general ‘peaceful purposes’ principle has coincided with the extensive militarisation of outer space below GEO,Footnote ⁴³ the Moon and other celestial bodies remain formally demilitarised under the ‘exclusively peaceful purposes’ principle. Cislunar space occupies an intermediate position and this mismatch generates a normative tension within the treaty framework.

Activities conducted in cislunar space, such as communication relay, surveillance, navigation support and infrastructure protection, can have direct and decisive implications for operations on the lunar surface. If military-related activities are permitted in cislunar space under a ‘non-aggressive’ interpretation of peaceful purposes, they may indirectly undermine the demilitarised status of the Moon by facilitating military advantages in its vicinity.Footnote ⁴⁴ In such circumstances, the legal separation between the Moon and its surrounding space risks becoming artificial, allowing the objectives of Article IV(2) to be circumvented without formal violation.

The experience in relation to traditional orbital regions reinforces this concern. In LEO and GEO, interpreting the ‘peaceful purposes’ principle as allowing non-aggressive use has not been adequate to curb the securitisation of outer space or to prevent its gradual framing as a potential arena of armed conflict.Footnote ⁴⁵ If the same regulatory logic is applied to cislunar space as it becomes more heavily utilised, similar dynamics are likely to emerge. By contrast, the continued demilitarised status of the Moon, however contingent on its historically limited use, demonstrates that the ‘exclusively peaceful purposes’ principle retains normative significance as a security preserving mechanism. Accordingly, treating cislunar space and the Moon as separate security zones governed by fundamentally different principles is increasingly difficult to justify. As cislunar space becomes operationally integrated with lunar activities, the dual-track application of the peaceful use principle under the OST reveals a structural gap. Extending the ‘exclusively peaceful purposes’ principle to cislunar space would address this inconsistency and reinforce the integrity of the Treaty’s security objectives.

3.3. Doctrinal and systemic support for extending the ‘exclusively peaceful purposes’ principle to cislunar space

The extension of the ‘exclusively peaceful purposes’ principle to cislunar space would not represent a radical departure from existing space law, but rather a development consistent with its internal logic and broader regulatory trends.

First, the Moon Agreement provides important interpretative support. Although limited in ratification,Footnote ⁴⁶ the Agreement constitutes the most detailed treaty-level effort to elaborate the principles of the OST as they apply to the Moon and other celestial bodies.Footnote ⁴⁷ Notably, Article 1(2) clarifies that references to the Moon include ‘orbits around or other trajectories to or around it’.Footnote ⁴⁸ This formulation implicitly recognises that the effective demilitarisation of the Moon requires attention not only to the surface, but also to the surrounding spatial environment.Footnote ⁴⁹ From this perspective, cislunar orbits and trajectories fall naturally within the security logic underpinning the ‘exclusively peaceful purposes’ principle.

Second, developments in space governance more broadly reveal an emerging acceptance of functional and spatial differentiation within an otherwise unified legal regime. Debates within the expanding regulatory discourse on LEO reflect an increasing recognition that different space environments present distinct risks, operational conditions and governance challenges. Increased congestion, debris proliferation and the deployment of large satellite constellations have prompted calls for orbit-specific regulatory responses.Footnote ⁵⁰ These developments demonstrate that differentiated legal treatment of distinct spatial regions is neither unprecedented nor incompatible with the foundational principles of space law.

Finally, the regulatory logic embodied in the United Nations Convention on the Law of the Sea (UNCLOS)Footnote ⁵¹ offers a useful methodological reference for addressing governance challenges in outer space. UNCLOS did not fragment the law of the sea into separate legal regimes; rather, it introduced a system of zonal differentiation within a single, coherent treaty framework. This differentiation was legally justified by objective factors, including geographical characteristics, varying degrees of State control, differing patterns of use and distinct regulatory needs across maritime spaces.Footnote ⁵² By calibrating rights, obligations and jurisdictional competences according to these factors, UNCLOS enhanced the effectiveness and predictability of the legal order while preserving its systemic unity.

From a legal perspective, this zonal approach reflects a broader principle of international law: differentiated regulation may be warranted where materially distinct conditions give rise to different legal risks and governance objectives, provided that such differentiation is grounded in functional necessity and embedded within a common normative framework. UNCLOS demonstrates that spatial differentiation can serve as a technique for specifying general principles, rather than derogating from them. Core norms, such as the peaceful use of the seas, freedom of navigation and the protection of the marine environment, are not abandoned in some zones, but are applied with varying degrees of intensity and specificity depending on context.Footnote ⁵³

The relevance of this experience for outer space law lies not in transplanting maritime zones to the extraterrestrial domain, but in illustrating that functional and spatial differentiation can be a legally legitimate and stabilising regulatory method when a uniform legal standard proves insufficient to address heterogeneous operational realities. In this sense, proposing differentiated applications of the ‘peaceful purposes’ principle does not undermine the coherence of the OST; rather, it aligns with established treaty practice in international law, whereby general principles are progressively adapted to distinct spatial and functional contexts in order to maintain their normative effectiveness.

Taken together, these considerations support the conclusion that extending the ‘exclusively peaceful purposes’ principle to cislunar space is both legally plausible and normatively justified. As a relatively underutilised yet increasingly strategic domain, cislunar space presents a timely opportunity for preventative norm-setting. Applying a stricter security principle at this stage could help preserve its peaceful character, prevent the spillover of militarisation from traditional orbital regions and strengthen the coherence of the existing treaty framework governing outer space.

4.1. Geopolitical competition over space security governance: an opportunity to reconsider the ‘exclusively peaceful purposes’ principle

The governance of space security has emerged as a central arena of geopolitical competition, particularly between two major blocs: the US and its allies on one side, and China and Russia on the other. This rivalry is increasingly reflected in their divergent approaches to rulemaking concerning the peaceful use of outer space.

Since the end of the Cold War, efforts to develop norms governing space security have followed two broadly distinct tracks. The US and its allies have generally favoured a soft law approach, emphasising non-binding instruments, transparency and confidence-building measures, and voluntary norms of responsible behaviour.Footnote ⁵⁴ By contrast, China and Russia have consistently promoted a hard law model centred on legally binding prohibitions, most notably through initiatives such as the draft Treaty on the Prevention of the Placement of Weapons in Outer Space (PPWT).Footnote ⁵⁵ This structural divergence in normative preferences has shaped contemporary debates on space security and has contributed to persistent deadlock in multilateral fora.

The depth of this division was clearly illustrated during the 2024 debate in the UN Security Council on the ‘Draft Resolution on the Prevention of an Arms Race in Outer Space’, prepared by Japan and the US and co-sponsored by 65 Member States.Footnote ⁵⁶ The draft reaffirmed States’ obligations under Article IV OST and underscored the need for further measures, including political commitments and legally binding instruments with appropriate verification mechanisms, to prevent an arms race in outer space in all its aspects.Footnote ⁵⁷ Although the substance of the draft was not fundamentally incompatible with the objectives advanced by China and Russia under the PPWT framework, the resolution was vetoed by Russia and therefore failed to be adopted.Footnote ⁵⁸ At the same time, China and Russia proposed an amendment calling for urgent measures to prevent the placement of weapons of any kind in outer space, as well as the threat or use of force in, from, or against outer space and urging the early negotiation of reliably verifiable, legally binding multilateral agreements.Footnote ⁵⁹ This amendment, which closely reflected the logic of the PPWT, likewise failed to secure sufficient support.Footnote ⁶⁰

The mutual failure of both initiatives reveals not merely procedural disagreement, but a deeper strategic contest over leadership in space rule-making. Both sides perceive legal authority in space governance as closely linked to long-term strategic interests and international influence and consequently remain cautious of any framework shaped predominantly by the other.

To date, however, this competition has been concentrated in the domain of Earth-centric orbital space. Although the PPWT refers to celestial bodies, including the Moon, its substantive obligations are already covered by existing treaties such as the OST. Meanwhile, newly emerging legal instruments, such as the Artemis AccordsFootnote ⁶¹ led by the US, focus on civil cooperation and normative frameworks for lunar exploration and resource utilisation, rather than security governance.Footnote ⁶² China, while advancing its ILRS initiative, has not yet proposed a detailed legal framework for security or peaceful use in the lunar or cislunar environment.Footnote ⁶³ Importantly, neither of these major actors has so far explicitly addressed cislunar space as a distinct legal and strategic domain requiring specialised rules. With the US and China planning long-term lunar missions, cislunar space is emerging as a critical support region for sustained lunar surface operations, rendering the preservation of its security an increasingly unavoidable priority.

Significantly, there are recent institutional developments that could support this effort. In October 2023, the First Committee of the UNGA voted to support the establishment of two parallel working groups, one backed by the US and its allies, and the other by China and Russia, to explore further legal and normative frameworks for space security.Footnote ⁶⁴ While this parallel process reflects persistent divisions, it also creates a formal space for coexistence and negotiation between soft and hard law models.Footnote ⁶⁵

This development presents a timely opportunity for all major spacefaring nations, including new actors in lunar exploration, to collectively address the security governance of cislunar space. As an initial step, States could jointly commit to applying the ‘exclusively peaceful purposes’ principle in this domain, thereby establishing a shared foundation for future rule-making. Such a move would not only reinforce the peaceful character of outer space but also provide a buffer against the further extension of geopolitical tensions into cislunar space.

4.2. Improving the OST through the adoption of an optional protocol

This article supports the application of the ‘exclusively peaceful purposes’ principle to cislunar space as a legal measure to enhance security in this newly emerging domain. Rather than advocating the negotiation of a new comprehensive treaty, a process that is both politically contentious and procedurally burdensome, this article endorses a more pragmatic and incremental approach: the development of the OST through the adoption of an optional protocol.Footnote ⁶⁶ Such an approach allows for the clarification and progressive development of existing obligations while preserving the foundational consensus that underpins the legitimacy and near-universal acceptance of the Treaty. There are several interrelated reasons why an optional protocol represents a particularly appropriate mechanism for advancing space security governance in cislunar space.

First, the broader trajectory of international space law strongly favours incremental development over comprehensive treaty-making. The limited acceptance of the Moon Agreement is widely regarded as marking the end of an era in which ambitious, binding multilateral treaties on space governance were politically feasible. Since then, norm development has proceeded primarily through soft law instruments, reflecting the difficulty of achieving consensus in an increasingly pluralistic and competitive international environment. Formal amendment of the OST under Article XV,Footnote ⁶⁷ never successfully invoked,Footnote ⁶⁸ faces similar structural obstacles, including the risk of fragmenting existing consensus and the possibility that amendment negotiations could undermine, rather than strengthen, the existing legal framework. In this context, an optional protocol occupies a middle ground between rigid treaty amendment and non-binding soft law, offering a legally binding yet politically flexible mechanism for norm development.

Second, optional protocols are a well-established technique under international law for the progressive development of treaty regimes. They are particularly suited to situations in which States agree on the desirability of strengthening certain norms but differ in their readiness to assume additional obligations. In fields such as international human rights law, optional protocols have been used to introduce new substantive rights, procedural mechanisms or enforcement arrangements without disturbing the integrity of the parent treaty.Footnote ⁶⁹ This opt-in structure respects State consent while enabling coalitions of willing States to advance legal standards that may later attract broader adherence. Applied to the OST, an optional protocol could allow States prepared to accept stricter constraints on military activities in cislunar space to do so, while avoiding the paralysis that often accompanies attempts at universal agreement.

Third, the structure and normative logic of the OST itself make it particularly amenable to supplementation through optional protocols. The Treaty establishes broad foundational principles, peaceful use, non-appropriation etc, rather than detailed regulatory rules. An optional protocol applying the ‘exclusively peaceful purposes’ principle to cislunar space would not contradict these principles but would instead specify their application in a distinct spatial context that was not fully contemplated at the time of drafting. In this sense, an optional protocol would function as an interpretative and implementing instrument, enhancing legal certainty while remaining faithful to the Treaty’s original objectives.

Finally, from a strategic perspective, an optional protocol offers a realistic pathway for advancing space security governance amid ongoing geopolitical rivalry. As demonstrated by the persistent division between soft law and hard law approaches to space security, comprehensive agreement on a new treaty is unlikely in the near term. An optional protocol allows norm development to proceed incrementally,Footnote ⁷⁰ potentially serving as a testing ground for stricter legal standards. If successful, such a protocol could gradually influence State practice and contribute to the crystallisation of customary norms applicable to cislunar space.

In outlining the proposed amendments to the OST, this article recommends that an optional protocol should focus on two key areas: (1) clarifying the definition of the ‘peaceful purposes’ principle and specifying the scope of activities prohibited under the ‘exclusively peaceful purposes’ principle; and (2) delineating the spatial scope of the ‘exclusively peaceful purposes’ principle, with particular emphasis on its applicability to cislunar space.

First, there is a pressing need to clarify the term ‘peaceful purposes’ and to specify activities that are either prohibited or permitted under the ‘exclusively peaceful purposes’ principle. Although the term ‘peaceful purposes’ is commonly interpreted to mean ‘non-aggressive’ activities, this understanding remains non-authoritative and subject to divergent interpretations.Footnote ⁷¹ Moreover, the absence of a precise legal definition has enabled spacefaring States to interpret the term in ways that accommodate their strategic interests, particularly by permitting military activities that fall short of overt aggression. This interpretative ambiguity undermines efforts to preserve outer space as a secure and cooperative environment, especially in sensitive and emerging regions such as cislunar space. To establish a more robust legal foundation, an optional protocol should therefore provide an authoritative definition of ‘peaceful purposes’, grounded in both legal and technical criteria. In this regard, recent scholarly work on the governance of military uses of outer space may serve as a useful reference.Footnote ⁷² Such a definition should, in particular, distinguish clearly between purely civilian activities and those with military or dual-use characteristics.

Furthermore, the protocol should enumerate specific categories of prohibited activities under the ‘exclusively peaceful purposes’ principle. As noted, Article IV(2) OST prohibits the establishment of military bases, installations and weapons testing on the Moon and other celestial bodies;Footnote ⁷³ however, it remains controversial whether this provision constitutes an exhaustive list. To address this gap, it is proposed that the optional protocol should include a clause modelled on Article I(1) Antarctic Treaty, incorporating the term ‘inter alia’ to indicate that any list of prohibited activities is illustrative, not exhaustive.Footnote ⁷⁴ Such language would provide flexibility for future developments while preventing loopholes that could be exploited to justify otherwise impermissible conduct. For example, emerging activities in cislunar space that may not directly take place on the lunar surface, but which affect its security or stability, could then be treated as falling within the scope of the prohibition.

Second, the optional protocol should clarify the spatial scope of application of the ‘exclusively peaceful purposes’ principle by expressly extending it to cislunar space. Admittedly, the OST neither defines the boundary of outer space nor differentiates among its various regions,Footnote ⁷⁵ and longstanding disagreement over airspace delimitation has impeded efforts at precise spatial classification. However, international legal regulation does not require absolute spatial precision where obligations are framed in functional or activity-based terms. In practice, space operations are already differentiated according to their operational environment, risk profile and functional relationship to other activities.

In view of this, it is recommended that the optional protocol adopt a working definition of cislunar space based on the prevailing understanding in the scientific community. The legal relevance of distinguishing between GEO and cislunar space may be limited at this stage and could be reserved for future discussion if necessary. More importantly, the protocol should emphasise the functional characteristics of activities in cislunar space, particularly the fact that most operations in this region are designed to support lunar exploration and utilisation. Recognising this functional nexus can help clarify the rationale for applying the ‘exclusively peaceful purposes’ principle in cislunar space and reinforce the idea that this zone should be governed with a view to protecting the integrity of lunar activities.

By clarifying both the substantive content and the spatial-functional scope of the ‘exclusively peaceful purposes’ principle, an optional protocol would significantly enhance legal certainty and preventive security governance in cislunar space, while remaining consistent with the structure and objectives of the existing space law framework.

4.3. Establishing soft law for operationalising the ‘exclusively peaceful purposes’ principle

In light of persistent geopolitical divisions and the limited prospects for comprehensive arms control agreements in outer space in the near term, soft law instruments play a crucial complementary role. Rather than serving merely as interim political compromises, soft law mechanisms can perform important legal functions by shaping State practice, clarifying normative expectations and facilitating the gradual consolidation of new legal standards.

In the context of cislunar space governance, soft law may serve three interrelated functions. First, it can play an interpretative role, providing practical guidance on how the ‘exclusively peaceful purposes’ principle should be understood and applied to emerging activities that are not expressly addressed by existing treaty provisions. Second, it may function as a preparatory mechanism, allowing States to test and refine behavioural norms before committing to binding legal obligations through an optional protocol. Third, soft law can operate as an operational tool, offering concrete standards of conduct and procedural safeguards to reduce security risks in a domain where legal rules remain underdeveloped. These functions are particularly relevant for cislunar space, where activities are rapidly evolving and where early norm-setting may help prevent destabilising patterns of behaviour from becoming entrenched.

Recent developments within the UN framework provide an institutional basis for advancing such soft-law-based implementation measures. The UNGA’s initiative on Reducing Space Threats through Norms, Rules and Principles of Responsible BehavioursFootnote ⁷⁶ and the establishment of the Open-Ended Working Group (OEWG)Footnote ⁷⁷ reflect a growing recognition that space security governance requires greater attention to behavioural regulation. Although the OEWG process has been characterised by significant disagreement, particularly over the relative merits of hard law versus soft law approaches and the definition of ‘responsible behaviour’,Footnote ⁷⁸ it remains the only inclusive multilateral forum in which major spacefaring States systematically engage on space security norms. As such, it offers a uniquely suitable platform for addressing the emerging security challenges of cislunar space.

Within this framework, the OEWG could be leveraged to promote soft law implementation measures specifically tailored to the application of the ‘exclusively peaceful purposes’ principle in cislunar space. A first priority should be the articulation of shared expectations regarding the peaceful character of activities conducted in this region. In particular, discussions should address the permissibility of dual-use systems, such as space situational awareness assets, logistics platforms and command-and-control infrastructure operating in cislunar orbits. Clarifying the boundaries between acceptable support functions and prohibited military activities would help operationalise the ‘exclusively peaceful purposes’ principle and reduce the risk of divergent national interpretations. Importantly, these efforts should be understood as complementary to, rather than substitutes for, the gradual development of binding obligations through an optional protocol to the OST.

A second and equally important area for soft law development concerns the advancement of transparency and confidence-building measures (TCBMs) adapted to the specific operational environment of cislunar space. TCBMs have long played a stabilising role in outer space governance by mitigating the risks of misperception, miscalculation and unintended escalation.Footnote ⁷⁹ The 2013 Report of the Group of Governmental Experts on Transparency and Confidence-Building Measures in Outer Space Activities identified a range of practical measures, including information exchange, prior notification of launches and manoeuvres and consultation mechanisms.Footnote ⁸⁰ While these measures were not designed with cislunar operations in mind, their underlying logic is particularly relevant to this emerging domain. Cislunar space presents distinctive security challenges that heighten the need for tailored TCBMs. Limited space situational awareness, extended transit times and the increasing reliance on dual-use infrastructure to support lunar missions create conditions in which benign activities may be misinterpreted as hostile. Moreover, the close functional relationship between cislunar operations and lunar surface activities means that security incidents in the surrounding space could directly undermine the demilitarised status of the Moon itself. Developing cislunar-specific TCBMs, such as enhanced information-sharing regarding mission objectives, transparency concerning the deployment and function of infrastructure in key cislunar locations and procedures for consultation in cases of anomalous behaviour, could significantly reduce these risks. State practice suggests that there is a degree of convergence on the value of transparency and confidence-building, even among actors that otherwise disagree on the preferred legal model for space security governance. The Artemis Accords focus on transparency, interoperability and the sharing of scientific and operational information in the context of civil lunar exploration,Footnote ⁸¹ while China and Russia have repeatedly acknowledged the role of TCBMs in preventing an arms race in outer space, including in their proposals under the PPWT framework.Footnote ⁸²

Building on this shared ground, the OEWG could facilitate the development of non-binding guidelines that operationalise the ‘exclusively peaceful purposes’ principle through concrete transparency and risk-reduction measures applicable to cislunar activities. Taken together, the development of soft law-based implementation measures and the pursuit of an optional protocol to the OST should be understood as mutually reinforcing tracks. Soft law can provide immediate guidance and foster convergence in State practice, while also generating the normative and political momentum necessary for future binding commitments. In this sense, extending the ‘exclusively peaceful purposes’ principle to cislunar space need not depend on the immediate success of formal treaty reform. Instead, a gradual and layered approach, combining interpretative guidance, confidence-building measures and incremental legal development, offers a realistic and legally coherent pathway toward preserving the security of this strategically significant domain.