UNIDIRSEPTEMBER 2024 – SPACE SECURITY LEGAL PRIMER 1

Outer Space &Use of ForceE RIN POBJIE · ALMUDENA AZCÁRATE ORTEGAO U T E R S PA C E & U S E O F F O R C E

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ACKNOWLEDGEMENTS
Support from UNIDIR’s core funders provides the foundation for all of the Institute’s activities.

This report was made possible thanks to the financial support of Norway. UNIDIR’s James
Revill, Sarah Erickson, Ching Wei Sooi, and Ritanshu Lohani provided invaluable advice,support, and assistance on this paper. The authors would also like to acknowledge inparticular Victoria Valdivia, Hitoshi Nasu, David Kuan-Wei Chen, and Guoyu Wang for theircritical feedback.

Photo credit for photograph in page 1,9 and 19: NASA.
Design and layout by Trifecta Content Studio.

ABOUT UNIDIRUNIDIR is a voluntarily funded, autonomous institute within the United Nations. One of the
few policy institutes worldwide focusing on disarmament, UNIDIR generates knowledge

and promotes dialogue and action on disarmament and security. Based in Geneva, UNIDIR

assists the international community to develop the practical, innovative ideas needed tofind solutions to critical security problems.

NOTE
The designations employed and the presentation of the material in this publication do not

imply the expression of any opinion whatsoever on the part of the Secretariat of the United
Nations concerning the legal status of any country, territory, city or area, or of its authorities,
or concerning the delimitation of its frontiers or boundaries. The views expressed in the

publication are the sole responsibility of the individual authors. They do not necessarilyreflect the views or opinions of the United Nations, UNIDIR, its staff members or sponsors.

CITATION
Erin Pobjie & Almudena Azcárate Ortega. “Space Security Legal Primer 1 – Outer Space & Useof Force”, UNIDIR, Geneva, 2024. https://doi.org/10.37559/WMD/24/Space/02.

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2UNIDIRTABLE OF CONTENTSABOUT THE AUTHORS

4

ABBREVIATIONS AND ACRONYMS

5

KEY TAKEAWAYS

6INTRODUCTION

8PART I: CONTEXT AND LEGAL FRAMEWORK10MILITARY USES OF OUTER SPACE

10

Peaceful purposes

10

Space weaponization

11
11

PAROS & THE MULTILATERAL SPACE SECURITY DEBATE
Which concerns to prioritize (capabilities and behaviours)

12
12

How to address concerns (legally binding instruments and non-legallybinding mechanisms)

13

INTERNATIONAL LEGAL FRAMEWORK

15International outer space law

15

General international law

16

Prohibition of the use of force

16

Thresholds below use of force

17

Lawful measures in response to harmful and hostile acts below use of force 17
PART II: THE PROHIBITION OF THE USE OF FORCE IN OUTER SPACE

20
20

THE PROHIBITION OF THE USE OF FORCE
The prohibition of the use of force under international law

20

Status of the prohibition under international law

20

Exceptions

21

Consequences of characterizing an act as a prohibited use of force

22

Legal consequences of violation

22

THE MEANING OF A PROHIBITED USE OF FORCE

23

Elements

24

Definitional framework

25

APPLYING THE PROHIBITION OF THE USE OF FORCE IN OUTER SPACE

25

Contextual Requirements

25

Elements of “use of force”

28

MOVING FORWARD: INTEGRATING THE PROHIBITION OF THE USE OF FORCE
35

INTO SPACE SECURITY DEBATESO U T E R S PA C E & U S E O F F O R C E

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UNIDIRABOUT THE AUTHORSERIN POBJIE
Assistant Professor of International Law at Essex Law School, SeniorResearch Affiliate at the Max Planck Institute for Comparative Public Law
and International Law and Fellow in the Space Security Programme at UNIDIR

Erin’s expertise is in public international law, with a focus on the illegal use of force by States
and new and emerging threats to international peace and security. She is co-Rapporteurof the International Law Association’s Committee on the Use of Force. Erin holds a PhD ininternational law (summa cum laude) from the University of Cologne and was a visiting
scholar at the Lauterpacht Centre for International Law at the University of Cambridge. She

is the recipient of the 2024–25 British Academy Global Innovation Fellowship (Technology
and International Affairs) at the German Council on Foreign Relations (DGAP). Her recent
monograph is Prohibited Force: The Meaning of ‘Use of Force’ in International Law (Cambridge
University Press, 2024).

ALMUDENA AZCÁRATE ORTEGA
Researcher for the Space Security and WMD Programmes at UNIDIR
Almudena has published widely and briefed UN Member States on the topics of space
security law and policy and has presented her research in multiple fora. She has led UNIDIR’s
participation in the UN Open-ended Working Group on Reducing Space Threats ThroughNorms, Rules and Principles of Responsible Behaviour, established pursuant to UN General
Assembly resolution 76/231. Prior to joining UNIDIR, Almudena was a Research Assistant at

Georgetown University Law Center, where she is currently a doctoral candidate. She holds

an LL.M. in National Security Law from the same institution, where she was the recipient ofGeorgetown’s Thomas Bradbury Chetwood, S.J. Prize for the most distinguished academic

performance in the programme. She received her LL.B. from the University of Navarra, Spain.

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ABBREVIATIONS
AND ACRONYMS
ASAT

Anti-satellite capability

EMP

Electromagnetic pulse

ENMOD

Environmental Modification Convention

GGE

Group of Governmental Experts

HPM

High-powered microwaves

ICJ

International Court of Justice

ISRIntelligence, surveillance and reconnaissance

ITU

International Telecommunication Union

LTBT

Limited Test Ban Treaty

NTM

National technical means

OEWG

Open-ended Working Group

OST

Outer Space Treaty

PAROS

Preventing an arms race in outer space

PPWT

Draft treaty on the Prevention of the Placement of Weapons in 		
Outer Space, the Threat or Use of Force against Outer Space Objects

RPO

Rendezvous and proximity operations

TCBM

Transparency and confidence-building measures

UNCOPUOS

United Nations Committee on the Peaceful Uses of Outer SpaceUNIDIR

United Nations Institute for Disarmament Research

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5UNIDIRKEY TAKEAWAYS• The 1967 Outer Space Treaty establishes that States shall carry out their activities inouter space in accordance with international law, including the Charter of the United

Nations (art. III). This includes the prohibition of the use of force between States in theirinternational relations enshrined in Article 2(4) of the Charter of the United Nations1 andalso established under customary international law.
•States have recognized and reaffirmed the prohibition on the threat or use of force inouter space. Acts in outer space below the threshold of a use of force remain regulated

by international law, including regulations on harmful interference and the principle ofnon-intervention.
•

A prohibited “use of force“ has a legal definition consisting of contextual requirements
and elements that make the act a “use of force”.

Contextual Requirements
By a StateIn “International relations”“Against the territorial integrity
or political independence of any

Elements of Use of Force
Means

Effects

Gravitystate or in any other mannerinconsistent with the Purposesof the United Nations”

Coercive or hostile intent

Figure 1: Elements of prohibited use of force under Article 2(4) UN Charter•

The contextual requirements must all be present for a “use of force“ to fall within thescope of the prohibition. But not all the elements of “use of force“ must be present foran act to meet the definition; instead, the elements should be assessed to determine
whether the threshold is met.2
12“All Members shall refrain in their international relations from the threat or use of force against the territorial
integrity or political independence of any State, or in any other manner inconsistent with the Purposes of theUnited Nations.” Art. 2(4) of the Charter of the United Nations and Statute of the International Court of Justice,
26 June 1945, 59 Stat. 1031; T.S. No. 993; 3 Bevans 1153 [hereinafter ‘UN Charter’].
See generally ERIN POBJIE, PROHIBITED FORCE: THE MEANING OF ‘USE OF FORCE’ IN INTERNATIONAL LAW (2024),
https://doi.org/10.1017/9781009022897.

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UNIDIR••

A prohibited “use of force“ is not necessarily the same as the use of a “space weapon“ or acounterspace capability. ”Use of force” does not depend on the definition of a “weapon”.
The prohibition of the use of force does not explicitly ban specific space capabilities orspace behaviours. However, these can be legally relevant to an assessment of whether
an act is a prohibited “use of force”.

•

Key areas lacking clarity regarding the meaning of a prohibited “use of force“ include:•

Contextual requirements: Whether article VI of the Outer Space Treaty could implicate

the State of registration in violations of ius contra bellum3 with respect to national
activities of their non-governmental entities (such as commercial actors) in outer•

Effects: whether “use of force“ includes acts with indirect effects, temporary and•Gravity: whether a prohibited “use of force“ has a gravity (i.e., severity) threshold.

••space.

reversible effects, and potential but unrealized harm.

Intention: what type of intent is relevant for an act to constitute a “use of force”, e.g. adeliberate, hostile or coercive intent or recklessness.

States should promote compliance with the prohibition of the use of force in the UN

Charter and customary international law and consider how to strengthen the prohibitionof the use of force in its application to outer space.

•

States should consider the interpretation of a prohibited “use of force“ in its application

to outer space security and take steps to build common understandings by addressing

the areas of uncertainty identified in this report.
•

States can increase clarity over the interpretation of a prohibited “use of force“ in its

application to outer space through non-legally binding measures and legally binding
instruments, which can be complementary, as well as through plurilateral or unilateralstatements.

•

As the definition of prohibited force can help clarify regulation of space capabilities andspace behaviours and already applies to all States, the prohibition of the use of force canserve as a useful basis for advancing common understandings for Preventing an Arms
Race in Outer Space, and for achieving and maintaining space security.

3

lus contra bellum refers to the international law rules prohibiting the recourse to force between States in theirinternational relations, and the exceptions to the prohibition. Ius contra bellum is a separate body of law fromius in bello (also known as international humanitarian law), which regulates the conduct of hostilities during
armed conflict, and will not be a topic addressed in this report.

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UNIDIRINTRODUCTION
Outer space is not a legal “wild west”. As the international community works towards

establishing mechanisms to ensure and maintain peace and security in outer space,
it is necessary to consider the legal framework already applicable to this environment.
Ascertaining existing obligations, limitations and prohibitions is key to determining whichparts of the legal framework need to be strengthened and which gaps need to be filled.

While space security is not the main focus of international space regulations, particularly
the Outer Space Treaty (OST),4 it is nonetheless an area covered by international law. In

this sense, international law applicable to outer space and activities conducted therein

establishes restraints through the prohibition of recourse to the use of force between Statesin their international relations.

The international community has engaged in multiple debates and proposed numerous

initiatives throughout the years with the aim of keeping outer space secure and preventing
an arms race in outer space. Over the course of these debates, the prohibition of the useof force has been raised, and States have underscored that this restraint —enshrined in

the Charter of the United Nations— applies to outer space.5 However, in their multilateral

discussions States have not delved into the details of what such a prohibition signifies forouter space activities.

What exactly is the prohibition of recourse to force (ius contra bellum),6 and how does it

apply to outer space? This report explores these questions, analysing the implications of thislegal restraint with respect to hostile actions in outer space and against space objects. Part

I contextualizes the relationship between space activities and the prohibition of use of force,

situating such proscription as determined by the ius contra bellum within the frameworkof the Prevention of an Arms Race in Outer Space (PAROS). Part II then underscores theapplicability of the prohibition of the use of force in outer space, highlights areas of ambiguity

around this issue, and illustrates these through examples, with a view to contributing tobuilding common understandings between States and other stakeholders on the meaningof prohibited force in outer space.
4

5

6

Despite this, it is worth noting that the OST served as a form of arms control agreement to ensure stabilityin space and included a key provision —its article IV— to this effect. See Jessica West & Almudena Azcárate
Ortega, Norms for Outer Space: A Small Step or a Giant Leap for Policymaking?, Space Dossier 7, UNIDIR 7(Mar. 2022), https://doi.org/10.37559/WMD/22/Space/01. See also FRANCIS LYALL & PAUL B. LARSEN, SPACE LAW:A TREATISE 453-454 (2nd ed. 2018). U.S. President Lyndon Johnson termed the OST as “the most important arms
control development since the Limited Test Ban Treaty of 1963”, see Lyndon Johnson, Statement by the President
Announcing the Reaching of Agreement on an Outer Space Treaty, 8 December 1966, in Lyndon B. Johnson:
Containing the Public Messages, Speeches, and Statements of the President: 1966 (in two books), [Book II] 1441
(1967), available online at https://babel.hathitrust.org/cgi/pt?id=miua.4731549.1966.002&seq=815.
Chairperson of the Open-ended Working Group on reducing space threats through norms, rules and principles
of responsible behaviours, Chairperson’s Summary, ¶20, U.N. Doc. A/AC.294/2023/WP.22 (1 Sept. 2023)[hereinafter ‘Chairperson’s Summary’], available on the OEWG webpage here: https://meetings.unoda.org/
meeting/57866/documents.
See supra note 3.

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PA RT I :CONTE XT AND LEGALFRAMEWORKO U T E R S PA C E & U S E O F F O R C E

9UNIDIRPART I: CONTEXT AND
LEGAL FRAMEWORKMILITARY USES OF OUTER SPACE
Outer space is a militarized environment, and has been for much of the history of space
exploration, which has had a decidedly military intent: both the United States and the Soviet
Union sought to use space for military purposes, namely intelligence, surveillance andreconnaissance (ISR). Over time, military reliance on space systems has only increased,

despite initiatives designed to regulate outer space, including the 1967 Outer Space Treaty.7
PEACEFUL PURPOSESThe OST establishes that States shall be guided by the principle of exploring and using outerspace for “peaceful purposes”.8 This principle, which has become customary internationallaw,9 has been understood to mean non-aggressive or non-hostile use, rather than nonmilitary.10 In practice, many space systems are used for applications that support military

operations —such as intelligence gathering, reconnaissance, navigation, early warning of

missile and air attacks or military communications— but which are not directly harmful
themselves.11

7Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including theMoon and Other Celestial Bodies, 27 January 1967, 18 U.S.T. 2410; 610 U.N.T.S. 205; 6 I.L.M 386 [hereinafter ’OST’].
The OST is the treaty relating to space matters that has gained the most acceptance among the international
community. The OST has, as of 1 January 2023, 112 State parties and has been signed by 23 other Statesaccording to the list on the status of international agreements relating to activities in outer space compiled anddistributed by the United Nations Office for Outer Space Affairs, https://www.unoosa.org/oosa/en/ourwork/
spacelaw/treaties/status/index.html.
8 This oft-quoted term only appears twice in the OST: firstly, in the non-binding, or hortatory, preambulatory text,
which states that there is a “common interest of all [hu]mankind in the progress of the exploration and use ofouter space for peaceful purposes,” and secondly, in article IV, which establishes that “[t]he Moon and othercelestial bodies shall be used by all State Parties to the Treaty exclusively for peaceful purposes.” Moreover, it
should be noted that art. IV OST does not explicitly establish the limitation of exclusive use for peaceful purposes
for the void of outer space as it does for the Moon and other celestial bodies. However, this obligation canbe inferred from the applicability of general international law to the space domain, established in art. III OST.
Under general international law, particularly under Art. 2(4) of the UN Charter, the use of force is prohibited.
See Almudena Azcárate Ortega & Hellmut Lagos Koller, The Open-Ended Working Group on Reducing Space
Threats through Norms, Rules and Principles of Responsible Behaviours: The Journey so Far, and the Road
Ahead, 48 Air and Space Law 19 (2023), https://kluwerlawonline.com/journalarticle/Air+and+Space+Law/48.
SI/AILA2023029.
9 The term’s consistent appearance in domestic laws and policies relating to outer space is indicative of
its prevalent recognition as a legal obligation. See P. J. Blount, Space Security Law, in OXFORD RESEARCHENCYCLOPEDIA OF PLANETARY SCIENCES (2018).
10 Shannon Orr, Peace And Conflict In Outer Space, 30 Peace Research 52, 58 (1998); Bhupendra Jasani & Maria A.
Lunderius, Peaceful Uses of Outer Space—Legal Fiction and Military Reality, 11 Security Dialogue 57, 58 (1980).
11 Michael N. Schmitt, International Law and Military Operations in Space, 10 Max Planck Y.B. U.N. L. 89, 102 (2006).

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10UNIDIRSPACE WEAPONIZATION
In light of the military relevance of outer space, States have sought to protect their national

security interests in this environment, sometimes through the development and use ofcounterspace capabilities. Space weaponization alludes to the proliferation, testing,deployment and use of weapons or counterspace capabilities located in or directed towards

space or space systems.12 The weaponization of space is not strictly prohibited by the OST andother space treaties, beyond article IV of the OST, which prohibits the “place[ment] in orbit

around the Earth [of] any objects carrying nuclear weapons or any other kinds of weaponsof mass destruction, install[ation of] such weapons on celestial bodies, or station[ing of]such weapons in outer space in any other manner” as well as the “establishment of militarybases, installations and fortifications, the testing of any type of weapons and the conduct of
military manoeuvres on celestial bodies”.13

While the OST as well as general international law prohibit the use of force or the threatof use of force in space or against space systems,14 international law applicable to space

is silent on the development and placement in orbit of any weapons that are not nuclear
weapons or other weapons of mass destruction. In the absence of an explicit prohibition,15

States have developed numerous forms of counterspace capabilities, ranging from debriscreating kinetic capabilities to non-kinetic counterspace assets which can deny, disrupt,

degrade, damage, destroy or otherwise harm a system through electronic or cyber means.16

12 “Militarization of outer space refers to any military activity in outer space (whether hostile or not, or whether
weapons-related or not) or any activity that supports military operations. Many argue that outer space has
been militarized since the early days of space exploration, thus highlighting that military uses of space are
not necessarily aggressive or hostile in nature, and therefore can be considered to be acceptable under theumbrella of peaceful purposes. It is generally understood that the concept of militarization of outer space must
be distinguished from the concept of weaponization of outer space. This distinction, however, is not universally
accepted, as a number of States argue that, due to the nature of space objects and the space environment,
it is not possible to develop a meaningful definition of a space weapon. Moreover, it is important to note that
there are languages which do not have a word for weaponization. In those instances, the word ‘militarization’ isoften used to refer to both of these ideas, which can create further confusion”. See Almudena Azcárate Ortega& Victoria Samson (eds.), A Lexicon for Outer Space Security, UNIDIR 3.3.3 (2023) [hereinafter ‘Space Security
Lexicon’], https://doi.org/10.37559/WMD/23/Space/05.
13 OST, supra note 7, art. IV.
14 Ibid., art. III, establishes that “States Parties to the Treaty shall carry on activities in the exploration and use ofouter space, including the Moon and other celestial bodies, in accordance with international law, including theCharter of the United Nations, in the interest of maintaining international peace and security and promotinginternational cooperation and understanding.” Art. 2(4) of the UN Charter, in turn, establishes that “All Members
shall refrain in their international relations from the threat or use of force against the territorial integrity orpolitical independence of any State, or in any other manner inconsistent with the Purposes of the United
Nations.”
15 Under the Lotus principle, named after the 1927 case between France and Turkey before the Permanent Courtof International Justice (S.S. ‘Lotus’ (Fr. v. Turk.), Judgment, 1927 P.C.I.J. (ser. A) No. 10 (7 Sept.), a consensual
approach to international law was established, suggesting that in the absence of a prohibition, a State is free
to act as it sees fit without the need for a specific basis that permits its action.
16 See generally Brian Weeden & Victoria Samson, Global Counterspace Capabilities, Secure World Foundation(2024), https://swfound.org/media/207826/swf_global_counterspace_capabilities_2024.pdf.

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UNIDIRPAROS & THE MULTILATERALSPACE SECURITY DEBATEAs a result of the international community’s efforts to ensure space security, the notion of
the “Prevention of an Arms Race in Outer Space” (PAROS) emerged in 1978.17 In 1982, PAROS
was added as an item to the Conference on Disarmament’s agenda, and since then, PAROS

has been present on the agenda in multilateral discussions, becoming the umbrella term
under which States discuss the maintenance and improvement of space security at theUnited Nations.18 The desire of the international community to keep space peaceful andsecure has manifested in the form of multiple initiatives over the years; however, these have

had limited success. This is due mainly to the international community’s inability to agree
on which issues to tackle as well as how best to address them.19

WHICH CONCERNS TO PRIORITIZE (CAPABILITIES AND BEHAVIOURS)At the core of PAROS is the desire to prevent arms racing. Paradoxically, States remain
unable to agree on a definition for a space “arms” or “weapon”, or even on whether thisis a term that should be defined at all. Generally, this term is used to refer to a capability
or system used to deny, disrupt, degrade, damage, destroy or otherwise harm a system,
infrastructure, person or group of people. For some, a weapon would only fit the definition of“space weapon” if it is located in space, whereas others understand that any object that cantarget any component of a space system, even those not located in space, could constitutea space weapon.20

Certain States have sought to define the term more clearly. For example, in the draft treaty

on the Prevention of the Placement of Weapons in Outer Space and of the Threat or Useof Force against Outer Space Objects (PPWT), introduced by the Russian Federation and
China to the Conference on Disarmament in 2008 and then again in 2014 after revisions,

proposed a definition of a “weapon in outer space”.21 This definition provides some clarity

on what could be considered a “space weapon”, though it does not account for dualpurpose space systems, which are “designed to fulfil a benign objective (such as debris

removal or on- orbit servicing), but [which] could potentially be repurposed to harm other

17 G.A. Res. S-10/2, 10th Special Sess., Final Document of the Tenth Special Session of the General Assembly, U.N.
Doc. A/RES/S-10/2, ¶80 (30 Jun. 1978), https://digitallibrary.un.org/record/218448?v=pdf.
18 Azcárate Ortega & Lagos Koller, supra note 8, at 23.
19 West & Azcárate Ortega, supra note 4, at 9.
20 Space Security Lexicon, supra note 12, at 3.3.10.
21 Art. I(b) draft treaty on the Prevention of the Placement of Weapons in Outer Space, the Threat or Use ofForce against Outer Space Objects, (16 June 2014) [hereinafter ‘PPWT’], defines “weapon in outer space” as“any outer space object or its component produced or converted to eliminate, damage or disrupt normal
functioning of objects in outer space, on the Earth’s surface or in the air, as well as to eliminate population,
components of biosphere important to human existence, or to inflict damage to them by using any
principles of physics.” Available online at https://docs-library.unoda.org/Conference_on_Disarmament_(2014)/1319%2BRussian%2BFederation%2BDraft%2BUpdated%2BPPWT%2B.pdf.

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space objects” without requiring any adaptation.22 The potential for certain space objects
to be used in this way is a significant cause for concern for the international community,23
however they cannot be considered a “weapon” on the basis of their capabilities alone.

Awareness of this has prompted States to seek to regulate not only capabilities but also how

space systems are used. In recent years, an approach to reduce threats to space systems
through mechanisms that focus on behaviours has emerged, introduced by the United

Kingdom in 2020 in what would eventually become General Assembly resolution 75/36 on
reducing space threats through norms, rules and principles of responsible behaviours.24

A focus on behaviours does not necessarily mean ignoring issues related to capabilities.
However, capabilities of space systems can often be neutral, and thus threats to space

security can result from how an actor behaves when using certain capabilities rather than
only from the capabilities themselves.25

HOW TO ADDRESS CONCERNS (LEGALLY BINDING INSTRUMENTS AND NON-LEGALLY BINDINGMECHANISMS)

States have also been unable to agree on how best to address space security

challenges. Some States have traditionally preferred to seek to establish new legallybinding instruments, due to the understanding that existing legal instruments, suchas the OST and the Charter of the United Nations, leave important security-relevant
gaps that need to be addressed.26 This, as well as the perception that legally binding

mechanisms provide a stronger foundation for security measures, led to the introduction

22 ‘Dual-purpose’ refers to “those space objects that are designed to fulfil a benign objective (such as debris
removal or on-orbit servicing), but [which] could potentially be repurposed to harm other space objects.” See
Space Security Lexicon, supra note 12, at 3.3.2.
23 OEWG on Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours, Threats to
the Security of Space Activities and Systems, UNIDIR ¶¶23–27, U.N. Doc. A/AC.294/2022/WP.16 (12 Sep. 2022),
https://documents.unoda.org/wp-content/uploads/2022/08/20220817_A_AC294_2022_WP16_E_UNIDIR.
pdf.
24 Resolution 76/231 on reducing space threats through norms, rules and principles of responsible behaviours
was the successor of resolution 75/36, and it convened an open-ended working group to “take stock of theexisting international legal and other normative frameworks concerning threats arising from State behaviours
with respect to outer space” and “consider current and future threats by States to space systems and actions,activities and omissions that could be considered irresponsible” in order to make recommendations on
possible norms, rules and principles of responsible behaviours to mitigate those threats and address any gapsthat may exist in currently applicable regulations, also taking into consideration “as appropriate, how theywould contribute to the negotiation of legally binding instruments, including on the prevention of an arms racein outer space.” See G.A. Res. 76/231, 76th Sess., on reducing space threats through norms, rules and principles
of responsible behaviours (30 Dec. 2021), https://undocs.org/A/RES/76/231.
25 OEWG on Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours, The Role ofNorms, Rules and Principles of Responsible Behaviour for Space Security, UNIDIR, ¶10, U.N. Doc A/AC.294/2023/
WP.3 (24 Jan. 2023), https://docs-library.unoda.org/Open-Ended_Working_Group_on_Reducing_Space_
Threats_-_(2022)/A_AC294_2023_WP3_UNIDIR.pdf.
26 Azcárate Ortega & Lagos Koller, supra note 8, at 23.

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UNIDIR
in the Conference on Disarmament of the Sino-Russian PPWT mentioned earlier, whichsought to obligate States to not “place in orbit around the Earth any objects carrying any
kinds of weapon” nor “resort to the threat or use of force against outer space objects”.27
Other States have argued that non-legally binding mechanisms, such as voluntarytransparency and confidence-building measures (TCBMs) could be easier to agree at themultilateral level due to their perceived greater flexibility, and their potential to complement
and augment legal measures.28 Examples of such non-legally binding mechanisms include

the TCBMs proposed in the 2013 report adopted by the Group of Governmental Experts (GGE)
on Transparency and Confidence-Building Measures in Outer Space Activities.29 The 2023

Open-ended Working Group (OEWG) on Reducing Space Threats through Norms, Rules andPrinciples of Responsible Behaviours was also mandated with making recommendations on
possible non-legally binding measures that would aid in achieving this goal; however, theOEWG was unable to agree on a consensus report.30

Although legally binding and non-legally binding mechanisms were previously perceived bysome as incompatible and mutually exclusive, their complementarity has been increasingly

recognized in multilateral debates, with States highlighting that although non-legallybinding measures are not substitutes for legally binding instruments, they can neverthelesscontribute to the development of concepts and proposals for the establishment of such
legally binding instruments. In some cases, non-legally binding mechanisms can eventually

become binding laws through crystallization into customary international law31 or through27 PPWT, supra note 21, art. II. See also Letter dated 12 February 2008 from the Permanent Representative of theRussian Federation and the Permanent Representative of China to the Conference on Disarmament addressedto the Secretary-General of the conference transmitting the Russian and Chinese texts of the draft “Treaty on
Prevention of the Placement of Weapons in Outer Space and of the Threat or Use of Force Against Outer Space
Objects (PPWT)” introduced by the Russian Federation and China, U.N. Doc. CD/1839 (29 Feb. 2008), https://digitallibrary.un.org/record/633470/files/CD_1839-EN.pdf.
28 Initially, these States advocated for voluntary measures as an alternative to a new treaty, (Benjamin Silverstein,
Daniel Porras & John Borrie, Alternative Approaches and Indicators for the Prevention of an Arms Race inOuter Space, Space Dossier 5, UNIDIR 11 (2020)), though eventually these non-legally binding mechanisms wereviewed as instruments that could complement legally binding measures, and could form the basis for them:
Report of the Group of Governmental Experts on Transparency and Confidence-Building Measures in Outer
Space Activities, U.N. GAOR, 68th Sess., ¶¶33, 69 U.N. Doc. A/68/189* (29 July 2013), https://www.unoosa.org/
oosa/oosadoc/data/documents/2013/a/a68189_0.html. This GGE was formed as mandated by resolution
65/68, on the proposal of the Russian Federation and China. See G.A. Res. 65/68, 65th Sess., on transparency
and confidence-building measures in outer space activities (13 Jan. 2011), https://undocs.org/A/RES/65/68.
29 Ibid. U.N. Doc. A/68/189*.
30 Almudena Azcárate Ortega & Sarah Erickson, OEWG on Reducing Space Threats: Recap Report, UNIDIR 30 (15Mar. 2024), https://doi.org/10.37559/WMD/24/Space/01. This OEWG was formed as mandated by resolution
76/231, on the proposal of the United Kingdom. See G.A. Res. 76/231, 76th Sess., on reducing space threats throughnorms, rules and principles of responsible behaviours (24 Dec. 2021), https://undocs.org/A/RES/76/231.
31 Two elements of customary international law are required to determine whether a customary rule has formed:(1) the general practice / widespread repetition of international acts by States over time (State practice); and(2) the requirement that the acts must occur out of a sense of legal obligation (opinio juris). See Report of theInternational Law Commission, 73 U.N. GAOR Supp. No. 10, U.N. Doc. A/73/10 (2018), reprinted in [2018] 2 Y.B. Int’l
L. Comm’n 152, U.N. Doc. A/CN.4/SER.A/2018/Add.1 (Part 2), https://legal.un.org/ilc/texts/instruments/english/
commentaries/1_13_2018.pdf.

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codification in legal agreements32 —a prime example being the OST, which was basedon two United Nations resolutions.33 Additionally, work on legally binding and non-legallybinding mechanisms can be pursued in a complementary manner, without undermining
existing legal obligations.34

INTERNATIONAL LEGAL FRAMEWORKThe discussions on PAROS outlined above do not occur in a legal vacuum. Although nospace security-specific international treaty yet exists, the legal framework applicable tospace activities establishes rights, obligations, limitations and prohibitions regarding outerspace activities,35 prime among them the prohibition of the threat or use of force between
States.

INTERNATIONAL OUTER SPACE LAW
The 1967 Outer Space Treaty serves as the foundation for all space law —both internationaland domestic. Only article IV explicitly addresses space security concerns by establishingthat States shall not place in orbit around the Earth any weapons of mass destruction,or install them on celestial bodies, as well as not test any type of weapons or conduct of
military manoeuvres on celestial bodies.36 However, neither the OST nor the subsequentinternational space treaties negotiated under the auspices of the United Nations37

provide further clarification regarding the placement of other types of weapons inspace.38 This issue has caused significant concern in the international communityand eventually led to the creation of PAROS, as seen above. However, this does notmean that international law establishes no other limitations beyond those provisions.

32 U.N. Doc. A/AC.294/2023/WP.3, supra note 25, 16.
33 The OST was based on G.A. Res. 1884 (XVIII), 18th Sess., on Question of General and Complete Disarmament (17Oct. 1963), https://digitallibrary.un.org/record/203960, which called on States to not station weapons of mass
destruction in outer space, and G.A. Res. 1962 (XVIII), 18th Sess., on Declaration of Legal Principles Governing
the Activities of States in the Exploration and Use of Outer Space (13 Dec. 1963), https://www.unoosa.org/pdf/
gares/ARES_18_1962E.pdf, which adopted that Declaration.
34 Chairperson’s Summary, supra note 5, ¶¶4, 5.
35 See generally OEWG on Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours,
Existing Legal and Regulatory Frameworks Concerning Threats Arising from State Behaviours with Respect toOuter Space, UNIDIR, U.N. Doc. A/AC.294/2022/WP.1 (3 Feb. 2022), https://documents.un.org/doc/undoc/gen/
g22/248/57/pdf/g2224857.pdf.
36 OST, supra note 7, art. IV.
37 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into
Outer Space, 22 Apr. 1968, 19 U.S.T. 7570; T.I.A.S. 6599; 672 U.N.T.S. 11 [‘Rescue Agreement’]; Convention on the International Liability for Damage Caused by Space Objects, 3 Mar. 1972, 24 U.S.T. 2389; T.I.A.S. 7762; 961 U.N.T.S. 187[‘Liability Convention’]; Convention on Registration of Objects Launched Into Outer Space, 12 Nov. 1974, 28 U.S.T.
695; T.I.A.S. 8480; 1023 U.N.T.S. 15 [‘Registration Convention’]; Agreement Governing the Activities of States on theMoon and Other Celestial Bodies, 5 Dec. 1979, 18 I.L.M. 1434; 1363 U.N.T.S. 3 [‘Moon Agreement’].
38 Cassandra Steer & Dale Stephens, International Humanitarian Law and Its Application in Outer Space, in WAR
AND PEACE IN OUTER SPACE 23, 26 (Cassandra Steer & Matthew Hersch eds., 2021), https://doi.org/10.1093/
oso/9780197548684.003.0002.

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Article III of the OST stipulates that “States Parties to the Treaty shall carry on activities inthe exploration and use of outer space, including the Moon and other celestial bodies, inaccordance with international law, including the Charter of the United Nations, in the interestof maintaining international peace and security and promoting international co-operation
and understanding”.39 The applicability of international law to the space environment is also

highlighted in article I of the OST, which indicates that the use and exploration of outer spaceshall be carried out “in accordance with international law”.40
GENERAL INTERNATIONAL LAW
These references to international law in the OST confirm that the full corpus of internationallaw applies to outer space when relevant.41 Arms control treaties and international law
applicable to military operations or security-related matters are of particular relevancefor the regulation of space security activities and for the pursuit of PAROS, even when they

may not solely concern space. Examples of this are the 1963 Limited Nuclear Test Ban
Treaty (LTBT)42 and the 1978 Convention on The Prohibition of Military or Any Hostile Use ofEnvironmental Modification Techniques (ENMOD).43 Even when rules do not explicitly refer

to outer space, international law remains applicable. This includes customary international
law: long-established rules such as the concept of good faith and the principle of pacta

sunt servanda44 apply to space.45 Some of these applicable principles were codified in

the UN Charter, as explicitly highlighted by article III of the OST. Among such tenets are thesovereign equality of States, the principles of non-intervention and non-aggression, and thefocus of this report: the prohibition of the use of force.46

PROHIBITION OF THE USE OF FORCEThe applicability of the UN Charter to space means that States ”shall refrain in theirinternational relations from the threat or use of force”47 in this environment. This is afundamental limitation to take into account when carrying out space activities. The

39 OST, supra note 7, art. III.
40 Ibid., art. I.
41 Olivier Ribbelink, Article III, in 1 COLOGNE COMMENTARY ON SPACE LAW 67 (Stephan Hobe, Bernhard SchmidtTedd & Kai-Uwe Schrogl eds., 2009).
42 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, 5 Aug. 1963, 14 U.S.T.
1313; 480 U.N.T.S. 6964 [hereinafter ‘LTBT’]. The LTBT explicitly mentions outer space in article I.1(a) which prohibits
the testing of nuclear weapons “in the atmosphere; beyond its limits, including outer space; or under water,
including territorial waters or high seas.”
43 Convention on the Prohibition of Military or Any Hostile Use of Environmental Modification Techniques, 18 May
1977, 31 U.S.T. 333; 1108 U.N.T.S. 151. The ENMOD prohibits States parties to “engage in military or any other hostile
use of environmental modification techniques having widespread, long-lasting or severe effects as the meansof destruction, damage or injury to any other State Party” in article I, similarly extending this prohibition to outerspace in article II.
44 A Latin expression meaning ‘pacts must be respected’. This is an international law principle by which treaties
are to be observed and agreements are to be kept; https://www.oxfordreference.com/display/10.1093/oi/
authority.20110803100300496.
45 The principle of good faith is “[o]ne of the basic principles governing the creation and performance of legal
obligations, whatever their source”. See Nuclear Tests (Austl. v. Fr.), Judgment, 1974 I.C.J. Rep. 253, ¶46 (20 Dec.).
46 Ribbelink, supra note 41, at 67.
47 See UN Charter, supra note 1, Art. 2(4).

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prohibition of the use of force is set out in Article 2(4) of the UN Charter and also binds

all States under customary international law. The prohibition of the use of force and its
application to hostile acts in space and against space objects is discussed in detail in Part II.
THRESHOLDS BELOW USE OF FORCESpace activities that do not reach the threshold of a “use of force” remain regulated byinternational law. For example, activities below the threshold of a “use of force“ may constituteharmful interference under article IX of the OST,48 unlawful intervention49 under customaryinternational law,50 or a prohibited “threat of force” between States under Article 2(4) of the
UN Charter.

HARMFULINTERFERENCEUNLAWFULINTERVENTIONTHREAT
OF FORCE

Figure 2: Spectrum of harmful and hostile acts below “use of force”
LAWFUL MEASURES IN RESPONSE TO HARMFUL AND HOSTILE ACTS BELOW “USE OF FORCE”
In relation to the space activities highlighted above that do not reach the threshold of ause of force, States are entitled to carry out different types of measures depending on
the threshold reached by the space activity in question. For example, States can engage

in consultations, such as under article IX of the OST in response to “potentially harmfulinterference with activities in the peaceful exploration and use of outer space”;51 retorsion
as a political means to urge another State to modify its conduct52 through lawful but

48 The OST does not define harmful interference, though the concept is defined (with respect to disruptionof radiofrequency signals) in both No. 1.169 of the Radio Regulations and in No. 1003 of the International
Telecommunication Union (ITU) Constitution, which prohibit “interference which endangers the functioningof a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedlyinterrupts a radiocommunication service operating in accordance with Radio Regulations”. See Constitutionof the International Telecommunication Union, Dec. 22, 1992, T.I.A.S. No. 97-1026; 1825 U.N.T.S. 330; International
Telecommunication Union, Radio Regulations of the International Telecommunication Union, complementing
the Constitution and the Convention of the International Telecommunication Union (2020), https://www.itu.
int/pub/R-REG-RR/en.
49 Unlawful intervention is the use of coercion by a State to interfere “directly or indirectly in internal or externalaffairs of other States”, for example, regarding its free “choice of a political, economic, social and cultural
system, and the formulation of foreign policy”. See Military and Paramilitary Activities in and against Nicaragua(Nicar. V. U.S.), Merits, Judgment, 1986 I.C.J. Rep. 14, ¶205 (27 June).
50 Ibid., ¶202.
51 See Space Security Lexicon, supra note 12, at 3.2.5. This consultations process is also recognized as a prerequisite
for the effective environmental protection of outer space, as well as potentially harmful interference moregenerally. However, it has never been used and there is no guidance on what constitutes a consultation.
52 Jana Robinson et al., Europe’s Preparedness To Respond To Space Hybrid Operations, The Prague Security
Studies Institute 18 (July 2018), https://www.pssi.cz/download/docs/8252_597-europe-s-preparedness-torespond-to-space-hybrid-operations.pdf.

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discourteous or unfriendly acts vis-à-vis that State,53 or non-forcible countermeasures.54
Countermeasures must comply with strict international legal requirements.55 Examples ofcountermeasures relating to space may include denying certain space services that had
been agreed upon in a treaty provided that these acts meet the conditions stated earlier.

Consultations

Retorsions

HARMFULINTERFERENCEUNLAWFULINTERVENTIONTHREAT
OF FORCE

Non-forceful Countermeasures

Figure 3: Lawful measures in response to harmful and hostile acts
by another State below “use of force”

53 Forms of retorsion include diplomatic protests or bans (such as not granting visas to diplomats of specificStates), withdrawal of aid, or reduced military and intelligence cooperation.
54 Countermeasures are non-forcible acts by a State that would otherwise be unlawful but are exceptionallypermitted in response to the breach of international law by another State of an obligation owed to the first State,
and which have the objective to induce the wrongdoing State to comply with the obligation in question. See
Responsibility of States for Internationally Wrongful Acts, art. 22 & arts. 49-53, annexed to U.N. Doc. A/RES/56/83(12 Dec. 2001) and corrected by U.N. Doc. A/56/49(Vol. I)/Corr. 4 (6 June 2007), https://legal.un.org/ilc/texts/
instruments/english/draft_articles/9_6_2001.pdf.
55 Ibid., Part three, chapter II.

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PA RT I I :THE PROHIBITION OF THE
USE OF FORCE IN OUTER
S PA C E
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UNIDIRPART II: THE PROHIBITION
OF THE USE OF FORCE IN
OUTER SPACE
THE PROHIBITION OF THE USE OF FORCETHE PROHIBITION OF THE USE OF FORCE UNDER INTERNATIONAL LAW
As noted in Part I, the UN Charter —including its cornerstone provision prohibiting the use offorce— applies in space. The prohibition of the use of force is set out in Article 2(4) of the UN
Charter and binds all States under customary international law. Article 2(4) provides:“All Members shall refrain in their international relations from the threat or use offorce against the territorial integrity or political independence of any state, or in
any other manner inconsistent with the Purposes of the United Nations.”

STATUS OF THE PROHIBITION UNDER INTERNATIONAL LAW
The International Court of Justice (ICJ) has recognized that the prohibition of the use offorce is a rule of customary international law56 and a “cornerstone“ of the UN Charter.57 The

obligation to refrain from the threat or use of force in Article 2(4) of the UN Charter prevails
over other obligations of Member States under any other international agreement in theevent of a conflict between the obligations.58 The prohibition of the use of force is widely

considered to be a peremptory norm of international law (ius cogens),59 that is, “a norm
accepted and recognized by the international community of States as a whole as a norm

from which no derogation is permitted and which can be modified only by a subsequent
norm of general international law having the same character”.60

56 Military and Paramilitary Activities in and against Nicaragua (Nicar. V. U.S.), Jurisdiction of the Court andAdmissibility of the Application, Judgment, 1984 I.C.J. Rep. 392, ¶73 (26 Nov.).
57 Armed Activities on the Territory of the Congo (Dem. Rep. Congo v. Uganda), Merits, Judgment, 2005 I.C.J. Rep.
168 ¶148 (19 Dec.).
58 UN Charter, supra note 1, art. 103.
59 The UN International Law Commission stated in its commentary on the Draft Articles on the Law of Treatiesthat “the law of the Charter concerning the prohibition of the use of force” is “a conspicuous example” of aperemptory norm: Draft Conclusions on Subsequent Agreements and Subsequent Practice in Relation to theInterpretation of Treaties, conclusion 7(3), annexed to U.N. Doc. A/RES/73/202 (3 Jan. 2019). The ICJ referred tothis statement in the Case concerning Military and Paramilitary activities in and against Nicaragua, supra note
49, 94, 190.
60 Vienna Convention on the Law of Treaties art. 53, opened for signature 23 May 1969, 1155 U.N.T.S 331 (entered into
force 27 Jan. 1980).

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EXCEPTIONSThere are two recognized exceptions to the prohibition of the use of force. The first is a useof force in self-defence under Article 51 of the UN Charter and customary international law.
The second exception is a use of force authorized by the UN Security Council (UNSC) acting
under Chapter VII of the UN Charter.61

With respect to self-defence, Article 51 provides:“Nothing in the present Charter shall impair the inherent right of individual orcollective self-defence if an armed attack occurs against a Member of the United
Nations, until the Security Council has taken measures necessary to maintaininternational peace and security. Measures taken by Members in the exercise ofthis right of self-defence shall be immediately reported to the Security Council

and shall not in any way affect the authority and responsibility of the Security
Council under the present Charter to take at any time such action as it deems
necessary in order to maintain or restore international peace and security.”

The exercise of the right to self-defence is subject to the customary international law
requirements of necessity and proportionality.62

Consultations

Retorsions

HARMFULINTERFERENCEUNLAWFULINTERVENTIONTHREAT
OF FORCE

Non-forceful Countermeasures

USE OFFORCEARMED
ATTACK

Self-defence

Figure 4: Spectrum of harmful or hostile acts by another State and lawful responsive
measures

61 UN Charter, supra note 1, arts. 39 and 42.
62 Military and Paramilitary Activities in and against Nicaragua, supra note 49, ¶176; Legality of the Threat or Use of
Nuclear Weapons, Advisory Opinion, 1996 I.C.J. Rep. 94, ¶41 (8 July); Oil Platforms (Iran v. U.S.), Merits, Judgment,
2003 I.C.J. Rep. 161, ¶51 (6 Nov.).

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UNIDIRCONSEQUENCES OF CHARACTERIZING AN ACT AS A PROHIBITED “USE OF FORCE”
A prohibited use of force cannot be legally excused by circumstances precluding

wrongfulness, for example, necessity, force majeure, distress or countermeasures. Use offorce as a countermeasure63 and armed reprisals in peacetime64 are prohibited underinternational law. In the Nicaragua case, the ICJ distinguished “the most grave forms of
the use of force (those constituting an armed attack) from other less grave forms”.65 Thismeans that if an act is a prohibited use of force but not an “armed attack”, it may not be

used in response to a similar act because the right to self-defence arises only in responseto an “armed attack”. Furthermore, “[a] treaty is void if its conclusion has been procured by

the threat or use of force in violation of the principles of international law embodied in theCharter of the United Nations”.66

LEGAL CONSEQUENCES OF VIOLATION
A prohibited use of force gives rise to international State responsibility and the obligation

to cease the unlawful act,67 make full reparation68 and the right of the victim State to takenon-forcible countermeasures.69 There are additional consequences if the violation is aserious breach of a peremptory norm, namely, that other States shall cooperate using lawfulmeans to bring the violation to an end, shall not recognize the situation as lawful and shallnot render aid or assistance in maintaining the situation.70

A prohibited use of force may trigger the collective security provisions under the UN Charter.
If a prohibited use of force meets the threshold of an armed attack, it will give rise to aright of individual and collective self-defence under Article 51 of the UN Charter. The Security

Council may determine that a prohibited use of force is a “threat to the peace, breach of

the peace, or act of aggression”, and make recommendations or decide on measures tomaintain or restore international peace and security in accordance with Articles 41 and 42of the UN Charter.71

63 Draft articles on Responsibility of States for Internationally Wrongful Acts, with commentaries, art. 50(1)(a),Report of the International Law Commission to the General Assembly, 56 U.N. GAOR Supp. No. 10, U.N. Doc.
A/56/10 (2001), https://legal.un.org/ilc/texts/instruments/english/commentaries/9_6_2001.pdf.
64 Legality of the Threat or Use of Nuclear Weapons, supra note 62, ¶46.
65 Military and Paramilitary Activities in and against Nicaragua, supra note 49, 101, ¶191.
66 Vienna Convention on the Law of Treaties, supra note 60, art. 52. The ICJ held that this reflects customaryinternational law: Fisheries Jurisdiction (U.K. v. Ice.), Jurisdiction, Judgment, 1973 I.C.J. Rep. 3, ¶14 (2 Feb).
67 Responsibility of States for Internationally Wrongful Acts, supra note 54, art. 30.
68 Ibid., art. 31.
69 Ibid., art. 22.
70 Draft articles on Responsibility of States for Internationally Wrongful Acts, with commentaries, supra note 63, art.
41.
71 See UN Charter, supra note 1, art. 39.
Art. 41 provides: “The Security Council may decide what measures not involving the use of armed force are to be
employed to give effect to its decisions, and it may call upon the Members of the United Nations to apply suchmeasures. These may include complete or partial interruption of economic relations and of rail, sea, air, postal,telegraphic, radio, and other means of communication, and the severance of diplomatic relations.”
Art. 42 provides: “Should the Security Council consider that measures provided for in Article 41 would beinadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may benecessary to maintain or restore international peace and security. Such action may include demonstrations,blockade, and other operations by air, sea, or land forces of Members of the United Nations.”O U T E R S PA C E & U S E O F F O R C E

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If a prohibited use of force meets the threshold of an act of aggression,72 it may give rise tointernational criminal responsibility for the crime of aggression. This criminal responsibility

may arise for individuals who are “in a position effectively to exercise control over or to direct
the political or military action of a State“ and who plan, prepare, initiate or execute “an actof aggression which, by its character, gravity and scale, constitutes a manifest violation of
the Charter of the United Nations”.73

THE MEANING OF A PROHIBITED“USE OF FORCE“
The prohibition of the use of force binds all States under customary international law, is a“cornerstone“ of the UN Charter, is applicable in outer space, and the consequences for its

violation are serious. It is therefore important to clarify the meaning of a prohibited use offorce and how it applies in outer space. The ICJ has not defined a prohibited “use of force“
in its jurisprudence74 and as yet there is no general agreement on how to define the term,

which has received far less scholarly attention than the definition of an “armed attack“ under
Article 51 of the UN Charter.75 However, the rules of treaty interpretation76 can be applied to
Article 2(4) of the UN Charter77 to distil a set of elements of a prohibited use of force, which

72 See Annex of G.A. Res. 3314 (XXIX), 29th Sess., on Definition of Aggression (14 Dec. 1974), https://undocs.org/A/
RES/3314(XXIX).
73 Rome Statute of the International Criminal Court arts. 5(d) and 8bis, https://www.icc-cpi.int/sites/default/
files/NR/rdonlyres/ADD16852-AEE9-4757-ABE7-9CDC7CF02886/283503/RomeStatutEng1.pdf.
74 The ICJ has considered the interpretation and application of Article 2(4) directly and indirectly in a number ofits decisions, but has not set out its content. The most relevant decisions are Corfu Channel (U.K. v. Albania),
Merits, Judgment, 1949 I.C.J. Rep. 4; Fisheries Jurisdiction (Federal Republic of Germany v. Iceland), Merits,
Judgment, 1974 I.C.J. Rep. 175; United States Diplomatic and Consular Staff in Tehran (U.S. v. Iran), Judgment,
1980 I.C.J Rep. 3, Case concerning Military and Paramilitary Activities in and against Nicaragua, supra note 49;
Fisheries Jurisdiction (Spain v Canada), Jurisdiction, Judgment, 1998 I.C.J Rep. 432; Legality of the Threat or Useof Nuclear Weapons, supra note 62, Oil Platforms, supra note 62, Legal Consequences of the Construction of aWall in the Occupied Palestinian Territory, Advisory Opinion, 2004 I.C.J. Rep. 136; Armed Activities on the Territoryof the Congo, supra note 57.
75 UN Charter, supra note 1, Art. 51.
76 These principles are enshrined in the Vienna Convention on the Law of Treaties, supra note 60. The general rulesof treaty interpretation and the rule on supplementary means of interpretation are set out in articles 31 and 32
of the Vienna Convention on the Law of Treaties, which also apply as rules of customary international law. These
rules require:• Looking at the “ordinary meaning to be given to the terms of the treaty in their context and in the light of its
object and purpose” (art 31(1));• Taking into account “subsequent agreements between the parties regarding the interpretation of thetreaty or the application of its provisions“ and “subsequent practice in the application of the treaty which
establishes the agreement of the parties regarding its interpretation”, together with “any relevant rules ofinternational law applicable in the relations between the parties” (art. 32); and• Where appropriate, considering preparatory work of the UN Charter (travaux préparatoires) and “othersubsequent practice“ as a supplementary means of interpretation (art. 32).
77 Vienna Convention on the Law of Treaties, supra note 60, art. 5 states that “[t]he present Convention applies
to any treaty which is the constituent instrument of an international organization … without prejudice to any
relevant rules of the organization”; see also Legality of the Threat or Use of Nuclear Weapons, supra note 62, ¶19
and Certain Expenses of the United Nations (Article 17, paragraph 2, of the Charter), Advisory Opinion, 1962 I.C.J.
151 Rep., ¶157.

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can serve to identify such uses of force in practice. This provides guidance to policymakers,

legal advisors and practitioners on how to determine if an act is a prohibited use of force inviolation of the UN Charter and customary international law.78
ELEMENTS
Article 2(4) requires two types of criteria to be met for a prohibited use of force: first,
contextual requirements which must all be fulfilled for a “use of force“ to fall within the scopeof the prohibition; and second, that the act is an actual “use of force”.79
Contextual requirements

The contextual requirements of a prohibited use of force are set out in the text of Article 2(4).
The “use of force” must be:•

By a State.

•“Against the territorial integrity or political independence of any state or in any other•In “international relations”.

manner inconsistent with the Purposes of the United Nations”.

Elements of “use of force”

As the elements of a “use of force“ are not set out in the text of Article 2(4), they must bederived through the process of treaty interpretation set out above. The following elementsof “use of force“ have been identified by scholars:80
•

Means

•

Gravity••

Effects

Coercive or hostile intent

78 Since the rule in Article 2(4) is the origin of the customary rule and the meaning is the same under both sourcesof law, it is logical to focus on interpreting Article 2(4) to derive the meaning of a prohibited use of force between
States: See POBJIE, supra note 2, chapters 1-3.
79 For a detailed analysis including the content of each element, see ibid., chapters 4-6 and 8.
80 See Ibid., Part II.

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Contextual Requirements
By a StateIn “International relations”“Against the territorial integrity
or political independence of any

Elements of Use of Force
Means

Effects

Gravitystate or in any other mannerinconsistent with the Purposesof the United Nations”

Coercive or hostile intent

Figure 5: Elements of prohibited use of force under Article 2(4) UN Charter
DEFINITIONAL FRAMEWORKThe elements of a “use of force” set out above should be weighed and balanced to determine

whether the threshold of the definition is met.81 Not all elements must be present for an act
to constitute prohibited force if they are compensated by other elements. For example, ahostile or coercive intent may turn a forcible act into a use of force even if other elements
are relatively weak, such as if the effects are temporary and reversible or if the gravity ofeffects is less severe.

APPLYING THE PROHIBITION OF THE USE
OF FORCE IN OUTER SPACE
This section applies the elements of a prohibited “use of force“ to outer space, identifies
areas of legal uncertainty and sets out possible legal interpretations of Article 2(4) of the
UN Charter.

CONTEXTUAL REQUIREMENTS
By a State

Attribution of a “use of force“ in outer space to a State raises factual and legal issues. Factually,

it can be challenging to attribute acts to a State in the space environment (discussed later
as well with respect to intent). The legal issue is whether a special regime of attribution
applies generally in outer space due to article VI of the OST, which provides:

81 See Ibid., chapter 8.

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UNIDIR“States Parties to the Treaty shall bear international responsibility for national

activities in outer space, including the Moon and other celestial bodies, whether
such activities are carried on by governmental agencies or by non-governmental

entities, and for assuring that national activities are carried out in conformity
with the provisions set forth in the present Treaty.”

Article VI of the OST – State responsibility for national activities in outer space
Article VI of the OST expresses that any act carried out by a non-governmental entity
in outer space is imputable to the State as if it were its own, making the State directly

responsible for such act. Article VI’s stipulation that a State is responsible for its national
activities in outer space marks a unique development in public international law, thatdifferentiates itself from the regime of State responsibility applicable to activities on

Earth. In the context of space law, a State cannot avoid responsibility by disclaiming

responsibility for the acts of its private persons. The way many States implement theirarticle VI responsibilities is through the enactment of national laws and regulations.82

The extent to which article VI of the OST displaces the customary international law rules for

attribution of conduct to a State under the law of State responsibility83 applicable to otherdomains, and in particular ius contra bellum, is unsettled.84 Experts have debated whether
article VI could implicate States in violations of ius contra bellum with respect to activities of

non-governmental entities in outer space.85 This could potentially result in a “use of force“

by a non-State actor (such as a commercial actor) being construed as a violation of theprohibition of the use of force by the State responsible. The increasing presence of dual-

purpose objects in space (many of which are operated by commercial actors), coupled withthe concern they pose to States, makes the determination of responsibility a matter of great
relevance. An interpretation aligned with article VI of the OST in relation to the use of force
would have clear legal and policy implications and may also result in State responsibility

for national activities of non-State actors in outer space under other applicable legal
frameworks.

82 See Space Security Lexicon, supra note 12, 3.2.10.
83 Responsibility of States for Internationally Wrongful Acts, supra note 54, chapter II.
84 Different positions in the debate are canvassed in Frans G. von der Dunk, The Origins Of Authorisation: Article VI
Of The Outer Space Treaty And International Space Law, in NATIONAL SPACE LEGISLATION IN EUROPE 3–28 (2011).
85 Erin Pobjie, Military Uses of Outer Space, in MAX PLANCK ENCYCLOPEDIA OF PUBLIC INTERNATIONAL LAW ¶16 (2024);
Tara Brown, Ukraine Symposium – The Risk of Commercial Actors in Outer Space Drawing States into Armed
Conflict, LIEBER INSTITUTE WEST POINT (8 July 2022), https://lieber.westpoint.edu/commercial-actors-outerspace-armed-conflict/; Dale Stephens, cited in Charlie Dunlap, Are Commercial Satellites Used for IntelligenceGathering in Attack Planning Targetable?, Lawfire (5 Mar. 2021), https://sites.duke.edu/lawfire/2021/03/05/
are-commercial-satellites-used-for-intelligence-gathering-in-attack-planning-targetable/:“inthe
Woomera Manual process we note this has the great potential to implicate States in ad bellum or in bello
issues in situations that would not otherwise be the case under general international law rules on attribution”.
For a different view, see Hitoshi Nasu, Targeting a Satellite: Contrasting Considerations between the Jus Ad
Bellum and the Jus in Bello, 99 International Law Studies 142, 151-152 (2022).

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UNIDIRIn “international relations”

To fall within the scope of the prohibition of the use of force, a use of force must be in“international relations”. This is clearly fulfilled when the use of force is against anotherState or against persons or objects with a sufficient legal nexus to another State (discussed

below in relation to the “victim State”). Article 2(4) of the UN Charter may also cover usesof force against non-State objects, such as malicious damage to the space environment
including the Moon and other celestial bodies, if it occurs in “international relations“ and is“inconsistent with the Purposes of the United Nations”,86 although as yet there is no State
practice to confirm this interpretation.

Malicious damage to the space environment
Although States have not categorized it as such, intentional debris creation throughthe use of kinetic counterspace capabilities (even in the context of testing) could beconsidered a form of malicious damage to the space environment. The evolution ofState practice and opinio iuris regarding the intentional creation of debris shows anincreasingly negative perception of these activities, which have led over the years

to the launch of multiple initiatives to address the problem debris causes. In theCommittee on the Peaceful Uses of Outer Space (UNCOPUOS), measures such as theDebris Mitigation Guidelines87 are an example of this.

Space sustainability has become an increasing priority in the context of space
security discussions,88 and the intentional creation of debris —one of the threats tospace security that raises the most concern among States— has become a worrisome

matter. This is not only because of the damage that uncontrollable debris could

cause to other space objects, but also because of the negative impact to the spaceenvironment, which would hamper space activities both in the present and future.

Resolution 77/41 on destructive direct-ascent anti-satellite missile testing89 which“calls upon all States to commit not to conduct destructive direct-ascent anti-satellite

missile tests” emphasizes the importance of space sustainability for peace andsecurity. The commitment undertaken so far by 39 States highlights that intentional
debris creation is seen as increasingly harmful and unwise. States could therefore
perceive intentional debris creation as malicious damage to the space environment.

86 For an analysis, see POBJIE, supra note 2, at 220-1.
87 Debris Mitigation Guidelines, supra note 40.
88 Sarah Erickson & Almudena Azcárate Ortega, To Space Security and Beyond: Exploring Space Security, Safety,
and Sustainability Governance and Implementation Efforts, Space Dossier 9, UNIDIR 7, 38-39 (2023), https://doi.
org/10.37559/WMD/23/Space/06. “Space sustainability is commonly understood to mean stakeholders’ ability
to continue to be able to use and benefit from space. Space sustainability requires that space be kept safe andsecure, so that stakeholders may be able to use, explore, and benefit from space ‘without discrimination of any
kind, on a basis of equality and in accordance with international law’ (article I of the OST). Space sustainability
therefore seeks to preserve the usability of space.” See Space Security Lexicon, supra note 12, at 3.3.9.
89 G.A. Res. 77/41, 77th Sess., on destructive direct-ascent anti-satellite missile testing, U.N. Doc. A/RES/77/41 (7 Dec.
2022), https://undocs.org/A/RES/77/41.

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UNIDIR“Against the territorial integrity or political independence of any state, or in any othermanner inconsistent with the Purposes of the United Nations”

There is no specific rule to determine which, if any, State is the victim of a “use of force“

in outer space. If a satellite is targeted, several States may have an affected interest:for example, through legal ownership, registration as a launching State90 or by suffering

harm as a result of the interference or damage to the satellite due to their utilization of theservices the satellite provides. The definition of an injured State under the international lawof State responsibility can be applied to determine which State or States are the victim.91
This may result in more than one victim State92 and also means that a use of force against

a commercial satellite could fall within the scope of the prohibition of the use of force.
The reference in Article 2(4) to force “in any other manner inconsistent with the Purposesof the United Nations,” is a catch-all provision that makes it clear that the prohibition in

Article 2(4) is all-encompassing and was intended to avoid loopholes.93 This is supportedby the travaux préparatoires of the UN Charter and was confirmed by States during thedrafting of the 1970 Friendly Relations Declaration.94

ELEMENTS OF “USE OF FORCE”
Means

Counterspace technologies or capabilities can be offensive and defensive, and can befurther classified into different groups including kinetic physical (hostile actions against

ground stations, direct-ascent ASATs, co-orbital ASATs), non-kinetic physical (lasers, high-powered microwaves (HPM), and electromagnetic pulses (EMP)), electronic, and cyber.

This is not a closed list, nor are these terms universally used by all States, and there aresome lists that include other categories.95 Another common classification is the division into

hard-kill (which generally refers to physical hostile operations —especially those that use
kinetic force— that result in the destruction of space objects) and soft-kill counterspacetechnologies (which refers to non-physical interference that renders space assets impaired,ineffective, or inactive).96

90 Registration Convention, supra note 37, art. 2.
91 Nasu, supra note 85, at 164.
92 Ibid., 166-7: “There are circumstances in which multiple States find themselves specially affected on different
grounds, for example when the missile early warning capability of one State is disabled by a laser employed toneutralize its sensors hosted on a commercial communications satellite on the register of another State. The
former State can claim an injury for the damage caused to the missile early warning system, whereas the latter
State might consider the physical damage caused to the sensors on board the satellite as sufficient evidenceof a special impact on it as the State of registry.”
93 See POBJIE, supra note 2, 101-103; Tom Ruys, The Meaning of Force and the Boundaries of the Jus ad Bellum: Are
Minimal Uses of Force Excluded from UN Charter Article 2(4)?, 108 Am. J. Int’l L. 159, 163-164 (2014).
94 POBJIE, Ibid.
95 For other terms used to refer to these capabilities, see Report of the Secretary-General, Reducing space threats
through norms, rules and principles of responsible behaviours, U.N. Doc. A/76/77 (13 July 2021) [hereinafter‘Report of the Secretary-General A/76/77’], https://undocs.org/en/A/76/77.
96 See Space Security Lexicon, supra note 12, at 3.1.2.

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A prohibited “use of force“ is not necessarily the same as the use of a “space weapon“ or acounterspace capability.97 The ICJ has stated that the prohibition of the use of force “doesnot refer to specific weapons“ and applies “to any use of force regardless of the weapons
employed”.98 The ordinary meaning of the term “use of force“ refers to “physical strength or

power exerted upon an object; esp. the use of physical strength to constrain the action of

persons; violence or physical coercion“ and “violent means”, and does not refer to the useof a weapon.99 Such physical strength or power can be exerted by a dual-purpose object aswell. Legal scholars emphasize the effects of an act rather than its means when assessing if
an act is a “use of force“ for the purposes of the prohibition.100

The release of kinetic energy is not required for an act to constitute a “use of force” underarticle 2(4) of the UN Charter.101 This is relevant to outer space since many counterspacecapabilities are non-kinetic. Non-kinetic counterspace capabilities include electronic andcyber counterspace technologies. The interruption of services through jamming or dazzlinga satellite or a cyberattack against a segment of a space system would be examples of this.

97 The definition of a weapon is relevant to certain rules of international humanitarian law and may be relevant forarms control treaties and verification measures, but does not form part of the international legal prohibition of
the use of force between States (jus contra bellum).
98 Legality of the Threat or Use of Nuclear Weapons, supra note 62, ¶39.
99 “Force, n.1.” Oxford University Press, OED Online, https://www.oed.com/dictionary/force_n1?tab=meaning_
and_use#4005322.
100 EG HENDERSON, THE USE OF FORCE AND INTERNATIONAL LAW 59 (2018): “a consideration of the effects of the action
takes on a greater importance the further one moves away from what we might consider to be conventional
weapons”; POBJIE, supra note 2, 125-130. See also the Tallinn Manual’s commentary on the definition of theuse of force with respect to cyber operations, which sets out indicative factors for whether a cyber operation
is a “use of force” and focuses on effects rather than means: TALLINN MANUAL 2.0 ON THE INTERNATIONAL LAWAPPLICABLE TO CYBER OPERATIONS, Commentary to rule 69, 333, ¶9 (Michael N. Schmitt ed., 2017).
101 POBJIE, supra note 2, 128-130, noting that “[s]ince the prohibition of the use of force in article 2(4) undoubtedly
covers the use of chemical, biological and nuclear weapons, a kinetic release of energy is clearly not always
required” (128-129, footnote omitted).

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UNIDIRElectronic Counterspace Capabilities“Electronic counterspace technologies, sometimes also known as electromagnetic

technologies, can target the electromagnetic spectrum used by space systems totransmit and receive data, causing harmful interference. Jammers generate noiseon the same radio frequency band as a space system in order to block or interferewith the signal travelling from Earth to a satellite (uplink) or from a satellite to Earth(downlink).”102

Cyber Counterspace Capabilities“These technologies can target data and the systems that use, transmit, and control
the flow of data. Information and communication technologies can be used to target

satellites as well as ground stations or even end-user components, such as modems,with the objective of interfering with services (such as Internet coverage), intercepting

information, or inserting false or corrupt data into a system. Hostile operations thatuse cyber means or methods are generally reversible; however, a malicious or hostile
operation that targets the command and control system of a satellite could render

it inoperable in an irreversible way, as the hostile party could cause a stoppage ofthe satellite’s functions permanently, and cause it to waste its fuel or damage its

sensors. Such a step could have a large impact radius and potentially affect criticalinfrastructure.103 The use of information and communication technologies against

space systems can be conducted in a relatively cheap manner, compared to othercounterspace capabilities. Cyber counterspace capabilities can be difficult to predict,detect, and attribute.”104

102 Space Security Lexicon, supra note 12, at 3.1.3.
103 There is no universally accepted definition of critical infrastructure in the context of space security. In thecontext of cybersecurity, the General Assembly has highlighted that critical infrastructures include “those used
for, inter alia, the generation, transmission and distribution of energy, air and maritime transport, banking andfinancial services, e-commerce, water supply, food distribution and public health—and the critical information
infrastructures that increasingly interconnect and affect their operations”. Critical infrastructure is consideredof fundamental importance and “the backbone of a society’s vital functions, services and activities. If thesewere to be significantly impaired or damaged, the human costs as well as the impact on a State’s economy,
development, political and social functioning and national security could be substantial”. See Space Security
Lexicon, supra note 12, at 3.3.1. See also G.A. Res. 58/199, 58th Sess., on the creation of a global culture ofcybersecurity and the protection of critical information infrastructures (30 Jan. 2004), available online at:https://digitallibrary.un.org/record/509571; Report of the Group of Governmental Experts on Advancing
Responsible State Behaviour in Cyberspace in the Context of International Security, U.N. Doc. A/76/135 (14 July2021), https://undocs.org/A/76/135.
104 Space Security Lexicon, supra note 12, at 3.1.4.

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Effects•

Directness of effects and object of harm: the relevant harmful effects are physical effects105
with sufficient proximity to the application of force (directness).106 Sufficient proximity

refers to the nexus between the act and its result, not how long it takes for the harm to

manifest.107 Indirect or reverberating effects are more likely to lead to characterization of
the act as a “use of force“ if they are of high gravity108 or if there is a hostile or coercive
intent (discussed under “Coercive intent” below). The extent to which indirect effects arelegally relevant to characterizing an act as a prohibited “use of force“ depends on thecontext.

A forcible act that interferes with, damages or destroys a satellite can have indirect orreverberating effects, which may include damage or destruction to other space objects

caused by the resulting debris from kinetic ASAT, terrestrial effects including deathor injury of persons,109 as well as the permanent impact on terrestrial biomass andirreversible effects on ecosystems such as oceans, impoverishing the living conditionsof large human populations. It can also cause interruption of services that the satellite

provides. It would be particularly grave if these services were related to the operability of•critical infrastructure.

Temporary and reversible effects: it is legally open whether the prohibition of the use offorce under Article 2(4) and customary international law covers temporary and reversible

effects.110 Many counterspace capabilities can produce temporary and reversible effects,

for example jamming or dazzling, high-powered microwaves, electronic pulses and theuse of cyber for counterspace.111 Due to military, commercial and civilian reliance on

space systems and services, harmful effects that are temporary and reversible may still•

be significant particularly if they affect critical infrastructure.

Potential effects: it is also textually ambiguous whether the physical effect (i.e. harm)must actually occur for the act to be a “use of force”, or if potential but unrealized physicaleffects/harm will suffice.112 The latter —a risk of potential harm— can be characterized asa hazard, which may be distinguished from an immediate risk of direct or indirect harm
which actually ensues (discussed earlier). For example, studies have demonstrated
the likelihood of long-lasting debris generation by the use of kinetic ASATs,113 which risks

105 POBJIE, supra note 2, 133: “non-physical effects alone (such as psychological, economic or more abstract formsof harm) are not likely to be legally relevant to the determination of whether an act is a ‘use of force’.” See also
Claus Kreß, The State Conduct Element, IN THE CRIME OF AGGRESSION: A COMMENTARY 412, 425 (Claus Kreß and
Stefan Barriga eds., 2017).
106 POBJIE, supra note 2, 134.
107 The commentary to Rule 69 in the Tallinn Manual 2.0 (which defines a “use of force” with respect to cyber
operations) suggests that the criterion of directness relates to States’ perception of the military nature of theact, since “[i]n armed actions … cause and effect are closely related”: Schmitt, supra note 100, at 333, ¶9.
108 See Nasu, supra note 85, at 159.
109 The present report confines its analysis to the prohibition of the use of force between States (ius contra bellum).
Uses of force with harmful effects on individuals may also be regulated by other applicable legal frameworks
such as international humanitarian law and international human rights law.
110 POBJIE, supra note 2, 135-136.
111 Clayton Swope et al., SPACE THREAT ASSESSMENT 2024, Center for Strategic & International Studies 3-5 (2024),
https://www.csis.org/analysis/space-threat-assessment-2024.
112 POBJIE, supra note 2, 136-137.
113 MICHAEL BYERS & AARON BOLEY, WHO OWNS OUTER SPACE?: INTERNATIONAL LAW, ASTROPHYSICS, AND THE
SUSTAINABLE DEVELOPMENT OF SPACE 274 (2023).

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UNIDIR
potential harm to other space objects. The report of the Secretary-General on reducing
space threats through norms, rules and principles of responsible behaviours, notes that:“Increasing debris poses a collision risk to space objects. Objects as small as 1
cm in diameter can damage the functions of an active satellite; objects between
1 and 10 cm can disable or destroy a satellite. Impacts involving larger objects

generate hundreds or thousands of pieces of debris. Thus, increasing debris also
poses a risk to future access to space, as the cascading generation of debris
could render orbits unusable for generations.”114

The 2019 GGE on Further Practical Measures for PAROS placed “some emphasis on

prohibiting in particular intentional acts that could result in the generation of longlasting debris in Earth orbit.”115
Gravity

Whether there is a gravity threshold of a prohibited “use of force“ is controversial andunsettled in legal scholarship.116 The question is significant because a de minimis (i.e.
lower) gravity threshold under Article 2(4) could also affect the size of the gap between aprohibited “use of force“ and an “armed attack“ giving rise to the right to self-defence under
Article 51. Under the framework set out in this report, there is no gravity threshold for a “useof force” under Article 2(4) but the gravity of an act remains relevant to whether that act is a“use of force” (in combination with other elements), and is also important to the contextual
requirement of whether the act occurs in “international relations”.117

114 Report of the Secretary-General A/76/77, supra note 95, ¶12.
115 Annex II to the Report of the Group of Governmental Experts on Further Practical Measures for the Prevention of
an Arms Race in Outer Space (procedural), ¶41 U.N. Doc. A/74/77 (19 Apr. 2019) [hereinafter ‘Annex II’], https://
undocs.org/Home/Mobile?FinalSymbol=A%2F74%2F77.
116 Scholars taking the position that there is a de minimis gravity threshold include OLIVIER CORTEN, THE LAW
AGAINST WAR: THE PROHIBITION ON THE USE OF FORCE IN CONTEMPORARY INTERNATIONAL LAW (2021) and Mary
Ellen O’Connell, The Prohibition of the Use of Force, in RESEARCH HANDBOOK ON INTERNATIONAL CONFLICT AND
SECURITY LAW 89 (Nigel White & Christian Henderson eds., 2013). A proponent of the opposite view is Tom Ruys,
see supra note 93. Other scholars who have examined this issue frame it differently. For example, Henderson,supra note 100, 74 argues that that “the gravity of the use of force against such private actors does not by itself
determine the applicability of the prohibition … Indeed, it is more a qualitative – State or private – as opposed
to quantitative – small- or large-scale – distinction, making a clear de minimis threshold hard to discern“ andthat “when the ‘international relations’ between states are engaged there is little state practice supportive ofa de minimis threshold in the context of incidences involving armed force.“ Similarly, POBJIE, supra note 2, 143
argues that there is no gravity threshold for a “use of force “ but it remains relevant to whether the act is a “useof force“ (in combination with other elements), and more importantly, to the contextual element of whether theact occurs in “international relations”.
117 See POBJIE, supra note 2, 143.

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The gravity of forcible acts is especially relevant in outer space, because of the wide range

in intensity of effects. In the discussions of the 2019 GGE on PAROS, “[i]t was considered thatthreats exist on a continuum from low intensity, characterized by reversible and disruptive
impacts, to high intensity, characterized by irreversible and destructive impacts.”118

REVERSIBLE AND DISRUPTIVE IMPACTS

IRREVERSIBLE AND
DESTRUCTIVE IMPACTS

Figure 6: Increasing gravity of a use of force
The gravity of harm caused by a specific act must be assessed on a case-by-case basis
and will vary depending on the nature and function of the targeted object, the type andseverity of interference or damage and the circumstances of the particular case, as well as
any legally relevant indirect effects (as discussed earlier).
Coercive or hostile intent

There are differing views on whether a coercive or hostile intent is required for a prohibited“use of force”. Intention is not a necessary requirement for an act to be internationallywrongful; whether intention is necessary depends on the obligation in question.119 The

majority of scholars argue that an intention to use force is required for a prohibited “useof force”.120 It is unclear from the text of Article 2(4) if a prohibited “use of force“ requires aparticular kind of intention. This is relevant to accidents, mistakes as well as situations wherean alternative legal framework may apply to the activity (such as law enforcement within aState’s own territory) and situations in which it may be difficult or impossible to discern thesponsoring intent of a forcible act.

118 Annex II, supra note 115, para. ¶35; see also Draft Report of the Group of Governmental Experts on Further
Practical Measures for the Prevention of an Arms Race in Outer Space, in Recommendations to promote thepractical implementation of transparency and confidence-building measures in outer space activities with thegoal of preventing an arms race in outer space, in accordance with the recommendations set out in the reportof the Group of Governmental Experts on Transparency and Confidence-Building Measures in Outer Space
Activities, ¶38, U.N. Doc. A/CN.10/2019/WP.1 (25 Apr. 2019) [hereinafter ‘Draft GGE Report’], https://undocs.org/A/
CN.10/2019/WP.1
119 See Draft articles on Responsibility of States for Internationally Wrongful Acts, with commentaries, supra note
63, commentary to article 2, at ¶¶3 and 10. Para 10: “In the absence of any specific requirement of a mental
element in terms of the primary obligation, it is only the act of a State that matters, independently of any
intention”.
120 Ian Brownlie argued that no specific intention is required: IAN BROWNLIE, INTERNATIONAL LAW AND THE USE OF
FORCE BY STATES 377 (1963). Scholars taking the opposite view include Corten, supra note 116, 76 and Henderson,supra note 100, 75. Tom Ruys argues that a hostile intent can exclude some acts of law enforcement andunintentional or harmless small-scale incursions from the scope of the prohibition: Ruys, supra note 93, 172-3,
190-1. Erin Pobjie argues that a hostile intent is a relevant (though not necessary) element to determine whether
an act is a prohibited “use of force”: POBJIE, supra note 2, 145-157.

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Nascent State practice supports the view that only acts which intentionally cause damage
to or interfere with space objects are likely to be characterized as a prohibited “use offorce”. The 2008 draft PPWT defined the “use of force” as “any hostile actions against outerspace objects including, inter alia, those aimed at their destruction, damage, temporarilyor permanently injuring normal functioning, deliberate alteration of the parameters of theirorbit, or the threat of these actions”.121 One analysis of the definition set out in the 2008 draft

PPWT did not object to the requirement that the action be “hostile“ but appeared to express
reservations regarding the interpretation of this term.122 The revised definition of “use offorce“ in the 2014 draft PPWT replaced the term “hostile actions“ with “intended action to

inflict damage”.123 Some experts in the 2019 GGE on PAROS emphasized the desirability of“prohibiting various types of intentionally harmful or destructive acts”.124 State submissions
to the OEWG which distinguish between irresponsible behaviours and security threats also

place emphasis on “deliberately causing non-consensual interference“ to space systems

for the latter.125 Given the well-known dangers posed by space debris,126 deliberately creating

long-lasting space debris through kinetic ASATs is likely to be considered reckless; however it

may be questioned whether such recklessness would suffice for a “use of force“ in this context.
A specific intent may be difficult to discern in outer space. This is particularly the case fordual-purpose space objects, which are not designed for military application but whosecapabilities may be repurposed to interfere with or damage other space objects, such as on-orbit servicers that carry out rendezvous and proximity operations (RPOs), and capabilities
designed for active debris removal.127

121 U.N. Doc. CD/1839, supra note 27, art. 1(e), emphasis added.
122 Letter Dated 19 August 2008 from the Permanent Representative of the United States of America Addressedto the Secretary-General of the Conference Transmitting Comments on the Draft Treaty on Prevention of thePlacement of Weapons in Outer Space and of the Treaty or Use of Force Against Outer Space Objects (PPWT)
as Contained in Document CD/1839 of 29 February 2008, ¶¶4,, 5, U.N. Doc. CD/1847 (26 Aug. 2008). The analysis
notes that the term “hostile“ in the definition “appears to be intended to capture only actions which are taken
against another country’s satellite(s), which are not part of a mutually-agreed cooperation program.“
123 PPWT, supra note 21 (emphasis added). Art. 1(d): “the terms ‘use of force’ or ‘threat of force’ mean, respectively,
any intended action to inflict damage to outer space object under the jurisdiction and/or control of otherStates, or clearly expressed in written, oral or any other form intention of such action. Actions subject to special
agreements with those States providing for actions, upon request, to discontinue uncontrolled flight of outerspace objects under the jurisdiction and/or control of the requesting States shall not be regarded as use offorce or threat of force“.
124 Annex II, supra note 115, ¶41 (emphasis added); see also Draft GGE Report, supra note 118, ¶68: “The Group reaffirmed concerns … over the deliberate and intentional use of force in space”.
125 Canada’s Views on Reducing Space Threats through norms, rules and principles of Responsible Behaviour, submission in response to note verbale, 2-3, U.N. Doc. A/AC.294/2022/WP.7(6 May 2022), emphasis added.
126 Report of the Secretary-General A/76/77, supra note 95, ¶12: “Many States express concern about space debris
as the most significant threat to the space environment.”
127 Annex II, supra note 115, ¶39.

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MOVING FORWARD: INTEGRATING THE
PROHIBITION OF THE USE OF FORCE
INTO SPACE SECURITY DEBATESAs States continue their efforts to address space security concerns and establish measuresand mechanisms to mitigate potential threats to space security and to achieve the goalsof PAROS, the analysis of the prohibition of the use of force laid out in this report can aid in

shedding light on the interpretation of the prohibition of the use of force, and how it applies
to outer space and the activities carried out therein. Key takeaways from the report toenable States to effectively integrate the prohibition of the use of force into space security
debates are set out below.

States should promote compliance with the prohibition of the use offorce in the UN Charter and customary international law and consider
how to strengthen the prohibition of the use of force in its application toouter space.

The prohibition of the use of force is a key legal mandate that Statesshould take into account as they work to develop mechanisms andinstruments for PAROS.

States should consider the interpretation of a prohibited “use of force“ inits application to outer space security and take steps to build common
understandings by addressing the areas of uncertainty identified in thisreport.
States can increase clarity over the interpretation of a prohibited“use of force“ in its application to outer space through non-legallybinding measures and legally binding instruments, which can becomplementary, as well as through plurilateral or unilateral statements.

The prohibition of the use of force is a universally applicable rule that applies in outer space
and is a key element for the pursuit and achievement of space security.

In the context of multilateral debates, States have the opportunity to increase clarity on

the interpretation of Article 2(4) of the UN Charter in its application to outer space. As thedefinition of prohibited force can help to clarify regulation of space capabilities and space
behaviours and already applies to all States, the prohibition of the use of force can serve asa useful basis for advancing common understandings for Preventing an Arms Race in Outer
Space.

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