A. Context
1 This entry sets out the international law applicable to military uses of outer space. As well as being a physical domain, outer space is a legal domain in which international law applies (Space Law). Its precise physical and legal boundaries with airspace remain subject to debate and are internationally undefined (see Outer Space for an overview of the debate and attempts to delimit outer space). In November 2019, NATO declared outer space to be an operational military domain. Several States have recently established a space force military branch, including the United States (‘US’), Russia, the People’s Republic of China, Iran, France, and Spain. The United Kingdom (‘UK’) established its Joint Space Command in 2021. An increasing number of States are developing counterspace capabilities including anti-satellite (‘ASAT’) weapons. At the 2021 Brussels Summit, NATO leaders recognized that ‘attacks to, from, or within space present a clear challenge to the security of the Alliance, the impact of which could threaten national and Euro-Atlantic prosperity, security, and stability, and could be as harmful to modern societies as a conventional attack’ (NATO [2021] para. 33).
2 Military uses of outer space at present principally concern satellites (Spacecraft, Satellites, and Space Objects) and the testing and use of ASAT and other space weapons and their stationing in outer space or on celestial bodies. Satellites have important military functions and are also of increasingly critical importance to civilians.
1. Military Functions of Satellites
3 Military functions of satellites include intelligence, surveillance, and reconnaissance (‘ISR’) and positioning, navigation, and timing (‘PNT’). Satellite ISR provides States with situational awareness prior to or during an armed conflict, including troop movement, deployment of heavy weaponry, and intercept communications, and may provide early warning of an impending attack. Satellite PNT allows for coordination of military activities including navigation and weapons guidance. Satellites are also central to the verification of adherence to arms control treaties and are referred to in some treaties as national technical means of verification.
2. Civilian Dependence on Satellites
4 Civilians worldwide are increasingly reliant on space systems, including for food production, health care, search and rescue, communications, environmental science, and the global navigation satellite systems such as GPS, Beidou, Galileo, and GLONASS, which themselves underpin banking, financial markets, and energy grids. Satellites also play an essential humanitarian role including in disaster emergency response and humanitarian relief, due to their function in providing information about weather during and after a disaster, communication and navigation satellites to support logistical operations, and Earth observation satellites to allow the planning and coordination of emergency and humanitarian relief (ICRC [2021] 2). The International Committee of the Red Cross (ICRC) has noted ‘[t]he human cost of using weapons in outer space that could disrupt, damage, destroy or disable civilian or dual-use space objects is likely to be significant’ (ibid 1).
3. Vulnerability of Satellites to Attack
5 Due to their visibility, predictable paths, limited manoeuvrability, fragility. and low defensibility, satellites are highly vulnerable to attack and other forms of interference (Wright Grego and Gronlund [2005] 109). The high speed of satellites (about 17,500 km/hr in low Earth orbit) renders them vulnerable to destruction by collision with small objects on different orbits. Satellite systems have several components that are each vulnerable to interference and attack, namely the ground station on Earth which operates and controls the satellite, the satellite in orbit and the links between the two (ibid). These interventions can be launched from Earth (ground) or space, and may consist of ground-to-ground (Earth-based attacks on satellite ground stations), ground-to-space (attacks launched from Earth to space), or space-to-space (on-orbit ASAT attacks, eg a satellite releasing an object which will collide with another satellite). ASAT interference or attacks may be kinetic (eg direct ascent ASAT [‘DA-ASAT’]) or non-kinetic, such as the use of lasers, electromagnetic interference (including orbital jamming, terrestrial jamming, hijacking, spoofing, or scanning), and cyber operations. Attacks and interference may render the satellite permanently impaired or destroyed (‘kill’ or destruction) or have temporary and reversible effects (‘soft kill’, or denial, disruption. or degradation).
4. ASAT Tests
6 China, the US, India, and Russia (and before that, the Soviet Union) have conducted destructive ASAT tests. China conducted such a test in 2007, destroying one of its own weather satellites and creating several thousand trackable pieces of space debris. The US conducted a DA-ASAT test in 2008, creating more than 400 pieces of trackable debris. India did so in 2019, creating a similar amount of trackable debris. In 2021, Russia launched an unannounced DA-ASAT missile test to destroy one of its own defunct satellites, forcing astronauts and cosmonauts aboard the International Space Station to seek shelter in their hardened Crew Dragon and Soyuz capsules from the thousands of trackable pieces of space debris that were created.
5. Space Debris
7 A major problem with kinetic ASAT weapons is the side effect of creating space debris. Even small pieces of space debris can destroy other space objects due to the often-high relative velocities of objects in orbit. Risks from space debris are increasing due to a rapidly changing orbital environment characterized by higher congestion including from abandoned rocket bodies and satellite mega-constellations. In the worst case, space debris can trigger the Kessler Syndrome, a collisional cascade that could make some orbits unsafe to access and use for decades (Kessler and Cour-Palais [1978]). The clear dangers of space debris have led both to calls for a treaty banning kinetic ASAT testing and to unilateral declarations by a growing number of States including the US, Canada, Germany, New Zealand, the UK, Japan, and Australia committing to refrain from such testing. In December 2022 the UN General Assembly adopted a resolution calling on States to commit not to conduct destructive DA-ASAT missile tests (UNGA Res 77/41 [2022] para. 1; United Nations, General Assembly).
B. International Law Applicable to Military Uses of Outer Space
8 International law, including the 1945 United Nations Charter (‘UN Charter’), is applicable to outer space and celestial bodies. This has been affirmed in numerous treaties and UN General Assembly resolutions, beginning with Resolution 1721A (XVI) of 20 December 1961 and recently in Resolution 75/36 of December 2020 on reducing space threats through norms, rules, and principles of responsible behaviours. In Resolution 75/36, the General Assembly affirmed that ‘all States must conduct their activities in the exploration and use of outer space, including the Moon and other celestial bodies, in conformity with international law, including the Charter of the United Nations’ (para. 1). Article III Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and other Celestial Bodies (‘Outer Space Treaty’; ‘OST’) similarly provides that
Article 103 UN Charter provides that ‘[i]n the event of a conflict between the obligations of the Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail’.
9 Of the five main outer space treaties, the 1967 OST and the 1979 Agreement Governing the Activities of States on the Moon and Other Celestial Bodies (‘Moon Agreement’) contain explicit provisions on the military uses of outer space. The other outer space treaties are: the 1968 Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space (‘Rescue Agreement’); the 1972 Convention on the International Liability for Damage Caused by Space Objects (‘Liability Convention’); and the 1974 Convention on Registration of Objects Launched into Outer Space (‘Registration Convention’).
1. Outer Space Treaty
10 The key provisions of the Outer Space Treaty relating to military uses of outer space are Arts III (mentioned above), IV (1) and (2), and IX.
(a) Nuclear Weapons and Weapons of Mass Destruction
11 Under Art. IV (1)
The OST does not prohibit placement of weapons in space apart from nuclear weapons and weapons of mass destruction and does not prohibit the testing or use of conventional weapons in outer space.
(b) Peaceful Purposes
12 Article IV (2) states that the Moon and other celestial bodies shall be used ‘exclusively for peaceful purposes’. Expressly forbidden under this article are ‘[t]he establishment of military bases, installations and fortifications, the testing of any type of weapons and the conduct of military manoeuvres on celestial bodies’, whereas ‘[t]he use of military personnel for scientific research or for any other peaceful purposes’ is not prohibited.
13 There is a clear distinction between Art. IV (1) and (2) OST in terms of zone of applicability and prohibited activities in that zone. Whereas Art. IV (1) refers to the entirety of outer space, Art. IV (2) only applies to ‘the Moon and other celestial bodies’ and not to ‘outer void space’ (the expanses between celestial bodies) (Cheng [1997] 518). The travaux préparatoires indicate that this omission was intentional (Christol [1982] 24). The effect of this difference is that ‘under both general international law and Article IV(1) of the 1967 Treaty, States are perfectly entitled to use the whole of outer space for military purposes, bar the stationing of nuclear weapons and weapons of mass destruction’ (Cheng [1997] 518) but the Moon and other celestial bodies are reserved ‘exclusively for peaceful purposes’.
14 There is an ongoing controversy over whether peaceful purposes in the OST means non-military or non-aggressive (Cheng [1997] 528–32; Tronchetti [2015] 339; Wolfrum [1984] 787–88); in other words, whether Art. IV (2) completely demilitarizes (Demilitarization) the Moon and other celestial bodies, or if it merely confirms the applicability of the ius ad bellum in outer space. Those taking the former position point to the meaning of this term in Art. 1 Antarctic Treaty (1959) as ‘non-military’ and conclude that the meaning appears to be the same (Cheng [1997] 519; Antarctica). The Statute of the International Atomic Energy Agency (1956) also supports this interpretation due to distinguishing between peaceful and military use of atomic energy (Wolfrum [1984] 788; International Atomic Energy Agency [IAEA]). Those taking the position that ‘peaceful purposes’ means ‘non-aggressive’ argue that the ‘non-military’ interpretation is not supported by the subsequent practice of States parties or the travaux préparatoires (Tronchetti [2015] 339). Michael Schmitt states that ‘[m]ost space-faring nations take the position that “peaceful” means “non-aggressive or non-hostile”’ (Schmitt [2006] 101).
(c) Due Regard and Harmful Interference
15 Article IX sets out the obligation of States parties to conduct their activities in outer space with due regard for the interests of other States parties. However, the interpretation of ‘due regard’ in this context is legally uncertain. States must undertake prior consultations before proceeding with an activity which may cause ‘potentially harmful interference with activities of other States Parties in the peaceful exploration and use of outer space’. The rules of the International Telecommunication Union (ITU) set out in its 1992 Constitution and Radio Regulations (Art. 1.169) prohibit harmful radio interference, ‘which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with Radio Regulations’ (emphasis in original). However, this does not apply to military radio installations due to exceptions in the Constitution of the ITU (Arts 6 (1) and 48 (1)).
(d) Attribution
16 Article VI OST sets out a special regime of attribution in outer space. It holds that
It is legally uncertain whether this rule displaces the customary international law rules of State responsibility regarding attribution of conduct to a State (ILC Articles on Responsibility of States for Internationally Wrongful Acts [2001] Chapter II; ‘ARSIWA’), including with regard to uses of force and armed attacks in outer space. If it does, this could result in the acts of non-State actors (including commercial satellite operators) giving rise to violations by their State of registration of ius ad bellum or the law of neutrality or amount to hostilities under international humanitarian law (Neutrality, Concept and General Rules). Alternatively, Art. VI can be interpreted to establish a separate regime of State responsibility under the OST solely for the purposes of liability in the sense of compensation for damage. Under this regime, a State will be liable for damage to another State’s space objects resulting from the former’s military activities in outer space (Outer Space, Liability for Damage). This interpretation is consistent with the adoption of the Liability Convention, which elaborates on the OST, just five years later (on the overlap between State responsibility and liability in outer space, see von der Dunk [1992]).
2. Moon Agreement
17 The Moon Agreement applies to the Moon and other celestial bodies in the solar system other than Earth (Art. 1 (1)). Article 3 (1) states that ‘[t]he moon shall be used by all States Parties exclusively for peaceful purposes’. Article 3 (2) expressly applies the prohibition of the threat or use of force on the moon and prohibits using the moon itself to commit a threat or use of force or to threaten a hostile act ‘in relation to the earth, the moon, spacecraft, the personnel of spacecraft or man-made space objects’. Article 3 (3) prohibits placement of nuclear weapons and other weapons of mass destruction on or in orbit around the moon. Article 3 (4) forbids ‘[t]he establishment of military bases, installations and fortifications, the testing of any type of weapons and the conduct of military manoeuvres on the moon’. It is important to note that the Moon Agreement has a very low number of parties: 18 States have ratified the treaty as of November 2023 (although one of them, Saudi Arabia, has given notice of withdrawal [Treaties, Withdrawal]), and major spacefaring nations such as US, China, and Russia are not signatories.
C. Ius ad bellum
20 The prohibition of the threat or use of force between States is set out in Art. 2 (4) UN Charter and is also binding on States under customary international law. The recognized exceptions to the prohibition are a use of force in self-defence when an armed attack occurs (under Art. 51 UN Charter and customary international law), and a use of force authorized by the UN Security Council acting under Chapter VII of the Charter (United Nations, Security Council). Customary international law rules and the UN Charter apply in outer space, including the Moon and other celestial bodies, as recognized in Art. III OST. Article 3 (2) Moon Agreement expressly applies the prohibition of the threat or use of force on the moon and prohibits using the moon itself to commit a threat or use of force or to threaten a hostile act ‘in relation to the earth, the moon, spacecraft, the personnel of spacecraft or man-made space objects’. The issue is therefore not the applicability, but the application of ius ad bellum rules in outer space.
1. Threat of Force
21 The possession by a State of space weapons is not unlawful. In the Legality of the Threat or Use of Nuclear Weapons (Advisory Opinion) (1996) (‘Nuclear Weapons Advisory Opinion’; see Nuclear Weapons Advisory Opinions) the International Court of Justice (ICJ) held that mere possession of weapons is not necessarily an unlawful threat to use force (Use of Force, Prohibition of Threat), but depends on
22 Regarding placement or stationing of weapons in outer space, under Art. IV (1) OST, it is prohibited ‘to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner’. The OST does not prohibit the placement of other kinds of weapons in space and does not prohibit the testing or use of conventional weapons in outer space.
23 Regarding explicit threats to use force, the ICJ has held that ‘if the use of force itself in a given case is illegal—for whatever reason—the threat to use such force will likewise be illegal’ (Nuclear Weapons Advisory Opinion para. 47). The definition of a prohibited ‘use of force’ in outer space is therefore relevant to both the prohibition of the threat and the use of force.
2. Use of Force
24 There is no agreed definition of a prohibited use of force in outer space. Issues include determining whether the use of force is against a State, if temporary and reversible harm such as jamming or dazzling of satellites suffices, and difficulties of attribution and ascertaining intent.
25 To fall within the scope of the prohibition of the use of force, the use of force must be in ‘international relations’ and ‘against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations’ (Art. 2 (4) UN Charter). When the object or target of a forcible act is a space object, it is unclear if the nexus to another State requires legal ownership by the State or if registration to a launching State (required by the Registration Convention Art. 2) would suffice. The concept of the injured State under the international law of State responsibility should be applied to determine whether the required nexus is met (Nasu [2022] 164).
26 The travaux préparatoires of the UN Charter affirm that a prohibited ‘use of force’ in Art. 2 (4) refers to armed force and not to economic or other forms of coercion (Economic Coercion). The prohibition of the use of force applies ‘to any use of force, regardless of the weapons employed’ (Nuclear Weapons Advisory Opinion para. 39). What counts is the physical effects of the application of force. This has implications for the creation of space debris by a State (for example, through testing or use of a kinetic ASAT weapon): in certain circumstances, the intentional creation of space debris which has the effect of damaging or destroying another State’s satellite could constitute a ‘use of force’.
27 It is legally uncertain whether temporary and reversible effects would suffice to fall within the definition of a prohibited ‘use of force’ (for a discussion, see Pobjie [2024] 135–37). This is especially relevant to military uses of outer space, because many counter-space capabilities rely on such effects, such as electromagnetic attacks to impair function without destruction, including jamming satellite signals, or dazzling a satellite with a laser to temporarily ‘blind’ it. These acts may not permanently damage or destroy the satellite but may nevertheless cause substantial harm due to military and civilian dependence on some space systems. Harm may extend beyond the denial or destruction of a satellite to include the effects on civilians, for example, interfering with food production or disaster relief. It is not clear in how far such secondary effects are relevant to a determination of whether any gravity threshold for a prohibited use of force is met. In this regard, developing State practice with respect to cyber operations may prove useful by analogy.
28 There appears to be consensus among States that deliberately causing harm is an element of a ‘use of force’ in outer space. Attempts to define this term for the purposes of outer space—including the 2008 draft treaty sponsored by Russia and China on the Prevention of the Placement of Weapons in Outer Space and of the Threat or Use of Force against Outer Space Objects (‘PPWT’); the 2014 Draft PPWT; and the 2019 Report of the UN Group of Governmental Experts on Further Practical Measures for the Prevention of an Arms Race in Outer Space (UNGA [2019] para. 32)—all refer to hostile or intentional acts. It is likely that States will consider intent as an element of a ‘use of force’ in outer space, whereas accidental, mistaken, or recklessly caused damage will be perceived as irresponsible or unsafe behaviours. The difficulty is that a deliberate or hostile intent is difficult to discern in outer space. In outer space, certain acts may give rise to a threat perception but the affected State may not be able to verify or exclude a hostile intent, for example, with respect to rendezvous and proximity operations.
3. Self-Defence
29 States have a right to exercise self-defence under Art. 51 UN Charter and customary international law in response to an armed attack. This right extends to armed attacks that occur in outer space. The 2008 draft PPWT (Art. V) and 2014 draft PPWT (Art. IV) sponsored by Russia and China (referred to in para. 28 above) recognize the right to self-defence under Art. 51 UN Charter, although only the 2014 draft PPWT expressly refers to both individual and collective self-defence. NATO has stated that ‘attacks to, from, or within space’ could lead to the invocation of Art. 5 North Atlantic Treaty (also called the Washington Treaty), which provides for the exercise of individual or collective self-defence by NATO Member States (NATO [2021] para. 33). However, some authors have argued that the ‘peaceful purposes’ requirement in Art. IV (2) OST (see paras 12–14 above) should be interpreted to prohibit all forms of military force in outer space, including self-defence (eg Markoff [1976]).
30 States may exercise their right of self-defence by using force in outer space subject to the customary international law requirements of necessity and proportionality, even if the armed attack took place on Earth, or vice versa. The use of force in self-defence does not need to take the same form as the armed attack it is defending against. Any measures in self-defence must be reported by the victim State to the UN Security Council under Art. 51 UN Charter.
31 Issues that arise in relation to the right to self-defence in outer space include attribution and proportionality. As discussed above, it is legally uncertain whether Art. VI OST displaces the customary international rule of attribution under the international law of State responsibility, especially with regard to uses of force. The exercise by a State of its right to self-defence must respect the customary international law requirements of necessity and proportionality. Proportionality is particularly relevant with respect to the creation of space debris: carrying out a kinetic ASAT attack in self-defence may be unlawful because the widespread creation of destructive space debris would likely violate the principle of proportionality.
D. International Humanitarian Law
1. Applicability
32 International humanitarian law (‘IHL’) (Humanitarian Law, International) applies in situations of armed conflict. As Art. III OST makes clear, States must conduct their activities in outer space in accordance with international law. The applicability of IHL to hostilities in outer space is affirmed by common Art. 1 1949 Geneva Conventions, in which ‘[t]he High Contracting Parties undertake to respect and to ensure respect for the present Convention in all circumstances’. The ICJ has stated that IHL applies ‘to all forms of warfare and to all kinds of weapons, those of the past, those of the present and those of the future’ (Nuclear Weapons Advisory Opinion para. 86). Common Art. 2 1949 Geneva Conventions and Art. 1 (3) Additional Protocol I (‘AP I’) provide that those treaties apply ‘to all cases of declared war or any other armed conflict which may arise between two or more of the High Contracting Parties’. In addition to specific treaties regulating the conduct of hostilities, ‘near universal’ agreement exists that customary IHL principles apply in outer space (Schmitt [2006] 116).
33 For IHL to apply to particular hostilities in outer space in the absence of a preexisting armed conflict, certain thresholds must be met. For international armed conflicts (‘IACs’), hostilities must take place between the armed forces of two or more States (Armed Conflict, International). For non-international armed conflicts (‘NIACs’), the intensity threshold for an armed conflict must be reached (Armed Conflict, Non-International). It is not clear whether non-destructive attacks against space objects (for example, targeting military satellites using non-kinetic means that have temporary and reversible effects) would amount to ‘hostilities’ for IACs or reach the required threshold of intensity for NIACs.
2. ‘Attack’
34 A similar question relates to when the threshold of an ‘attack’ is reached under IHL. This is relevant because many rules in IHL apply to ‘attacks’, such as the rules of distinction, proportionality, and precautions (Precautions in Attack). ‘Attacks’ are defined as ‘acts of violence against the adversary, whether in offence or in defence’ (Art. 49 (1) AP I). It is unclear under which circumstances non-kinetic operations against space objects would be considered an ‘attack’ under IHL. The ICRC’s position is that these IHL rules apply ‘not only to kinetic operations against space objects, but also to non-kinetic operations that would disable space objects without necessarily damaging them physically’ (ICRC [2021] 3). Bill Boothby argues that ‘jamming operations that do not cause injury or physical damage are unlikely to be classified as an attack’ ([2017] 210, footnote omitted). This legal issue is also highly debated with respect to analogous cyber operations (Cyber Warfare).
3. Means and Methods of Warfare
(a) Space Weapons
35 While the OST prohibits placement of nuclear weapons in outer space, it does not prohibit the placement or use of conventional weapons. It is debated what constitutes a space ‘weapon’ due to the unique characteristics of outer space discussed in Section A above (see Boothby [2017]). Attempts to define a space ‘weapon’ include the 2008 draft PPWT (Art. I (c)) and the 2014 draft PPWT (Art. 1 (b)). The definition of space ‘weapon’ is relevant to several IHL customary and treaty rules relating to weapons, for example, the prohibition to use weapons of a nature to cause superfluous injury or unnecessary suffering (ICRC Customary Study [2005] Rule 70; Art. 35 (2) AP I) or weapons that are by nature indiscriminate (ICRC Customary Study [2005] Rule 71; Art. 51 (4) AP I) and the obligation to review new weapons to ensure compliance with IHL (Art. 36 AP I).
(b) Principle of Distinction
36 Under the principle of distinction, ‘[t]he parties to the conflict must at all times distinguish between civilian objects and military objectives. Attacks may only be directed against military objectives. Attacks must not be directed against civilian objects’ (ICRC Customary Study [2005] Rule 7; Arts 48 and 52 (2) AP I; Military Objectives; Civilian Objects). This is a ‘cardinal principl[e]’ of IHL and an ‘intransgressible principl[e] of international customary law’ (Nuclear Weapons Advisory Opinion paras 78–79). A challenge in the space environment is the dual use nature of many space objects, such as hosted military payloads on civil or commercial satellites, or the reliance of civil and commercial infrastructure on military satellites (eg GPS and equivalents). In each case, the question is whether the object by its ‘nature, location, purpose or use make[s] an effective contribution to military action’ and if its ‘partial or total destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage’ (ICRC Customary Study [2005] Rule 8; Art. 52 (2) AP I). If so then it is a military objective; if not then the object is a civilian object (ICRC Customary Study [2005] Rule 9; Art. 52 (1) AP I).
37 In the space environment there is a further problem of identifying the nature, purpose, or use of an object. Registering a military satellite as civilian and then using it to facilitate an attack to kill or injure (and in some versions, capture) an adversary will violate the prohibition of perfidy (ICRC Customary Study [2005] Rule 36; Art. 37 (1) AP I). In case of doubt, the presumption shall be in favour of it being a civilian object (Art. 52 (3) AP I). In situations where only part of the object meets the definition of a military objective, there is controversy over whether the entire object qualifies as a military objective and may therefore be targeted (see International Law Association Study Group on the Conduct of Hostilities in the 21st Century [2017] 333 ff; International Law Association [ILA]). This also relates to the assessment of proportionality and collateral damage (see ICRC International Expert Meeting Report: The Principle of Proportionality [2018] 38–40).
(c) Astronauts
38 Also related to the principle of distinction is the status of military astronauts and civilians working in military space commands or for commercial space entities providing services to the military. The status of military astronauts under IHL is relevant to the rules applicable to their treatment during an international armed conflict, ie whether they may be captured and detained, or if they must be rescued and returned. There is tension between the rules of IHL defining combatants and the status of astronauts under space law as envoys of humankind in the OST and Rescue Agreement (see Steer and Stephens [2021]).
(d) Indiscriminate Attacks
39 The prohibition of indiscriminate attacks under IHL (ICRC Customary Study [2005] Rule 11; Art. 51 (4) AP I) greatly restricts the lawfulness of kinetic ASAT attacks due to the creation of destructive space debris. This is because the effects of such an attack cannot be limited as required by IHL ‘and consequently … are of a nature to strike military objectives and civilians or civilian objects without distinction’ (ICRC Customary Study [2005] Rule 12; see also Art. 51 (4) AP I). Attacks ‘which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated’ violate the principle of proportionality (ICRC Customary Study [2005] Rule 14) and are considered indiscriminate (ibid Rule 12; Art. 51 (5) (b) AP I).
(e) Precautions
40 The parties to an armed conflict are required to ‘take all feasible precautions in the choice of means and methods of warfare with a view to avoiding, and in any event to minimising, incidental loss of civilian life, injury to civilians and damage to civilian objects’ (ICRC Customary Study [2005] Rule 17; Art. 57 (2) (a) (ii) AP I) and ‘to protect the civilian population and civilian objects under their control against the effects of attacks’ (ibid Rule 22; Art. 58 (c) AP I). Parties to an armed conflict must therefore avoid a kinetic ASAT attack when an alternative, less harmful means of attack is possible, such as non-kinetic ASAT.
(f) Environmental Protections under International Humanitarian Law
41 Kinetic ASAT attacks may also be restricted by specific protections of and obligations in relation to the environment under IHL, including under Arts 35 (3) and 55 AP I and customary international law (Environment, Protection in Armed Conflict). Kinetic ASAT attacks that create destructive space debris could violate the prohibition ‘to employ methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment’ under Art. 35 (3) AP I. States parties to the ENMOD Convention are prohibited from engaging ‘in military or any other hostile use of environmental modification techniques having widespread, long-lasting or severe effects as the means of destruction, damage or injury to any other State Party’ (Art. I (1)). ‘[E]nvironmental modification techniques’ include ‘any technique for changing—through the deliberate manipulation of natural processes—the dynamics, composition or structure’ of outer space (Art. II). Tronchetti argues that the ENMOD Convention ‘may create restrictions on the use of’ ASAT weapons ([2015] 344–45).
E. Outlook and Assessment
42 Current developments in the use of outer space have significant implications for militaries, commercial actors, and civilians. Advances in military counterspace capabilities and the increasingly embedded dual military and civilian uses of space infrastructure are taking place in a rapidly changing orbital context due to the growth in number of commercial space actors and the placement in orbit of satellite mega-constellations. Increased congestion in low Earth orbit and intensifying competition between actors for access to and control of outer space greatly escalates the risks of and from space debris from accidental collisions and deliberate targeting of satellites. Scientific research on the Kessler Syndrome makes clear the danger of space debris rendering outer space inaccessible to humans for long periods. Due to heavy reliance of civilians on satellites, the ICRC has underscored the ‘potentially significant human cost for civilians on earth of the use of weapons in outer space’ (ICRC [2021] 4).
43 In light of the potentially catastrophic risks of military uses of outer space, international efforts to prevent an arms race in outer space (‘PAROS’) are underway at the United Nations. In 2022, pursuant to UN General Assembly Resolution 76/231, the Open-Ended Working Group (‘OEWG’) on reducing space threats through norms, rules and principles of responsible behaviours was convened:
The OEWG held four working sessions in 2022 and 2023 but ultimately failed to reach consensus on even a procedural report on the meetings and produced no formal report or set of recommendations. In 2023, the UN General Assembly requested the Secretary-General to establish a Group of Governmental Experts on Further Practical Measures for PAROS ‘to consider and make recommendations on substantial elements of an international legally binding instrument on the prevention of an arms race in outer space, including, inter alia, on the prevention of the placement of weapons in outer space’ (UNGA Res 77/250 [2023] para. 8).
44 So far UN negotiations on PAROS have not reached consensus on binding rules to govern military activities in outer space. This stalemate reflects deep divisions in the international community between the desirability and effectiveness of pursuing a hard-law approach (such as Russia and China’s draft PPWT), or a soft law approach focusing on enhancing ‘safety, security, and sustainability of all outer space activities’ and establishing transparency and confidence building measures (such as the EU Draft International Code of Conduct for Outer Space Activities [31 March 2014]). Key substantive issues at stake include effective verifiability, legal gaps in definitions and framing of rules which could be exploited by adversaries, and the desire for flexibility. In October 2023, the UN First Committee (Disarmament and International Security) voted to put forward two draft resolutions to the General Assembly that would create parallel Open-Ended Working Groups related to PAROS with different goals: a UK-sponsored draft resolution titled ‘Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours’, and another, backed by Russia, titled ‘Further Practical Measures for the Prevention of an Arms Race in Outer Space’, which would make recommendations on an international legally binding instrument. Delegates have warned that ‘parallel processes would lead to further polarization and fragmentation of efforts to preserve space security’ (UN Press Release [2023]).
45 Negotiations to develop new binding rules or soft law norms for outer space security must recognize that military uses of outer space ‘do not occur in a legal vacuum but are constrained by existing law, notably the Outer Space Treaty, the UN Charter and IHL’ (ICRC [2021] 4). Either approach risks undermining existing international law rules that apply to military uses of outer space. The international law rules on the prohibition of the use of force (ius ad bellum) and IHL already apply in outer space and can provide useful guidance and important limitations on military uses of this domain. Establishing clarity and consensus on how to apply ius ad bellum and IHL rules in the unique environment of outer space is a useful foundation for establishing agreement on norms, rules, and principles and eventually a binding treaty to prevent an arms race in outer space.
![Max Planck Encyclopedia of Public International Law [MPEPIL]](/view/covers/epil.jpg)
