1. Introduction
Space activities underpin all instruments of national power.Footnote 1 For militarily powerful states, space is an integrated part of their national security. In recent years, states like the United Kingdom and the United States have established dedicated space commands and articulated space related strategies that reflect the significance of space, as well as the dangers associated with space activities.Footnote 2 NATO recognizes space as an operational domain, alongside air, land, sea, and cyberspace.Footnote 3 Outside of NATO, the EU has likewise identified space as a strategic domain and set out a space strategy that includes protecting its space assets, maximizing the use of space for security and defence purposes, and deterring hostile activities in space.Footnote 4 China and Russia are both major spacefaring nations. China also designates space as a military domain,Footnote 5 and both states continue to develop their already significant space capabilities.Footnote 6 India is a rising space power and other states like Iran, North Korea, Australia, France, Japan, Israel, and South Korea are focusing on their own space programmes and developing space technologies.Footnote 7 Other states will undoubtedly follow and, meanwhile, they lease satellite services or are otherwise reliant on the services that satellites provide. Increasingly, therefore, space is hugely significant to all states.
Space is also an increasingly contested domain, with the potential to be a source and place of armed conflict. The prospect of war in space is of real concern and states assert their right to act to defend their interests in that domain.Footnote 8 Unease over the militarization, or ‘weaponization’ of spaceFootnote 9 is accordingly at the top of the international agenda, with bodies such as the UN General Assembly (UNGA) consistently emphasizing the need for international cooperation on the peaceful uses of outer space, as well as expressing serious concern about an arms race in that domain.Footnote 10 In particular, there is great unease over states developing counterspace weapons that might threaten access to, and freedom to operate in, space.Footnote 11 To date no state has used such a weapon against a satellite of another stateFootnote 12 but, given their importance to military operations, satellites might be considered very attractive targets for states in situations of conflict. This fact is evidenced by the testing and possible future use of offensive and defensive anti-satellite (ASAT) weapons capable of disrupting or destroying both civilian/commercial and military satellites.Footnote 13 ASAT weapons are a persistent feature of the discourse among states and international organizations relating to the threat environment in space.Footnote 14 Indeed, a recent EU statement to the UN Conference on Disarmament on preventing an arms race in space referenced Russia’s test of a kinetic direct ascent ASAT missile in 2021 as a ‘strong reminder’ of the need for development in the legal governance of space activities.Footnote 15
The call for legal regulation of ASAT weapons is particularly urgent given the physics of space and the potential enduring effects of space debrisFootnote 16 that might result from ASAT weapon use.Footnote 17 Space debris imperils other satellites in orbit, many of which are fundamental to the operation of human societies and to the functioning of global economies. The modern world relies on the essential services that these space-based assets provide.Footnote 18 States view space debris as a significant threat to this space environment, with the intentional destruction of satellites exacerbating the threat.Footnote 19 Yet, despite the importance of satellites, international law pertaining to state activities that might threaten them is scarce.Footnote 20 Multilateral attempts to restrain the escalating weaponization of space have failed.Footnote 21 Legally regulating the targeting of satellites using ASAT weapons relies, therefore, on bodies of international law that were not originally designed for space but naturally pertain to the use of ASAT weapons.
To date, aside from failed attempts at weapons control, much of the deliberation surrounding military activities in space has centred on international humanitarian law (IHL).Footnote 22 This focus is entirely understandable given that IHL governs how states conduct hostilities, including laying down targeting rules that apply to ASAT weapon use during armed conflict. It is axiomatic that these IHL precepts apply equally to all belligerents and regardless of a state’s reason for entering into hostilities.Footnote 23 Yet, the separate question of legality under the jus ad bellum, which governs when states may lawfully use force in their international relations, is equally important. The legal justification for states using force against satellites and what limitations apply as a result of that justification must also be appraised to establish the legality of targeting satellites. Although states assert their right of self-defence in space, however, this essential jus ad bellum issue has received little attention.Footnote 24 This is an opportunity lost for the examination of the legality of ASAT weapon deployment. This article addresses this gap by investigating how the jus ad bellum applies to ASAT weapons and restricts their use, including in self-defence.
This article begins in Section 2 by considering the types of ASAT weapons that states have developed to date and might use in self-defence, as well as providing an overview of contemporary international space law that relates to the development and use of such weapons. Section 3 proceeds to examine the application of the jus ad bellum to space and, more specifically, how the rules of the jus ad bellum regulate the targeting of satellites using ASAT weapons in self-defence. This author argues that when a state is exercising its right of self-defence in space, civilian and military planners are restricted by the jus ad bellum in their choice of targets in that domain. Sections 4 and 5 explain this contention through the exploration of the jus ad bellum requirements of necessity and proportionality that condition the exercise of a state’s right of self-defence. Section 4 emphasizes that, despite the potential military advantage that might be gained by targeting satellites, jus ad bellum necessity restricts the target options available to a defending stateFootnote 25 to ensure that its military response is confined to the defensive and is directed solely at an aggressor. Even if IHL and jus ad bellum necessity do not prevent a state from damaging or destroying a satellite, Section 5 illustrates how the operation of jus ad bellum proportionality might nevertheless preclude ASAT weapon use because of the ensuing effects of targeting a satellite on civilians and/or on the interests of other states and the international community more broadly.
Applying the jus ad bellum rules analyzed in this article has implications at both the strategic level (regarding how states develop space related policies and ASAT technologies) as well as the operation and tactical levels (in terms of how military planners execute military operations in space). A clearer understanding of the jus ad bellum rules as they apply in space helps decision makers avoid putative acts of self-defence being characterized as unlawful uses of force. Ultimately, the jus ad bellum regulation of ASAT technologies addresses state concerns regarding protecting their space assets, while also helping to avoid conflict and the escalation of conflict in space. Adherence to the jus ad bellum rules promotes and helps to secure international peace and security on Earth and beyond the Earth’s atmosphere.
2. ASAT weapon types and applicable international law
To examine how the jus ad bellum applies to the use of ASAT weapons, we must first consider the types of such weapons. The development of counterspace weapons that can disrupt, degrade, or destroy satellites and related infrastructure has a long history, going back to the dawn of the space age itself.Footnote 26 The USA, China, and Russia have developed the most advanced ASAT technologies, although other states possess counterspace capabilities.Footnote 27 ASAT weapons can be placed into four broad categories.Footnote 28 The first category is kinetic physical ASAT weapons, comprising anti-satellite missiles or other methods of physical kinetic attacks directed against satellites. Such attacks may be launched from the ground (direct ascent ASAT weapons) or from space (co-orbital ASAT weapons). The latter type of weapons are sent into orbit and, by way of rendezvous and proximity operations, placed near their intended target. Examples include satellite-launched projectiles,Footnote 29 kinetic kill vehicles, chemical sprayers, and the use of space-based robotic arms to grab target satellites.Footnote 30 Suspicious use of inspection and repair satellites has also been identified as a potential threat to space-based assets.Footnote 31 To date, the USA, China, Russia, and India have all demonstrated kinetic ASAT capabilities.Footnote 32 Most recently, in November 2021, Russia tested a direct-ascent ASAT missile that destroyed one of its own satellites called Cosmos 1408 in orbit, creating a large and long-lived debris field.Footnote 33
The second category comprises non-kinetic physical ASAT weapons, which have reversible or permanent physical effects on satellites or other space systems, but which make no physical contact with them. They include directed-energy weapons that can be used to damage, disrupt, or destroy space systems. For example, lasers can temporarily dazzle or permanently blind satellite sensors or cause components to overheat. High-powered microwaves can likewise disrupt or permanently damage electronics. Nuclear devices could also be detonated in space, creating a high-radiation environment and electromagnetic pulses that can harm satellites. Such non-kinetic physical counterspace weapons may be launched from other satellites, or from land, sea, or airborne weapons platforms on Earth. Electronic ASAT weapons, such as jamming devices that interfere with the transmission of signals to and from satellites and spoofing devices that can falsify such signals, are the third category of weapons. Finally, cyberattacks that target data (as opposed to communications signals) and the systems that use data are the fourth type. Such attacks may be used to monitor data, or to intercept, falsify, or corrupt it, in each case on a temporary or permanent basis.
Specific legal regulation of the use of these ASAT technologies is scarce. Contemporary international space law that governs military activities in space is centred on the Outer Space Treaty of 1967 (OST)Footnote 34 and, to a much lesser extent, the Moon Agreement of 1979 (Moon Agreement).Footnote 35 Article IV of the OST explicitly bans the placing of nuclear weapons and other weapons of mass destruction in orbit, on celestial bodies, and otherwise stationing such weapons in outer space. According to the rules of treaty interpretation,Footnote 36 the ordinary meaning of the term ‘weapons of mass destruction’ confines this prohibition to weapons aimed at causing widespread devastation and loss of life, being chemical, biological, or nuclear weapons.Footnote 37 The use or testing of nuclear weapons in space is also prohibited by the Partial Test Ban Treaty.Footnote 38 However, other types of weapons, including conventional ASAT technologies placed in orbit, are not prohibited by current international space law.Footnote 39 The caveat to this partial weapons ban is that the moon and other celestial bodies are fully demilitarized by Article IV and may only be used for peaceful purposes.Footnote 40 Regardless, and important for this article, ASAT weapon use in lawful self-defence is not affected by these OST and Moon Agreement provisions.Footnote 41
In the absence of specific regulation by current international law, the apprehension associated with states testing and potentially using ASAT weapons persists. Efforts by the UN to forestall the weaponization of space and to preserve it for peaceful purposes continue, but have not yet borne fruit.Footnote 42 The UNGA, in a widely supported resolution, has called upon all states to commit not to conduct destructive direct-ascent ASAT missile tests.Footnote 43 Several states, including the USA, Canada, France, Germany, Japan, New Zealand, the United Kingdom, and South Korea have unilaterally agreed not to conduct debris-generating direct-ascent ASAT weapons tests.Footnote 44 However, the prospect of securing multilateral support for such a comprehensive ban, including from Russia and China, is unlikely in the present geo-political climate.Footnote 45 It is notable that a draft treaty first proposed by China and Russia in 2002 and updated in 2008 and 2014Footnote 46 aimed at banning the placement and use of weapons in space has failed to be adopted by any state.Footnote 47 Instead, guiding principles governing space activities and other ‘soft law’ efforts have been pursued by international organizations such as the UNFootnote 48 and the EU.Footnote 49 Notably, under the EU’s Draft International Code of Conduct for Outer Space Activities, even if states lawfully exercise their right of self-defence, they undertake to do so in a manner which minimizes, to the greatest extent practicable, the creation of space debris.Footnote 50 Absent new international law banning or restricting ASAT weapon use, these endeavours and other efforts like the Woomera and MILAMOS manuals that seek to clarify rules pertaining to the use of ASAT weapons, are very welcome.Footnote 51 A clearer understanding of the jus ad bellum regulation of ASAT weapons is a significant part of the answer. As will be seen in the following sections, the raison d’être of the jus ad bellum is the maintenance of international peace and security and adherence to its requirements directly addresses the fears associated with ASAT weapon use.
3. Applying the jus ad bellum to the use of ASAT weapons in self-defence
General international law applies to human activities in space.Footnote 52 We see this principle reflected in Article III of the OST, which requires state parties to ‘carry on activities in the exploration and use of outer space, including the moon and other celestial bodies, in accordance with international law, including the Charter of the United Nations’.Footnote 53 Likewise, the UNGA continues to reaffirm the applicability of the UN Charter and general international law to activities in outer space, including in its resolutions specifically addressing the use of ASAT technologies.Footnote 54 State practice also reflects this position. For example, the USA, as the main power in space, together with all other NATO countries and the EU, have publicly expressed their commitment to responsible behaviour in space and to act in space in accordance with international law.Footnote 55 Even in the absence of a treaty banning ASAT weapon testing and use, therefore, all military activities in space, including the use of ASAT weapons, must comply with these generally applicable legal rules. The difficulty is determining how such rules developed for the terrestrial realm are necessarily interpreted to apply beyond Earth, particularly given the physics of space, as well as the nature of certain satellites and their importance for humanity. These factors are discussed in detail in the following sections.
Regarding ASAT weapon use specifically, the focus to date has naturally been on the detailed targeting rules of IHL and how they apply in space. Such rules are discussed further in Section 4. In addition, the rules of the jus ad bellum, grounded in the UN Charter and customary international law, apply to ASAT weapons. Indeed, the ICJ has confirmed that the UN Charter applies ‘to any use of force, regardless of the weapons employed’.Footnote 56 Under the jus ad bellum, states are generally prohibited from threatening or using force in their international relations. This prohibition is reflected in Article 2(4) of the UN Charter.Footnote 57 The two recognized exceptions to this cardinal rule are force authorized by the UN Security Council (UNSC) under Chapter VII of the UN Charter and force used in self-defence pursuant to Article 51 and customary international law.Footnote 58 State consent to a use of force against one of its own satellites would also preclude a potential breach of Article 2(4) as between the states concerned. Accordingly, absent such state consent or UNSC authorization, any actions against satellites that constitute a threat or use of force require justification as lawful acts of self-defence. Otherwise, these actions will contravene Article 2(4) and will be characterized as unlawful uses of force. The focus of this article is on ASAT weapon use in self-defence to respond to armed attacks that occur on Earth or in space.
We should note that states assert their right to act in self-defence in space.Footnote 59 While not universally accepted, the dominant view is that states may lawfully exercise that right in that domain and the jus ad bellum applies in space to condition the exercise of that right.Footnote 60 Although Article IV of the OST stipulates that the ‘moon and other celestial bodies shall be used by all States Parties to the Treaty exclusively for peaceful purposes’,Footnote 61 it is generally agreed that, in line with the UN Charter, peaceful purposes equates to ‘non-aggressive’ or ‘non-hostile’, rather than ‘non-military’ purposes.Footnote 62 This means that the OST and the Moon Agreement do not preclude the exercise of self-defence in space.Footnote 63 Regardless, the right of self-defence would take priority over any such treaty provisions that are incompatible with that right.Footnote 64 As such, states retain the right of self-defence where an armed attack occurs in or from, or is directed through or towards, space.Footnote 65
Several important issues are implicated by the starting premise that the jus ad bellum regulates ASAT weapon use in self-defence. Among them is the crucial question of what constitutes a use of force in space, as well as whether such force might amount to an ‘armed attack’, which triggers a state’s right of self-defence.Footnote 66 Establishing whether a state has in fact been the object of an armed attack based on the nature of the satellite (i.e., civilian or military, or both) and/or whether it is the state of registry of that satellite, is but one challenging issue.Footnote 67 Determining whether the so-called gravity threshold of violence is also surpassed to comprise an armed attack is likewise not straightforward. On this latter point, only ‘grave’ uses of force, based on their scale and effects, constitute armed attacks, thereby triggering a state’s right of self-defence.Footnote 68 As such, ASAT weapon use against a satellite falling below this notoriously indeterminate line of demarcation does not permit the target state to respond using force. Instead, state responses must be confined to the non-forceful, such as deploying countermeasures.Footnote 69 Therefore, although context dependent, non-kinetic ASAT weapon use against satellites that does not cause physical damage (for example, jamming or limited cyberattacks) is highly problematic for determining whether an armed attack has occurred and the right of self-defence is triggered.Footnote 70 If not armed attacks, using ASAT weapons might nonetheless amount to unlawful uses of force or constitute other internationally wrongful acts, such as unlawful intervention in the sovereign affairs of a state.Footnote 71
However, where the armed attack threshold is crossed either as a result of force used against a state in space or on Earth, targeting satellites in self-defence using ASAT weapons might be viewed as an attractive option. For this to be lawful, however, such targeting must comply with IHL and the requirements of jus ad bellum necessity and proportionality discussed in the following sections. Adherence to jus ad bellum necessity and proportionality ensures that targeting a satellite is capable of achieving a legitimate defensive purpose, being to halt, repel, or (if some form of anticipatory self-defence is acceptedFootnote 72 ) prevent an armed attack. For the purposes of the ensuing analysis, ‘targeting’ satellites in self-defence involves engagement or action to alter or neutralize the function it performs for the adversary.Footnote 73 This may mean physically damaging or destroying a satellite or otherwise permanently or temporarily disabling or neutralizing it.
4. Necessity in outer space
As noted in the previous section, a state’s right of self-defence arises when an armed attack occurs.Footnote 74 Jus ad bellum necessity and proportionality then apply to the entirety of a defensive military operation to condition the exercise of that right so that force is contained and confined purely to the defensive.Footnote 75 The ICJ has affirmed that jus ad bellum necessity and proportionality are requirements of customary international law that must be strictly adhered to in order for acts of self-defence to be considered lawful.Footnote 76 In terms of applying these requirements to ASAT weapons, the jus ad bellum requirement of necessity must be considered first. If necessity is established, whether or not defensive action is also proportionate may then be examined.
4.1. Defensive force as a measure of last resort
Necessity represents the fundamental idea that, given the general prohibition on states using force,Footnote 77 the resort to force even in self-defence is an exceptional and limited response to a genuine situation of emergency. To that end, necessity requires defensive force to be a measure of last resort, where alternatives to force are unavailable or unfeasible and/or, on their own, will be ineffective to halt, repel, or prevent an armed attack. Force used in self-defence must be the only reasonable choice of means available to the defending state in the circumstances.Footnote 78 Any acts of self-defence, whether they be in space or on Earth, must surpass this initial hurdle in order to be considered lawful under the jus ad bellum. Any use of force that is unnecessary will be unlawful. This jus ad bellum assessment is independent of lawfulness under IHL.
For defending states considering using ASAT weapons, therefore, the first question will always be whether options not involving military force are practical and will likely be effective to counter an armed attack, or have a reasonable chance of doing so.Footnote 79 If recourse to the UNSC results in adequate and effective measures that remove the threat to the defending state, exercising self-defence will not be necessary.Footnote 80 The same applies to non-forcible measures that resolve the situation, such as diplomacy, retorsion, or the application of countermeasures. Likewise, military action falling below the threshold of the use of forceFootnote 81 may stand as a reasonable alternative to force, whether directed at satellites or other assets belonging to the aggressor. Cyber operations that temporarily disable a satellite, but do not cause it to collide with other space objects or otherwise create debris, are a potential example. It is only where such non-forceful alternatives, on their own, are insufficient to respond to an armed attack that there is a necessity of using force in self-defence. Unless and until this requirement is satisfied, ASAT weapons may not be used.
4.2. Targeting satellites: Lawful objects of self-defence?
Targeting particular satellites is ostensibly an attractive option for defending states. Satellites are an integral part of enabling military operations on Earth. They are hardwired into the infrastructure of modern warfare, providing precise navigation, furnishing real-time targeting and weather data, allowing instantaneous global communications, warning of possible missile threats, collecting intelligence, and carrying out surveillance and reconnaissance.Footnote 82 ‘Operation Iraqi Freedom’ in 2003 is often cited as an exemplar of the pivotal role that satellites play to contemporary combat operations,Footnote 83 and this role is equally evident today in Russia’s ongoing invasion of Ukraine.Footnote 84 As discussed in Section 2, satellites might also be the source of an armed attack in space. As such, the defensive advantage of neutralizing a satellite that supports an adversary’s aggressive behaviour seems readily apparent. Yet, even if the prima facie necessity is established in the particular circumstances for a state to resort to force in self-defence in some form (because reasonable alternatives to force are unavailable or will be ineffective to respond to an armed attack), it does not follow that satellites, or any other targets, automatically become fair game from the perspective of the jus ad bellum. Jus ad bellum necessity goes further than the general premise set out in Section 4.1 by requiring that states confine any necessary defensive response to targets that serve a defensive purpose.Footnote 85 For policy makers and military planners who are considering whether they may lawfully target satellites in self-defence, therefore, jus ad bellum necessity constitutes a further hurdle to overcome, in addition to complying with IHL targeting rules that also govern whether or not satellites are targetable.Footnote 86
Under IHL, satellites may only be targeted in an armed conflict if they constitute ‘military objectives’. ‘Civilian objects’ may not be directly targeted.Footnote 87 These core tenets of IHL constitute customary international law that binds all states in both international and non-international armed conflicts.Footnote 88 Attacks against satellites must be cancelled or suspended under these IHL rules if it becomes apparent that the satellite is not a military objective or if destroying the satellite would violate IHL proportionality limitations.Footnote 89 As a consequence, IHL naturally has primacy for states considering using ASAT weapons in a defensive operation and will generally be the ‘first point of call’ in forming and carrying out targeting decisions against satellites.Footnote 90 In addition, jus ad bellum necessity also restricts these targeting decisions to ensure they remain defensive and do not take state acts into the realm of unlawful uses of force. This understanding of jus ad bellum necessity accords with the ICJ’s approach in Oil Platforms, as well as academic commentary and state practice.Footnote 91 For decision makers, this means that jus ad bellum necessity imposes an additional and separate targeting requirement that operates cumulatively and in parallel to IHL. This additional requirement is that defensive force should in principle be directed against the source of the armed attack(s) being halted, repelled, or preventedFootnote 92 and be limited to military targets connected with that armed attack.Footnote 93 Simply put, targeting something that has no military function cannot be necessary to achieve a lawful defensive purpose.Footnote 94 Furthermore, without a nexus or a ‘functional link’Footnote 95 between attack and defence, targeting a satellite risks being deemed punitive and unlawful, rather than being defensive and lawful.
The operation of this jus ad bellum targeting requirement as it applies to satellites as potential objects of self-defence raises some very important difficulties for states operating in the space domain. Yet, it also acts as an important guard rail to ward against conflict in space, as well as avoiding escalation of conflict on Earth into that domain. That jus ad bellum necessity requires acts of self-defence to be connected to an armed attack means that it is only necessary to target in self-defence military assets that belong to the authors of that attack. While perhaps facile to state, the challenges associated with legally attributingFootnote 96 armed attacks to their author (a requirement for any act of self-defence) are potentially exacerbated in the space domain. Attribution of an armed attack is particularly difficult when the attack itself is by way of ASAT weapons directed against a putative defending state’s own satellites operating in Earth’s orbit. The difficulty is being able to positively identify the aggressor where the object of the armed attack is in space and, by operation of necessity, to limit the defensive response accordingly. An incorrect assessment of this ‘who did it’ question might mean that an intended act of lawful self-defence is instead characterized as an unlawful armed attack because it is directed at the wrong object. As well as breaching Article 2(4) of the UN Charter, if rising to the level of an armed attack, misdirected action risks a military response by the target state in self-defence.
Tracing the use of ASAT weapons to their author depends heavily on the technology employed. Direct ascent ASAT missile attacks against satellites are more easily attributed than other counterspace weapons because launches of ASAT missiles from Earth are detectable and their effects can create detectable orbital debris.Footnote 97 Beyond that method of attack, identifying with any precision aggressive acts in space is generally challenging. Other technologies, such as non-kinetic directed energy ASATs weapons (notably lasers and high-powered microwaves), electronic ASAT weapons, and cyber operations, are often much less visible and are more difficult to attribute.Footnote 98 Likewise, co-orbital ASAT weapons of all types are harder to identify and track in orbit.Footnote 99 Behaviour by satellites may also look unusual or threatening to other satellites but, without further information, it is difficult to clarify what precisely is occurring so far from Earth.Footnote 100 All of these technical problems pose practical difficulties for state compliance with jus ad bellum necessity in terms of ensuring the object of self-defence is correctly identified as belonging to the aggressor. It should also be noted that not all threats to satellites come from states. Although military activities in outer space are likely to remain largely limited to states, at least in the near future,Footnote 101 threats like signal jamming and cyberattacks might also be carried out by non-state actors, including terrorist organizations.Footnote 102 This possibility further complicates the threat assessment and related response.
On a good faith assessment, if there is no reasonable and objective basis for concluding that a particular state is the author of an armed attack, jus ad bellum necessity precludes the targeting of that state’s satellites in response, as well as any other asset belonging to that state. Given the potential disastrous consequences of targeting satellites (see Section 5), states are advised to be extremely careful in attributing aggressive conduct in space to other states in order to allow for a defensive response in the space domain. Moreover, public justifications of defensive action against satellites, including reports of self-defence to the UNSC,Footnote 103 must be accompanied by sufficient information so that third party reviewers can judge whether the legal test of attribution is met and, therefore, whether acts of self-defence are necessary in that they are appropriately targeted. In cases not involving direct ascent ASAT missile attacks, this is likely to be a high evidential burden to meet.
In addition to the difficulty of attributing conduct in space, a further limitation of jus ad bellum necessity regarding whether satellites are lawful targets of self-defence relates to their nature and composition. It might be possible to establish that a satellite registered to an aggressor state is wholly owned or used by it and clearly serves a military purpose, such as surveillance and reconnaissance or providing targeting information for military operations. In cases where a satellite can be positively characterized in this way, and a factual connection with an armed attack being defended against is established, the jus ad bellum necessity requirement described in this section imposes no obvious restrictions on targeting it.Footnote 104 Yet, verifying whether a satellite is in fact military in nature is generally very difficult. It is nearly impossible to get a first-hand look at satellites in space, so a military planner must gather information on potential targets from a variety of intelligence sources,Footnote 105 including the very limited information contained in the UN Register of Objects Launched into Outer Space.Footnote 106 Notably, however, this Registry does not disclose whether a satellite is military or civilian in nature and not all satellites are actually registered.
Some satellites are purely civilian or commercial in nature and do not constitute lawful targets for the purposes of jus ad bellum necessity. Other satellites may not have one sole function, however. They may be dual use, serving both civilian/commercial and military clients. Notable examples include communication and navigation satellites. Additionally, satellites may host multiple payloads, which are also dual use in that they have both civilian/commercial and military users.Footnote 107 A celebrated example of a dual use satellite system is SpaceX’s Starlink service, which provides internet access to support Ukraine’s self-defence against Russia’s ongoing invasion. The Ukrainian government and military have depended on Starlink as the ‘linchpin’ of the Ukraine armed services’ command, control, communications, computers, intelligence, surveillance, and reconnaissance capabilities.Footnote 108 Civilians have likewise relied heavily on the service.Footnote 109 In addition to the potential for dual civilian and military use, SpaceX’s Starlink service demonstrates how satellites may not be exclusively owned, operated, or used by one state, but rather by a multiplicity of states, international organizations, and/or private entities.Footnote 110
Dual use and multiple owner/user satellites pose significant challenges for military planners, including assessing compliance with IHL targeting rules. Yet, whether a satellite or other object is a lawful target for the purpose of IHL is separate to the question of legality under the jus ad bellum.Footnote 111 The question for our purposes is whether, regardless of legality under IHL’s targeting rules, these types of satellites are generally targetable in self-defence because of the requirements of jus ad bellum necessity.Footnote 112 For certain states, it would seem that the answer is in the affirmative. In 2022, Russia publicly warned that commercial infrastructure in outer space used for military purposes can become legitimate targets for retaliation.Footnote 113 This is a deeply problematic view and, if indeed related to Russia’s appreciation of possible lawful targets of self-defence, does not accord with the obvious requirements of jus ad bellum necessity. As noted, for state responses in self-defence to be confined exclusively to a defensive purpose, any target selected in the course of defensive operations must be military in nature and connected with the armed attack being halted, repelled, or prevented.Footnote 114 Applying this general rule to targeting satellites which have a dual military/civilian function or which provide services to multiple states as well as non-state actors is complicated. The jus ad bellum does not comprise detailed rules of targeting that are found in IHL. For example, it is unclear whether the jus ad bellum requirement that the target be military in nature is identical or similar to the requirement under IHL that targets be limited to ‘military objectives’. The ICJ’s understanding of this requirement per its dictum in Oil Platforms certainly leaves room for debate.Footnote 115 However, even if connected with an armed attack, the fact that a satellite may not be exclusively a military target and/or may not be owned, operated, or used solely by the aggressor state must logically limit the responses that may be directed against that satellite using ASAT weapons in self-defence.
At a minimum, jus ad bellum necessity restricts which type of ASAT weapon may be used against a dual use or multiple owner/user satellite. Direct ascent or co-orbital kinetic physical ASAT weapons that are not capable of being directed at particular payloads and cannot avoid harm to other payloads on the same satellite would naturally seem to be ruled out by this requirement. This is because their effects cannot be limited solely to military targets connected to the armed attack. Anti-satellite missiles are the obvious case in point as their use is likely to destroy, or at least severely damage, the entire satellite and all payloads without distinction. Other types of ASAT weapons, however, do have the potential to comply with the rule that only military targets of the aggressor and not innocent third parties are targeted in self-defence, depending on how they are used. Such weapons might include co-orbital robotic arm technologies, as well as non-kinetic ASAT weapons like electronic devices that jam specific military signals, or cyber operations that can be specifically targeted and their effects contained. However, limiting the effects of these types of weapons is not guaranteed because of the dual use or multiple user nature of such satellites, the inherent difficulties of operating in space, and any action the victim of an intended attack might take to thwart it (which might result in kinetic impacts).
The legal risks to defending states of making dual use or multiple owner/user satellites targets of military action are manifest. Damaging or destroying space objects not exclusively owned, operated, or used by the aggressor exposes the defending state to potential legal claims from third states,Footnote 116 as well as possible private claims by non-state actors affected by military action. Moreover, the scale and effects of defending states using certain ASAT weapons might comprise an armed attack, giving rise to a right of the victim state to exercise self-defence against the (initial) defending state.Footnote 117 If multiple states are affected by the ASAT weapon use, the risk of military escalation becomes dangerously apparent and the fear of wars in space becomes very real. Given these risks, states using force in self-defence should abide by a broad understanding of jus ad bellum necessity by avoiding targeting dual use and multiple owner/user satellites entirely. Defending states that only target satellites that are (i) solely owned, operated, or used by the aggressor state, (ii) exclusively military, and (iii) factually connected with the armed attack, will most likely comply with this jus ad bellum requirement. Adherence to jus ad bellum necessity benefits the defending state by allowing it to avoid potential legal liability, as well as protecting itself from being a target of military responses by other states affected by ASAT weapon use.
A defending state may nevertheless choose to target in self-defence military satellites exclusively owned, operated, or used by the aggressor state. A defending state may likewise employ non-kinetic ASAT weapons against dual use or multiple user satellites that constitute uses of force, but which decision makers believe satisfy jus ad bellum necessity. If so, that is not the end of the story. Given the potential effects of damaging or destroying any type of satellite, even if the jus ad bellum necessity requirements are capable of being satisfied, jus ad bellum proportionality might nevertheless prevent targeting such satellites with ASAT weapons.
5. Proportionality in outer space
If some form of forceful military response in self-defence is necessary because force is the only reasonable option in the circumstances and the targeting requirements of both IHL and jus ad bellum necessity are also satisfied, it does not automatically follow that an ASAT weapon may be used in self-defence. As the following analysis reveals, jus ad bellum proportionality is likely to act as a strong limitation on whether or not a satellite may lawfully be targeted in self-defence and might even preclude entirely the targeting of certain types of satellites.
Jus ad bellum proportionality restricts how much total force a state may use in a military operation to achieve a legitimate defensive purpose (being to halt, repel, or prevent an armed attack). It allows states to effectively defend themselves, but requires that states do no more than that.Footnote 118 Jus ad bellum proportionality operates differently from IHL proportionality and compliance with each requirement is a distinct legal question. To be lawful, any act of defensive targeting of a satellite must comply independently with both sets of rules. In terms of how these requirements function, IHL proportionality operates at the micro or operational level of decision-making to regulate the specifics of individual attacks in order to minimize resulting harm to civilians and civilian objects. Jus ad bellum proportionality meanwhile operates at the macro or strategic level to limit a defending state’s total military response viewed as a whole. Jus ad bellum proportionality, therefore, acts as a prohibition against excessive overall military reactions by states that undertake necessary acts of self-defence.Footnote 119
Regarding appraising state compliance, jus ad bellum proportionality is not limited to an assessment of expected civilian harm resulting from particular planned attacks, which is at the core of IHL proportionality.Footnote 120 Jus ad bellum proportionality instead is concerned with the bigger picture and the totality of the wider defensive response, requiring states to balance uses of force and their outcomes primarily against a defensive purpose (being to halt, repel, or prevent an armed attack). Jus ad bellum proportionality also requires that defensive operations, viewed as a whole, are not excessive in terms of the overall negative impacts they might have on civilians and on the interests of other states and the international community more broadly.Footnote 121
In space, jus ad bellum proportionality has particular significance. This is partially due to the nature of certain satellites and the services that they provide, which are discussed further below in this section. More generally, the physical characteristics of space mean that there are serious consequences to targeting defensively all kinds of satellites.Footnote 122 Reports in February 2024 that Russia was pursuing the deployment of a nuclear-based ASAT weapon in space focused attention on the potentially devastating consequences to all states of a nuclear detonation in that domain.Footnote 123 Yet, even the deployment of conventional weapons in space raises particular concerns for third-party interests. Before moving to the detailed analysis, a familiar ASAT testing case study illustrates how targeting a satellite in self-defence might affect these interests. In January 2007, China employed a kinetic kill vehicle ASAT to destroy once of its own ageing weather satellites called Fengyun 1C.Footnote 124 This ASAT test caused international concern as the destruction of the satellite created a large cloud of debris, which spread out and travelled into different orbits above the Earth. The effects of this particular debris cloud have been long-lasting. It is estimated that more than 2,700 pieces of Fengyun 1C debris remain in orbit and most of these will continue orbiting the Earth for decades.Footnote 125 The threat of the debris to third parties materialized six years later when, in January 2013, fragments of Fengyun 1C collided with a small Russian laser-ranging retroreflector satellite called ‘Ball Lens in The Space’, knocking it from its orbit.Footnote 126 The satellites of other states, as well as the International Space Station, were also put at risk from this enduring cloud of space junk.Footnote 127
Other more recent examples of ASAT weapon use include India’s ‘Mission Shakti’ in March 2019, comprising the test of a missile to destroy one of its own satellites in low Earth orbit.Footnote 128 As with the destruction of Fengyun 1C, India’s test highlights the risks to other states’ space activities and assets in space caused by the debris that may result from the use of ASAT weapons. Indeed, following India’s test, NASA administrator Jim Bridenstine reminded us that ‘[d]ebris ends up being there for a long time; if we wreck space, we’re not getting it back’.Footnote 129 Russia’s destruction of Cosmos 1408 in 2021 also exemplifies this fear.Footnote 130 The USA continues to track approximately 1,500 pieces of the debris resulting from this incident, highlighting the ongoing concern that states have regarding the repercussions that flow from the use of ASAT weapons.Footnote 131
The risk of enduring space debris caused by damage to or destruction of a satellite is a factor that is peculiar to the space environment and the risk is growing. Space is a rapidly transforming and an increasingly congested domain. More than 7,500 satellites currently orbit the Earth,Footnote 132 and with satellite demand set to quadruple in the next decade, one estimate places 24,500 satellites in orbit by the end of 2031.Footnote 133 Debris-creating defensive ASAT weapon use could have long lasting and unforeseen consequences for the rights and interests of many spacefaring actors. Space debris does not discriminate,Footnote 134 so the risk of collision ‘is to all civilian, commercial, and government satellites of all nations’.Footnote 135 States therefore view space debris as ‘the most significant threat to the space environment’, with ‘the intentional destruction of satellites using kinetic force as exacerbating such threats’.Footnote 136 Any or all of their satellites, together with the essential services that rely on them, could be affected to varying degrees by ASAT weapons use. The weapons tests referred to above illustrate this concern and there have been efforts to address it. In the EU’s Draft International Code of Conduct for Outer Space Activities, for example, states resolve to minimize, to the greatest extent practicable, the creation of space debris, even when exercising their right of self-defence under the UN Charter.Footnote 137 Jus ad bellum proportionality is the positive law answer to this soft law aspiration. The following sections explain how these aftereffects factor in the jus ad bellum proportionality calculus.
5.1. Civilian harm
On Earth, ‘[i]t is the strategic impact of large-scale civilian casualties and damage that appears to influence what might constitute a disproportionate exercise of the right to self-defence by a State’.Footnote 138 That civilian harm stands as the clearest indicator that claims of self-defence might be deemed disproportionate under the jus ad bellum is clearly reflected in the reactions of states and international organizations.Footnote 139 A classic example is Israel’s invasion of Lebanon in 2006, justified as self-defence, but most frequently cited as disproportionate based primarily on the factor of excessive harm to civilians and to civilian infrastructure.Footnote 140 Applying this indicator to ASAT weapons use, damage to or destruction of a satellite may indeed lead directly to civilian death or personal injury. The rise of commercial space tourism puts private ‘space tourists’ in danger, although harm to astronauts on board a space station or transiting through space are perhaps more significant examples of the potential risk to civilians. The debris field from the destruction of Cosmos 1408 led to astronauts on the International Space Station to seek shelter, demonstrating the very real concern to life and limb of those operating in orbit.Footnote 141 That said, astronauts may have military or civilian backgrounds and they possess a unique status in international space law as ‘envoys of mankind in outer space’.Footnote 142 Consequently, astronauts are not automatically civilians for jus ad bellum proportionality assessment purposes. Yet, if not engaging in hostilities, astronauts should logically be regarded as civilians, meaning that death or injury to them will likely weigh more heavily in proportionality determinations. Conversely, harm to astronauts engaging in hostilities will be less significant for such assessment.Footnote 143 Regardless, given the relatively small numbers of both astronauts and private civilians operating in space, direct harm to individuals in that domain resulting from ASAT weapon use is likely to be limited, at least for now.
Of much greater significance to jus ad bellum proportionality is the potential harm to satellites owned and operated by civilians and corporations, together with the effects on Earth of damaging or destroying satellites that serve civilian populations. Such consequential civilian harm may result from a satellite being directly targeted by an ASAT weapon, or because satellites are damaged or destroyed by space debris that has resulted from targeting another satellite. Given the potential enduring nature of debris clouds, any number of satellites belonging to civilians or serving civilian needs may be at risk. That risk is hugely significant. As mentioned, satellites are central to the operation of human societies and to global economies. Even temporary disruption to a satellite that serves these vital civilian needs may have long lasting effects stretching and enduring well beyond the use of the ASAT weapon. An obvious example of the scale of this risk to civilians is any harm caused to the American Global Positioning System (GPS)Footnote 144 and its equivalents,Footnote 145 which provide military and civilian users with global positioning, navigation, and timing (PNT) services. It is no exaggeration to say that PNT systems are indispensable to the functioning of modern civilizations. Among other essential services, agriculture, aviation and other transport networks, financial markets, banking systems, logistics, communications systems and networks, power grids and other critical infrastructure, emergency services, environmental protection, disaster surveillance, humanitarian relief, military operations, and the preservation of national security more generally, all rely on PNT technology.Footnote 146
Even a temporary and reversible disruption to these essential services resulting from ASAT weapon use could have disastrous consequences for millions of civilians who rely on them, as well as for civilian infrastructure on Earth. The effects might be economic, caused by havoc wrought on financial markets. This possibility has been noted by the head of UK Space Command in reference to Russia’s potential use of jamming satellites that could ‘cut off the UK from the outside world’.Footnote 147 The effects could also be physical, for example, because emergency services or disaster relief teams are unable to respond, aircraft cannot fly safely or other transport systems cannot function properly, agricultural production is disrupted, and so forth. The International Committee of the Red Cross, among others, has voiced its concern about such potential severe human costs,Footnote 148 with civilian injury or death being readily foreseeable in many instances. Given the centrality of space-based PNT services to life on Earth, the potential effects on individuals and on human society caused by ASAT weapons are in many respects unforeseeable and unquantifiable.
Accordingly, even if regarded as military objectives for IHL purposes, given the jus ad bellum proportionality requirement to minimize civilian harm, it is arguable that the jus ad bellum generally rules out the direct targeting of satellites that provide PNT and equivalent essential services. The risk to civilians of disrupting such services, even temporarily, is too great. The use of non-kinetic ASAT weapons to disrupt PNT or other essential services temporarily seems equally impossible to justify from a jus ad bellum perspective. This is so despite a functional link between such a satellite and an armed attack that might satisfy jus ad bellum necessity. For jus ad bellum proportionality, the potential repercussions on Earth of these acts are too varied and potentially too significant to evaluate in any meaningful way that might justify the pursuit of a defensive purpose. The possible repercussions also mean that deploying ASAT weapons against space-based PNT and other essential services that prima facie fall below the threshold of a use of force is also a risky strategy. This is because the scale and effects of that act might mean that the threshold of violence is eventually crossed and the jus ad bellum requirements that states wished to avoid nevertheless end up applying. This conclusion counters the idea that temporary or reversible disruption to a satellite’s function would not generally amount to a use of force.Footnote 149 However, this may be true where satellites that provide non-essential services are targeted and/or where the scale and effects of targeting are in fact limited. Context is determinative and the wider effects of such targeting, including on Earth, must be considered for the purposes of the initial use of force threshold determination and the jus ad bellum proportionality analysis.
Additionally, the foregoing analysis applies where the targeting of other satellites would put PNT and other essential services at risk because of the debris created by that act of targeting. Targeting satellites in proximate orbits is the most obvious example. The only possible exception to these conclusions, as with the use of nuclear weapons, is a greater freedom to use ASAT weapons where the survival of the state is at stake.Footnote 150 Beyond such extreme and unusual circumstances, however, jus ad bellum proportionality acts as a significant limitation on defensive action in space in terms of directly targeting certain type of satellites and incidentally causing harm to them and the essential services they provide. Furthermore, states must still account for the impact on civilians resulting from defensively targeting any kind of satellite, even those that do not provide such essential services. Jus ad bellum proportionality requires decision makers to consider very carefully the possible resulting consequences in space and on Earth, in each case so as to minimize collateral civilian harm. In addition to any harm to civilians, in assessing whether the results of that targeting might be excessive, possible consequences for other states must also factor in the jus ad bellum proportionality assessment.
5.2. Third party rights and interests
Beyond civilian harm, state interests more broadly factor in the jus ad bellum proportionality assessment. This idea is rooted generally in international space law. Article I of the OST stipulates that the exploration and use of outer space shall be carried out for the benefit and in the interests of all states and shall be the province of all mankind.Footnote 151 Furthermore, states parties to the OST are obliged by Article IX to ‘conduct all their activities in outer space, including the moon and other celestial bodies, with due regard to the corresponding interests of all other States Parties to the Treaty’.Footnote 152 These general presumptions of how states should operate in space in times of peace encapsulate the essence of jus ad bellum proportionality, which is designed to limit the excesses of war. In particular, jus ad bellum proportionality requires that the legally protected rights of other states must not be unduly harmed when states use ASAT weapons.Footnote 153 Excessive harm to such rights risks defensive action being viewed as disproportionate under the jus ad bellum. Determining what harm to third-party rights and interests might be deemed disproportionate is not an exact science, however. The jus ad bellum provides no detailed rules on this point, and we do not have state practice and opinio juris to guide us that has resulted from ASAT weapon use in self-defence. Moreover, whether third party reviewers (be they other states, international organizations, or courts and tribunals) will regard defensive action as excessive will be largely fact dependent. Nevertheless, there are certain conclusions we can readily draw.
First, the risk of enduring space debris caused by ASAT weapon use poses a direct threat to satellites owned or operated by third states and to satellite provided services on which they rely, such as essential PNT services. Depending on the nature of the satellite targeted, the impacts of ASAT weapon use on interests of other states could be multiple and varied. They encompass effects on Earth and in space. Physical or non-physical harm might result, including significant economic loss resulting from the denial of access to a satellite provided service. A number of other legally protected rights might also be implicated, among them a state’s right to neutrality.Footnote 154 These outcomes of defensive ASAT weapon use are likely to factor highly in assessments of jus ad bellum proportionality by third states and other international actors. Moreover, the threat posed by damage or destruction to satellites reaches beyond individual state rights and interests, to rights and interests appertaining to all states and to the international community more broadly. Particularly notable are the possible effects of ASAT weapon use on both terrestrial and space environments. The ICJ has indicated that, for the jus ad bellum, ‘[r]espect for the environment is one of the elements that go to assessing whether an action is in conformity with the principles of necessity and proportionality.’Footnote 155 The UNGA has meanwhile underscored that ASAT technologies might generally threaten the ‘long-term sustainability of the outer space environment’.Footnote 156 The use of nuclear weapons in space clearly implicates this concern, given the potential perils for space and Earth, but dangerous conventional ASAT weapon use could likewise contaminate the space environment in the long term. ASAT weapon use has the potential to impair the ability of all states to operate in space and to benefit from the freedom to explore and use space peacefully, including placing satellites in orbit. Enduring clouds of space debris are the clearest example of this risk.
In terms of other community interests that might be affected by space debris, the UNGA also reminds us that ‘the creation of long-lived orbital debris arising from the deliberate destruction of space systems increases the risk of in-orbit collisions and the potential for misunderstanding and miscalculations that could lead to conflict’.Footnote 157 This statement speaks to the wider possible impact of ASAT weapon use on international peace and security, in which all states have an interest. This peace and security is legally protected by Article 2(4) of the UN Charter and by strict adherence to the requirements of jus ad bellum necessity and proportionality that condition a state’s right of self-defence. Indeed, the ultimate rationale of jus ad bellum proportionality has been described as the maintenance of international peace and security, by seeking to minimize disruption to the international order caused by an exercise of self-defence.Footnote 158 Consequently, ASAT weapon use against another state that directly threatens this peace and security by creating space debris that may affect all spacefaring states is likely to be regarded as disproportionate under the jus ad bellum.
These general conclusions have practical real-world repercussions for those contemplating using ASAT weapons in self-defence. That adherence to jus ad bellum proportionality requires the minimization of harm to the interests of other states and to the international community equates to an obligation on decision makers to consider the effects on Earth and in space of the methods they use. Given the consequences associated with the creation of space debris, outside of extreme situations of self-defence threatening the existence of the state,Footnote 159 it is arguable that jus ad bellum proportionality prima facie precludes the use of most, if not all, kinetic physical ASAT weapons. Non-kinetic alternatives, such as directed energy and electronic ASAT weapons and cyber operations, might also be problematic for jus ad bellum proportionality compliance. This will be so where the effects of such weapons on third-party interests are comparable to kinetic physical ASAT weapon use because of the effect on the targeted satellite and/or related service, or the effects cannot be quantified or confined to the aggressor state. An example is a cyber operation that causes a satellite to lose control and collide with another satellite or space object, resulting in damage and debris. Cyber operations, as well as lasers and high-powered microwaves, might also be used to impair satellites providing essential PNT services to a state or group of states temporarily or permanently. Unless other satellites in a constellation can ensure the continuation of the relevant service, catastrophic consequences might ensue, potentially including loss of life. Likewise targeting dual use and multiple owner/user satellites with such non-kinetic weapons might result in potentially unquantifiable harm to states other than the aggressor, as well as to other non-state entities. Cyber operations raise particular concerns because attacks on one system may have repercussions for various other systems and cause indiscriminate effects due to the interconnected nature of cyberspace.Footnote 160 With each example, affected third-party interests will weigh heavily on determinations of whether ASAT weapon use is disproportionate under the jus ad bellum.
Not all potential targets will be multiple owner/user or dual use satellites, however, or will obviously implicate third-party interests. Non-kinetic physical ASAT weapons might be used to target a military satellite wholly owned and operated by an aggressor state, or a commercial satellite that constitutes a military objective for the purposes of IHLFootnote 161 and also passes the jus ad bellum targeting rules discussed in Section 4.2. The aim might be to blind satellite sensors, cause components to overheat, damage electronics, or compromise data. Where the use of such weapons will not cause space debris, these types of satellites might appear to be obvious and potentially, legally, risk-free targets for defensive military operations. In certain cases that may be true, provided the effects of the targeting are contained and there are no excessive negative impacts on other states. Yet, such actions, because of their scale and effects, might nevertheless comprise a use of force, requiring a justification of self-defence, and may still prove problematic for jus ad bellum proportionality compliance. This is most likely where the satellite is targeted in self-defence in response to an armed attack on Earth.
For this author, jus ad bellum proportionality does not obviously restrict a defensive response to its immediate locality, meaning that there must be a geographical nexus between an armed attack and acts of self-defence.Footnote 162 Accordingly, defensively targeting a satellite in space in response to an armed attack on Earth should not be regarded as precluded by jus ad bellum proportionality simply because of the geographical distance between those acts. Instead, jus ad bellum necessity governs this scenario by requiring that the satellite targeted is connected to the armed attack on Earth. If this connection is established, as discussed in Section 4.2, there is a prima facie defensive necessity to such action. Yet, targeting a satellite in response to a terrestrial armed attack nevertheless risks being characterized as disproportionate under the jus ad bellum. This is not because of geography or distance, however. Self-defence of this kind will be regarded as jus ad bellum disproportionate if using an ASAT weapon against a satellite in response to an armed attack on Earth is regarded as a significant escalation of hostilities.Footnote 163
If defensive acts create space debris, they should always be regarded as an unwarranted escalation of hostilities because of the potential and unquantifiable harm to third parties discussed above in this section. Where non-kinetic physical ASAT weapons are used, arguably there is also a risk that taking the defensive responsive away from the Earth-based armed attack to space is unduly escalatory. A defensive response in space transports the conflict to a new domain of warfare and, given its peculiar physical characteristics, the rights and interests of other states are more likely to be negatively affected as a result. As NATO rightly cautions, ‘any conflict that extends into space has the potential to affect all users of space’.Footnote 164 The ensuing threats to third party interests and to international peace and security are thereby multiplied. Furthermore, a military response in space risks a further riposte by the aggressor in that domain. Therefore, a reciprocal escalation of the conflict arising from an armed attack on Earth exposes states to the prospect of wars in space that they fear so much.Footnote 165 Jus ad bellum proportionality directly addresses this danger of significant escalation that might take conflict on Earth to conflict in space. Jus ad bellum proportionality requires states to consider this macro picture and to limit defensive action accordingly.
Given the nature of the space environment and the potential negative effects of targeting a satellite using any form of ASAT weapon, it seems sensible to conclude that, where self-defence can be effectively achieved by striking a military target on Earth that has a nexus with the armed attack rather than targeting a satellite in space, the general requirement that flows from jus ad bellum proportionality must be that the former target be preferred over the latter. That terrestrial targets are generally preferred would also naturally apply in response to armed attacks in space.Footnote 166 It might be possible, for example, for a satellite to be neutralized by a strike against a ground-based control node in a remote area, rather than risking targeting the satellite directly. If so, this option must be taken.Footnote 167 Compliance with this general approach to targeting Earth-based objects avoids the risk of significant escalation and the threat to third parties and to international peace and security that jus ad bellum proportionality seeks to avoid.
In summary, for states seeking to abide by the tenets of jus ad bellum proportionality, targeting satellites with any form of ASAT weapon must be approached with extreme caution. Clearly, alternatives to ASAT weapons that cause physical damage and destruction should generally be preferred to avoid the problems of space debris and impinging unduly on third party interests. Weapons that only temporarily destabilize or render dysfunctional satellites, or electronic jamming or spoofing devices that are limited to interfering with or falsifying the transmission of signals to and from satellites, are most likely to comply with jus ad bellum proportionality. The same is true for targeted and limited cyberattacks on satellite-related computer networks. In each case, compliance depends on whether such ‘soft kill’Footnote 168 techniques can be used in a way that causes no, or only minimal, damage to non-aggressor states and to the interests of other third parties. States, like the USA, appear broadly to agree with this approach to minimizing civilian collateral harm and the effects of military action on space activities, albeit not expressed explicitly in jus ad bellum terms. American policy to negate enemy space capabilities is incremental, going in ascending order in the methods it adopts, from deception and denial to degradation and ending at destruction of space assets.Footnote 169 Of course, methods at the lower end of the spectrum of military activity might not constitute a use of force at all, because the scale and effects of their use do not cross that threshold of violence. If so, these operations do not require justification by reference to the jus ad bellum, albeit that their use might implicate the laws of state responsibility and/or other bodies of international law.Footnote 170
6. Conclusion
Space is the ‘province of all mankind’.Footnote 171 All states must be free to explore and use space peacefully, to benefit from satellites placed in space, and to have this communal resource protected from excessive use of military activities. Impingements on this freedom must be strictly controlled. Absent specific regulation of military activities in space, including a multilateral weapons control treaty pertaining to ASAT weapons, the jus ad bellum, alongside IHL, must be viewed as an essential part of the current international law framework limiting the use of ASAT weapons. Although states continue to develop new counterspace weapons and space is an ever-contested military domain, adherence to the requirements of jus ad bellum necessity and proportionality has the potential to limit ASAT use. A clearer understanding of these jus ad bellum requirements directly addresses pressing international concerns regarding the weaponization of space and the fear of wars between states in that domain.
Given the unique nature of the space environment and the importance of satellites to the functioning of states and human societies, jus ad bellum compliance means that ASAT weapon use in defensive military operations is heavily restricted, and it may even be denied in all but the most extreme circumstances of self-defence. Instead, targets of self-defence should generally be confined to Earth. Where satellites are targeted, in addition to IHL targeting limitations, the jus ad bellum requires that the methods employed to neutralize satellites be strictly controlled to minimize harm to civilians and to third party interests and to avoid conflict and the escalation of conflict in space. Adherence to the jus ad bellum requirements of necessity and proportionality underpins international peace and security and state aspirations of safeguarding space for peaceful purposes and ensuring its valuable resources continue to benefit all mankind.
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