The militarization of outer space remains at the top of the international agenda. Tests of anti-satellite (ASAT) weapons have fuelled fears over the weaponization of space, and the prospect of space being a source and place of armed conflict is a very real concern. Against this background, the U.N. General Assembly’s newly minted Open-Ended Working Group (OEWG) on the Prevention of an Arms Race in Outer Space is holding its first substantive session this week. Continuing its longstanding focus on the issue, and building on the work of previous OEWGs, the General Assembly has tasked the new OEWG to submit recommendations on the prevention of an arms race in outer space.
These U.N. efforts are laudable and must be supported. After all, space debris resulting from ASAT weapon use threatens other satellites in orbit, many of which underpin the operation of human societies and the functioning of global economies. Conflict in space could, therefore, have catastrophic effects on civilians and State interests, both on Earth and in space. Regulating military activities in space benefits us all. However, despite the importance of satellites and the need to protect space from the effects of military activities, multilateral attempts that have sought to restrain the escalating weaponization of space have largely failed. The OEWG endeavors and other multilateral attempts to limit or ban ASAT weapon use face an uphill battle, therefore, especially in the current geo-political climate. While we wait for any State consensus on this issue, we must look to existing international law and how it can be applied to address this pressing threat to international peace and security, while at the same time recognizing that states will act to defend themselves on Earth and in space.
To this end, the publication in 2024 of The Woomera Manual on the International Law of Military Space Activities and Operations constitutes a valuable contribution to clarifying the current state of international law regarding military operations in space. I had the pleasure of attending the recent UK launch of the Manual and I applaud its ambitious aim “to provide the first comprehensive, detailed analysis of the existing legal regime of space.” There is no doubt that it advances our understanding of the legal regulation of military activities in space by setting out 48 Rules, which are said by the authors to reflect how international law regulates military activities during i) peace time, ii) times of tension and crisis, and ii) armed conflict.
The Manual has already been the subject of discussion on Just Security and other fora (see, e.g., here and here) and my intention is not to provide a comprehensive review. For my part, despite the Manual’s important contribution to our understanding of how international law applies to military activities in space, much more can be said on the limitations that the law places on States acting in self-defense in space, including by using ASAT weapons. My purpose, therefore, is to focus attention on the huge potential for international law on the use of force, typically referred to as the jus ad bellum, to address the fears over ASAT use and the militarization of space. This body of law, grounded in U.N. Charter and customary international law, is the subject of my recently published article for the Leiden Journal of International Law, entitled “Self-defence in outer space: anti-satellite weapons and the jus ad bellum.”
On the topic of self-defense in space, the Woomera Manual does address the jus ad bellum in its Rules pertaining to “times of tension and crisis” and advances our understanding of the applicable law. Notably, its consideration in Rule 23 of the “armed attack” trigger for the right of self-defense for the purposes of Article 51 of the U.N. Charter is commendable in how it applies to satellites and other objects targeted in space. In contrast, however, consideration of how the jus ad bellum applies to ASAT use in self-defense is relatively light touch (more on this below). Similarly, much of the attention in the literature regarding ASAT use has focused on the targeting rules of international humanitarian law (IHL). This focus is entirely understandable. IHL is the “go to” body for targeting decisions, as it governs how states conduct hostilities, including laying down targeting rules that apply to ASAT weapon use during situations of armed conflict. It is axiomatic that these IHL rules apply equally to all belligerents and regardless of a State’s reason (lawful or otherwise) for entering into hostilities.
Yet, despite the undisputed importance of IHL for considering issues of State security and international peace and security in space, my argument is that the separate question of legality under the jus ad bellum, which speaks to why States are using 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 (separately, and in addition, to IHL) to establish the legality of targeting satellites. Most notably, absent State consent to use force against one of its own satellites or UN Security Council authorization authorising a State to use force, any actions against satellites that constitute a threat or use of force require justification as lawful acts of self-defense. Otherwise, such conduct will breach Article 2(4) of the U.N. Charter as an unlawful use of force.
This important issue of States using defensive force against satellites is the focus of my recent article. My argument is that, without a multilateral ASAT weapons control treaty, 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. As with IHL, the jus ad bellum was not originally conceived of as applying beyond the Earth’s atmosphere, but its rules can be interpreted and applied in a meaningful way to space, setting out limits regarding when and how states may lawfully target satellites using ASAT technologies, even in self-defense.
The principal jus ad bellum rules discussed in my article are the requirements of necessity and proportionality, which apply to the entirety of a defensive military operation to condition the exercise of the right of self-defense so that force (against satellites or otherwise) is contained and confined purely to the defensive. Although Rule 26 of the Woomera Manual acknowledges these requirements, relatively limited attention is given to how necessity and proportionality are operationalized in the specific context of self-defense in space. Beyond some high-level observations, the continued application of these requirements to defensive military operations is not explored in detail. Indeed, the Manual’s thematic structuring – which presents the Rules relating to military activities during i) peace time, ii) times of tension and crisis, and ii) armed conflict – may give the impression that the jus ad bellum ceases to apply once an armed conflict is underway, with legal regulation thereafter becoming the sole preserve of IHL.
Perhaps this lack of attention to how self-defense is constrained in space is a result of a perceived lack of space-specific State practice and opinio juris. Regardless, this is an opportunity lost when considering that other core jus ad bellum issues, like the meaning of an “armed attack,” are subjected to much more detailed commentary in the Manual. As I argue, the fact that jus ad bellum necessity and proportionality apply to the entirety of a defensive military operation means that ASAT weapon use in self-defense must comply with the targeting rules of IHL and (additionally and separately) with the targeting rules of the jus ad bellum. I examine these rules in detail, drawing on my existing research on necessity and proportionately and applying it to the space context.
As a general premise, 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 may even be denied in all but the most extreme circumstances of self-defense. First, beyond requiring that resort to force in self-defense is an exceptional and limited response to a genuine situation of emergency, I argue that jus ad bellum necessity operates to restrict which type of ASAT weapon may be used against satellites. Dual use or multiple owner/user satellites pose particular challenges for military planners from the perspective of necessity. I conclude that, depending on the ASAT technology used, 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 jus ad bellum necessity.
Even if it is deemed necessary to target a satellite using an ASAT weapon, jus ad bellum proportionality is likely to act as a strong limitation on whether or not a satellite may lawfully be targeted in self-defense 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 and has particular significance in space. This is due to the nature of certain satellites (those that provide services to both military and civilian users) and the essential services that they provide (including global positioning, navigation, and timing services). The physical characteristics of space also mean that there are serious consequences to targeting defensively all kinds of satellites. 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 poses a major risk to satellites and the essential services that they provide to states and civilian populations.
States seeking to abide by the tenets of jus ad bellum proportionality must approach targeting satellites with any form of ASAT weapon with extreme caution. I argue that alternatives to ASAT weapons that cause physical damage and destruction should generally be preferred to avoid the problems of space debris, including resulting civilian harm and impinging unduly on third party interests (including neutrality, sustainability of the space environment, and international peace and security more generally). 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. I also conclude that jus ad bellum proportionality logically requires that targets of self-defense should generally be confined to Earth. Where satellites are targeted, however, 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.
The authors of the Woomera Manual do not suggest that their work is the end of the story when it comes to how international law applies to military activities in space. On the contrary, one of the Manual’s great strengths lies in its candid recognition of the limits of current legal clarity. Where a lack of State practice means that the law does not provide answers, the Manual refrains from “filling the gaps” by projecting an academic opinion of what the law should be. The Manual leaves it to states as lawmakers to fill these gaps (or not) over time. In the context of a dynamic lawmaking environment, the authors express the desire that the Manual prompts states to engage with the issue of legally regulating military activities in space. I share this worthy goal and hope that my own work contributes to such engagement.
In considering how jus ad bellum necessity and proportionality apply to the use of ASAT weapons in self-defense, I hope that States and other interlocutors will engage with the subject of restraining military activities in space, even when States act legitimately to defend their national security in space. Such engagement allows the law to develop and for greater legal certainty and predictability to arise as a result. A clearer understanding of jus ad bellum necessity and proportionality directly addresses pressing international concerns regarding the weaponization of space and the fear of wars between States in that domain. Adherence to these requirements allows States to protect their vital space assets while also addressing State concerns regarding space debris, civilian harm, and avoiding conflict in space. As we all rely on satellite-based services in our daily lives, greater clarity regarding legal restraints on warfare in space benefits us all, as well as contributing more generally to securing international peace and security on Earth and in space.