Anti-Satellite Weapons and Self-Defence: Law and Limitations

Space is an increasingly militarized domain with the potential to be a source and place of armed conflict. In recent years, tests of anti-satellite (ASAT) weapons capable of neutralizing civilian and military satellites have fuelled fears of warfare in that domain. Satellites are potentially attractive targets during armed conflict, making ASAT weapons central to assessing the threat environment in space. Space debris resulting from ASAT weapon use is of particular concern, as it threatens other satellites in orbit, many of which underpin the operation of human societies and global economies. Although states recognize this threat, attempts at weapons control have failed. Instead, we must look to existing international law that governs military activities in space, including in the cyber domain. Yet, how the jus ad bellum (JAB), which regulates state uses of force, applies to ASAT weapons has received little attention. This is despite state assertions of their right to act in self-defence in space. This paper argues that JAB regulation of ASAT technologies addresses state concerns regarding protecting space assets and avoiding conflict in space. This author contends that states acting defensively in space are restricted in their choice of targets by the requirements of JAB necessity and proportionality, which protect civilians and the interests of other states. While defensive acts, including cyber operations, that do not cause space debris are most likely to be JAB-compliant, this is not guaranteed. Military actions of any kind against mixed-use or multi-user satellites raise particular concerns for JAB proportionality due to the potential resulting harm to civilians and to the interests of other states. A clearer understanding of how the JAB regulates ASAT weapons helps decision makers avoid lawful acts of self-defence being characterized as unlawful uses of force. Adherence to these JAB rules ultimately helps secure international peace and security on Earth and beyond Earth's atmosphere.