Self-defence in outer space: Anti-satellite weapons and the <i>jus ad bellum</i>

Abstract Space is an increasingly militarized domain, with the potential to be a source and place of armed conflict. Tests of anti-satellite (ASAT) weapons capable of neutralizing civilian and military satellites have fuelled fears of warfare in that domain. Resulting space debris 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 the functioning of 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. Yet, how the jus ad bellum , which regulates when states may use 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 article argues that jus ad bellum regulation of ASAT technologies directly addresses state concerns regarding protecting their space assets and avoiding conflict in space. This author contends that states acting defensively in space are restricted by the requirements of jus ad bellum necessity and proportionality in their choice of targets, thereby protecting civilians and the interests of other states. A clearer understanding of how these jus ad bellum requirements apply in space helps decision makers avoid putative defensive acts being characterized as unlawful uses of force. Adherence to these requirements ultimately helps to secure international peace and security on Earth and in space.