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On February 5, 2022, Russia launched Cosmos 2553 into orbit. On December 5, 2024, the world learned that the satellite carried a dummy nuclear warhead, designed to test components for a nuclear-armed anti-satellite (ASAT) weapon. That satellite, if detonated in space, would have the potential to destroy the infrastructure on which much of modern life depends. Although the prospect of a nuclear-armed anti-satellite weapon is new, the threat to satellites is not. As satellite technologies have proliferated to facilitate civilian and military operations around the globe, so too have ASAT weapons. China, Russia, India, and the United States have all tested ASAT weapons, which can create millions of pieces of space debris in a realm where anything larger than one centimeter may damage spacecraft.

These new threats raise pressing questions that legal scholarship is only beginning to answer: Does the deployment of some or all ASAT weaponry violate international law? What lawful responses are available to states facing such threats? Do threats from weapons positioned in space meaningfully differ from those posed by ground-based systems? At what point may states lawfully invoke the right of self-defense and use force to counter these threats? This Essay aims to fill this gap. It begins by describing the novel threats posed by the rise of ASAT weaponry and outlining the legal framework governing spacefaring and hostilities. It then presents a framework to determine the lawful measures states can take to respond to ASAT threats and examines the threshold at which states may lawfully invoke their right of self-defense. The Essay urges caution in responding to novel ASAT technology, arguing that overreacting to space-based threats might trigger the very global catastrophe that states hope to avoid.