New Perspectives on Nuclear Verification

by Raoul Cardellini Leipertz (Centre for Higher Defence Studies (CASD/SSU), Rome, Italy)

Last spring, a lone Russian satellite drifting through an unexpected low Earth orbit set off alarms from Washington to Brussels. Was it an innocuous on-orbit experiment or the first sign of an exo-atmospheric strike platform? Although officials later softened the most sensational claims, the incident exposed a deeper and far more persistent concern. Space law may prohibit nuclear weapons in orbit, but it provides no means of verifying whether that rule is being respected.

The treaties that anchor outer space security were never built with verification in mind. Article IV of the 1967 Outer Space Treaty forbids placing objects carrying nuclear weapons in orbit, and the 1963 Partial Test Ban Treaty outlaws nuclear explosions beyond the atmosphere. Yet, neither instrument explains how a State might demonstrate compliance, nor what evidence is required to judge whether a satellite is nuclear-free in its payload. More importantly, they remain silent on systems that only pass through space.

The Cold War Fractional Orbital Bombardment System (FOBS) exploited precisely this silence by sending a warhead into a low, partial orbit and then de-orbiting it before it completed a full revolution, approaching the target from an unexpected direction. Because the payload was never technically stationed in orbit, the Soviet Union, which developed such a system, could argue that it did not breach Article IV’s wording, a position sustained by the United States, which did not denounce FOBS. SALT II attempted to close this loophole by explicitly banning FOBS, but because the treaty never entered into force, the clarification never became binding. Even the transparency language in Article IX of the Outer Space Treaty, which calls for due regard and consultations where activities may cause harmful interference, offers little help. It reflects a general expectation of openness, but it was never designed as a nuclear verification provision. It provides no agreed procedure, no standard of proof and no technical mechanism for resolving doubts about a satellite’s payload or purpose.

This uncertainty now shapes every major space power’s interpretations of rival behaviour. Verification of nuclear activities in orbit relies on imperfect indicators such as thermal signatures, launch profiles and changes in orbital elements. These can reveal where an object is, but not what it contains or how it will be used. A manoeuvring satellite, a hypersonic glide vehicle and a missile defence interceptor can follow almost identical trajectories, making verification less a question of observing facts and more one of interpreting intent under conditions of doubt.

The Russian satellite incident showed this clearly. A single object in an unexpected orbital band generated competing narratives about its purpose and broader strategic motives: the problem was not the satellite itself, but the absence of a trusted method to determine what it was capable of or whether it violated the norms prohibiting nuclear weapons in outer space.

Today, the pressures are mounting. Major powers are once again debating space-based interceptors as part of their missile defence architecture. In the United States, for example, the proposed Golden Dome concept envisages a multilayered system in which space-based sensors and interceptors would play a central role. From a verification perspective, such systems risk further blurring the line between defensive technologies and nuclear-capable platforms. Satellites designed to track or intercept missiles in their boost phase would occupy orbits and employ manoeuvres that are difficult to distinguish, from the outside, from those of systems intended to deliver or support nuclear weapons.

The United Nations General Assembly recognised these dangers in 2024, when it adopted Resolution 79/18 on “Weapons of mass destruction in outer space,” which recalls the obligations of the Outer Space Treaty and explicitly calls for the development of “effective verification measures” to prevent an arms race in outer space and to reduce the risk of tensions arising from misperceptions and miscalculations. The resolution hints at a mix of legally binding obligations and political commitments, but the details remain open.
Without greater transparency in launch reporting, wider sharing of space situational awareness data, and clearer interpretative guidance on how key treaty terms apply to transit, fractional orbit, and space-based interception, mistrust will continue to fill the gaps left by sensors and legal text alike. After all, the central question remains unchanged. How do we verify what we cannot see?

Raoul Cardellini Leipertz is a PhD student in Defence and Security Studies at the Centre for Higher Defence Studies (CASD/SSU) in Rome, specialising in outer space security and the effects of emerging technologies on warfare. They focus on the legal and policy challenges of autonomous, dual-use systems, nuclear non-proliferation and the role of space in multi-domain operations.