A new modeling study published today in Nature proposes a practical method for verifying whether a satellite in orbit carries a nuclear weapon, addressing a long-standing gap in international space law. The approach, developed by Areg Danagoulian, a professor at the Massachusetts Institute of Technology, relies on miniature satellites equipped with neutron detectors that can identify telltale signals produced when uranium neutrons interact with protons in Earth's magnetic field.
The 1967 Outer Space Treaty, which forms the foundation of international space law and has been ratified by 118 nations including the United States, China, and Russia, prohibits the deployment of nuclear weapons in orbit. Such weapons could destroy most satellites in low Earth orbit. However, no practical methods have existed to verify compliance with the treaty and confirm the absence of such arms.
The study draws on historical precedent. In 1962, during the Starfish Prime test, the United States detonated a 1.4-megaton thermonuclear warhead in space, inadvertently destroying many early satellites. The explosion released enormous quantities of high-energy electrons, many of which became trapped in Earth's magnetic field, where they damaged any electronic components they encountered.
«When a nuclear detonation occurs in outer space, practically the entire mass of the bomb becomes ionized and almost every single electron in the weapon's mass becomes free,» Danagoulian explained. «These electrons are injected into the inner Van Allen radiation belt. Once there, they begin to strike everything that passes through these belts, causing ionization, radiation damage, and more.»
Five years after Starfish Prime, the Outer Space Treaty declared space the «heritage of all mankind» and, among other safeguards, banned the use of nuclear weapons. More recently, in 2022, weeks before the invasion of Ukraine, Russia launched the satellite Cosmos 2553, officially declared for surveillance and detection activities but immediately considered suspicious. U.S. authorities have expressed concern that it may serve as a test platform for a future nuclear anti-satellite weapon.
A nuclear detonation in that orbit, with Cosmos 2553's apogee at about 2,000 kilometers, could destroy many U.S. reconnaissance satellites, international satellite communication platforms, and Starlink satellites. «The Russians launched this satellite into a very strange and unusual orbit because it passes through the most hostile environment possible around the planet,» Danagoulian noted. «Nobody puts satellites there because it is a highly radioactive environment. So why put a satellite in that orbit? That position is probably the best place to trap electrons if one were to detonate a thermonuclear weapon.»
The proposed detection method involves a sensor system installed on a satellite that could orbit near a suspicious satellite and detect neutrons generated by high-energy protons colliding with radioactive material. According to calculations, a sensor system the size of a 9U cubesat, roughly the size of a large encyclopedia, could detect the presence of a nuclear weapon inside a suspicious satellite with 99 percent accuracy by maintaining a distance of about four kilometers for approximately one week. Detection time could be reduced to a few hours if multiple sensor satellites were used, or if the satellite could approach within one thousand meters of the suspicious object.
The theory behind the system is a reaction known as spallation, caused by extremely high-energy protons present in radioactive environments. When a high-energy proton strikes elements with a high atomic number, such as uranium or plutonium, it can expel about 40 neutrons. This is an incredibly high number, considering that in such an environment there are millions of protons per second per square centimeter. Normal satellites would not emit such a high number of neutrons, though protons, neutrons, and natural electrons are present in space, especially in low Earth orbit.
The system uses two panels made of neutron sensor pixels known as scintillators, which interact with radiation and emit light. The panels are enclosed between synthetic crystalline diamond detectors, which allow the system to distinguish neutrons coming from radioactive materials from natural protons and electrons. The two-panel structure can then be used to estimate the direction from which the neutrons are coming, enabling differentiation between neutrons from a nuclear weapon and those from natural background radiation.