!GPS Block II-F satellite in Earth orbit. Credit: NASA
For nearly 60 years, the Outer Space Treaty has banned nuclear weapons in orbit. But the treaty has always relied on an honor system with no way to verify compliance. That may be about to change.
A team of researchers led by MIT nuclear physicist Areg Danagoulian has proposed a detection system that uses cosmic rays to sniff out hidden nuclear weapons on satellites. The work, published July 8 in the journal Nature, outlines a method that could give the 1967 treaty its first objective verification mechanism.
The timing is critical. Rising geopolitical tensions and Russia’s suspicious satellite activities have heightened fears that the treaty may fail. Russia’s Kosmos 2553, launched in February 2022, has been alleged by U.S. officials to be part of a nuclear antisatellite weapon development program. The satellite was placed in an orbit passing through the inner Van Allen radiation belt.
“That’s a terrible place to put a satellite at. You’re going to damage your satellite with all that radiation,” Danagoulian said. But it would be an ideal location to detonate a nuclear weapon. Radiation from such a blast would accumulate in the belt, corralled by Earth’s magnetic field, and destroy thousands of satellites in lower orbits.
The threat has shifted dramatically from Cold War fears of space-based nukes targeting Earth. Experts now worry about nuclear antisatellite weapons designed for mass kill of orbital infrastructure.
“The reason this is under pressure is that the U.S. heavily depends on space capabilities for military power, and Russia, in particular, is exploring how to take those space capabilities away,” said Jeffrey Lewis, a nuclear nonproliferation expert at the Foreign Policy Research Institute.
“They seem to be considering mass kill of satellites in orbit, and if you think about it, what’s the easiest way to get rid of all those Starlink satellites? It would be to detonate a small number of nuclear weapons.”
Danagoulian’s proposed solution exploits a basic physics phenomenon called spallation. When high-energy cosmic ray protons strike uranium atoms in a nuclear weapon, they eject neutrons. A detector satellite positioned within about 4 kilometers (2.5 miles) of a suspect spacecraft could pick up those telltale neutrons.
“If you detect those neutrons, that itself can be a telltale sign that there is an unusual amount of uranium on the satellite, and it’s most likely to be a nuclear weapon,” Danagoulian said.
The inspector satellite would use a pixel array detector covered with diamond layers. Diamonds can detect charged particles like electrons and protons but remain transparent to neutrons, providing a natural filter. A neutron scatter camera would trace the trajectory of detected neutrons over fractions of a second to distinguish weapon-signature neutrons from background neutrons bouncing off Earth.
The simulations suggest the detector would need to observe a target satellite for up to one week to obtain reliable readings. With a closer approach or multiple inspector satellites, the job could be done in hours during a single pass.
“It’s not easy, but we believe that it can be done,” Danagoulian said.
Practical challenges remain. The 4-kilometer (2.5-mile) proximity requirement means the inspector satellite must fly “essentially right next to” the target, as Lewis put it. Such close shadowing could itself be seen as an aggressive act, raising political complications. But Danagoulian noted that his work has received encouragement from national security colleagues on “the other side of the fence.”
“Our hope is that as we are publishing this paper, people who work on classified research can take this and can modify it. Hopefully this leads us to a working solution,” Danagoulian said.
The stakes are substantial. Commercial and military satellite constellations now number in the thousands, and modern civilization depends on them for communications, navigation, weather forecasting, and national security. A single nuclear detonation in orbit could cripple that infrastructure for years.
The Nature paper presents a technical path forward, but the geopolitical will to act remains the larger question. For the first time, however, the technical community has offered a credible answer to a question that has gone unanswered since 1967: how to actually check whether the treaty is being kept.