It claimed that Russia was thinking of putting a nuclear weapon in orbit around Earth.

Soon after, suspicions fell on Kosmos-2553. This Russian satellite, launched just a few weeks before the invasion of Ukraine in 2022, was placed on a highly unusual trajectory 1,300 miles above the planet’s surface — a region home sparsely populated by other, largely defunct, satellites.

Several experts opined that although it didn’t have a nuclear bomb on board, Kosmos-2553 was testing technology designed to be used for a space-based nuclear weapons system.

At the time, Russia’s president denied that he had any schemes to place a nuclear weapon in space, and there was some debate between intelligence agencies as to the true nature of the satellite. Either way, this development caused a great deal of geopolitical anxiety.

Although a nuclear detonation in Earth’s orbit wouldn’t directly harm anyone down below, it would severely degrade or take out thousands of satellites. Such a weapon, then, represents another version of a nuclear deterrent, a self-destructive, pyrrhic tool that would only be used out of sheer desperation.

But if Russia (or any other nuclear-armed nation for that matter) smuggled a nuclear bomb into orbit using an innocuous-looking satellite, how would we know for sure?

A new study published this month in Nature offers a potential solution. If a uranium-powered bomb is situated at a similar altitude to Russia's suspicious Kosmos-2553 satellite, it would interact with Earth’s natural radiation belts, causing it to cough out energetic particles. The study calculates that, if detectors were placed on a CubeSat and flown close enough to the suspected nuke, it could detect those telltale particles escaping from the concealed uranium.

This idea is entirely theoretical; there are currently no plans to build a fleet of nuke-sniffing small satellites. But the science behind the concept is, according to outside experts, sound.

“My reaction is cautious optimism,” says Angela Di Fulvio, a nuclear scientist at the University of Illinois Urbana-Champaign and who wasn’t involved with the new paper. Concealing a nuclear weapon in space would be deeply troubling. But for those hoping to deter nations from doing just that, this research suggests that, ironically, “space itself may help.”

As you might expect, placing nuclear weapons in space is illegal, per the United Nations’ 1967 Outer Space Treaty. There are myriad sensible reasons for this, including the fact that a nuclear detonation in Earth’s orbit could trigger a satellite apocalypse. Any satellites nearby would be severely damaged, likely fatally so, by the bomb’s eruption of X-rays and gamma rays. But the peril remains several weeks or months after the blast has faded away: its unleashed radiation would continue to around Earth’s magnetic belts, splashing onto countless other satellites and chipping away at their solar panels, communication arrays and internal electronics.

Satellites aren’t (usually) designed to withstand such a radiative onslaught, and many of those at a similar altitude to the detonation would, in time, be rendered functionally dead. “A nuclear detonation in orbit would be a very crude but effective anti-satellite weapon,” says Victoria Samson, chief director of space security and stability for the U.S.-based Secure World Foundation.

In 1962, the U.S. test-fired a nuclear device 250 miles above the Pacific Ocean. The experiment, known as Starfish Prime, caused an eldritch aurora, some localized blackouts in Hawai‘i, and damaged or destroyed about one-third of all satellites in low-Earth orbit — which tops out at around 1,250 miles altitude.

Compared to the 1960s, when only a smattering of satellites existed, low-Earth orbit today is tremendously crowded. It’s home to 10,000 (and counting) Starlink satellites, which have, to Russia’s ire, been used by war-torn Ukraine to coordinate military operations. It’s also replete with many weather and climate satellites, plenty of spy satellites, the Hubble Space Telescope, and both the International Space Station and China’s Tiangong crewed outpost.

Understandably, when the Biden administration accused Russia in 2024 of plotting to violate the Outer Space Treaty, it triggered a furore. “The United States still believes that Russia is working on a nuclear warhead that could be placed in space as an anti-satellite weapon,” says Samson.

A curious plot twist emerged last year, when Kosmos-2553, the satellite alleged to be assisting in these plans, was reported tumbling in space. This suggested that Russia had lost control of it. If so, this would mark a setback for Russia’s orbital plans — but it doesn’t mean the purported program has been terminated. So if Russia did deploy a nuclear weapon at low-Earth orbit, how could it be definitively identified?

Areg Danagoulian, a nuclear science and security researcher at the Massachusetts Institute of Technology, has given it some thought. In his new, sole-authored Nature paper, he has spelled out a way to accomplish this. (At the time of writing, Danagoulian did not respond to a request for comment.)

The paper imagines that a nuclear bomb is hidden in a satellite at 1,250 miles, at the upper boundary of low-Earth orbit. In this region, Earth’s magnetic field traps high-energy electrons and protons. When these protons interact with heavy nuclei, for example, those of uranium and plutonium, the most common fissile elements in nuclear weapons, this can trigger a process known as nuclear spallation.

This is where a heavy atomic nucleus is struck by a high-energy particle (like a proton), causing it to break apart and emit several smaller particles. Those particles can include, among others, neutrons. Assuming the nuclear warhead contains a roughly 210-pound block of uranium, calculations show that nuclear spallation at this altitude would produce 30 million neutrons per second.

But if you managed to park a CubeSat inspector satellite orbiting 2.5 miles below the suspect satellite, the study estimates that it would be able to detect these neutrons by using a plastic scintillator, one that produces a flash of light when hit by radiation.

This alone wouldn’t offer an unequivocal detection of a hidden nuclear weapon. “The biggest technological hurdle is separating the signal we care about from everything else happening in space,” says Di Fulvio. A nuclear weapon’s emitted neutrons may not be the only thing that causes the CubeSat’s scintillator to spark up. It may also do so when it encounters other energetic particles coming from the natural radiation maelstrom around Earth.

One way around this would be to add thin layers of diamond to the scintillators. High-energy neutrons would pass straight through this layer and slam into the scintillator. On the other hand, high-energy protons are likely to hit and illuminate both the diamond layer and the scintillator.

Another potential issue is that some high-energy neutrons would come from far-flung cosmic rays and the sun’s own particles crashing into Earth’s atmosphere. But, according to the study, the CubeSat should be able to determine which direction these particles are being sourced from.

It’s also possible that if a nuclear weapon is placed in Earth’s orbit, it may be at a different altitude and be of a different design to that suggested in the study. But, at least on paper, a CubeSat tracking a weaponized satellite from below could provide an unambiguous detection of a clandestine nuclear weapon after roughly a week’s worth of reconnaissance.

But providing in-situ evidence of an illegal nuclear warhead may have a more potent effect. “Making it publicly known that a nuclear warhead in space can be detected might also deter other countries from attempting to do so as well,” says Samson.

Those not involved with the study were intrigued by Danagoulian’s idea of how to verify the Outer Space Treaty in Earth’s orbit. But making this a reality would be easier said than done.

“The study assumes an inspector satellite that can stay close enough to the suspect object and maintain a useful viewing geometry over time,” says Di Fulvio. “That may require more propulsion and control capability than a simple CubeSat can provide.”

Even if it were agile enough to perform these acrobatics, having to position itself just 2.5 miles away from the target satellite may blow its cover. “This is the kind of maneuver that the Russians would be able to see,” says Juliana Süß, a security and defense policy expert at the German Institute for International and Security Affairs.

Russia probably wouldn’t react well to this kind of orbital encounter. Ideally, those overseeing the inspector satellite on its mission would be from a neutral body — something akin to the International Atomic Energy Agency, perhaps. But as Süß points out, Russia is pretty much the only nation that would want to place a nuclear weapon in space, so if the United Nations announced plans to launch an inspector satellite, Russia would immediately feel targeted, no matter who was technically in charge of the endeavor.

So will we ever see a squadron of CubeSats skulking about in Earth’s orbit, trying to make sure everyone is adhering to the Outer Space Treaty? It’s difficult to say, and even if this does come to pass, the degree to which it will act as a disincentive to belligerent nations is unclear.

For the time being, says Süß, “the biggest deterrent is China and not wanting to — to use a technical term — piss them off.” China’s developed a sophisticated space program over the last couple of decades, which includes a proliferating array of satellites. “China would not be amused, to say the least, if Russia was suddenly like, by the way, we’ve put a nuclear bomb in orbit.”