The U.S. says Russia is building a nuclear weapon for space. But how would you prove there’s a warhead inside a satellite 1,200 miles up?
Nuclear explosions in space, conducted by the United States in the 1960s, produced brilliant auroras and knocked out power on Earth. If a similar experiment were carried out in orbit today, we would lose navigation, communications, and weather forecasting for a long time. Space is now densely populated with satellites that would not survive the electromagnetic pulse and particle streams from a nuclear explosion. But U.S. officials are concerned that the Russian military is considering such a strike against orbital constellations — which give the Ukrainian army an advantage — and is even launching spacecraft to test components of a future “nuclear mine.” How real is this threat? And has anyone actually put weapons in orbit? Areg Danagoulian, an American physicist, has set out to answer these questions.
The problem of finding nuclear weapons: Why is it so hard to find a bomb on Earth?
No one can detonate a nuclear bomb, even in the most remote corner of the globe, without being detected. The global network of monitoring stations run by the Comprehensive Nuclear-Test-Ban Treaty Organization — seismic, hydroacoustic, infrasound, and radionuclide — would pick up such an event immediately. In the absence of nuclear tests, those same stations track the atmospheric explosions of cosmic bodies, such as the Chelyabinsk meteor.
Detecting a nuclear warhead ready for use — or its components — is far more difficult. Weapons-grade uranium and plutonium are radioactive, emitting neutrons and gamma radiation. But that activity is very low. At just 10 to 20 meters (about 30 to 65 feet) — depending on whether the device is plutonium- or uranium-based — the radiation it gives off is barely distinguishable from the natural background. A consumer-grade dosimeter is of little use here. It reads only the overall background, cannot tell gamma rays from neutron flux, and is easily thrown off by other radiation sources, such as granite.
Detecting nuclear weapons therefore requires complex, specialized detectors: instruments that can build gamma-ray spectra with high precision and measure neutron energies and flux. That is what lets them tell a uranium- or plutonium-based warhead from something else entirely.
Even those are not always sufficient. In 2002, journalists from ABC News smuggled 15 pounds of depleted uranium — which, when shielded, gives off a radiation signature much like that of the highly enriched uranium used in bombs — past customs at the Port of New York, hidden in a shipping container. Customs officers flagged the container as high-risk and screened it anyway. Their equipment missed the uranium.
A more reliable approach uses active detectors, which emit a stream of neutrons or gamma radiation of their own. The neutrons “split” uranium or plutonium nuclei and generate new neutrons, revealing the presence of fissile material. If an object bombarded with neutrons throws back a powerful neutron flux, it is a nuclear device.
And did this method ever actually detect nuclear weapons?
In July 1989, Soviet and American physicists conducted a joint field experiment to detect a nuclear warhead aboard the Soviet cruiser Slava — later renamed Moskva, and now at the bottom of the Black Sea.
American specialists from the Natural Resources Defense Council used a high-resolution semiconductor detector built around a high-purity germanium crystal. It was placed on the ship beside the missile, three meters (about 10 feet) from the lid of its launch tube.
Soviet scientists from the Kurchatov Institute attempted to detect the warhead from a helicopter that circled the cruiser at 30 to 80 meters (about 100 to 260 feet). The helicopter carried the Sovetnik (“Adviser”) system, which used helium-3 neutron sensors.
The American detector found that the warhead contained little uranium-238 — no more than 4% — and some uranium-232, which indicated a reactor origin. The Soviet device detected neutrons coming from the warhead from 76 meters (about 250 feet). The experiment showed that nuclear warheads on sea-launched cruise missiles could be detected — which meant a strategic arms reduction treaty, the future START, could be verified.
If detecting a bomb on Earth is already difficult, is it even harder to do so in space?
Yes. Article IV of the Outer Space Treaty prohibits placing nuclear weapons in outer space or installing them on celestial bodies. But verifying compliance is much harder than for the test-ban treaty.
If it takes sophisticated equipment to find a bomb sitting a few dozen meters away, finding one aboard a satellite hundreds or thousands of kilometers up is harder still. And not only because of the distance: the faint radiation of a nuclear warhead drowns easily in the cosmic radiation background.
Areg Danagoulian, a physicist who specializes in nuclear arms control at the Massachusetts Institute of Technology, is one of the people working on it. In an interview with the Nature podcast, Danagoulian said the United States needs reliable ways to detect weapons aboard spacecraft — at a moment when suspicion among nuclear powers runs high and accusations of “militarizing” space are getting more frequent.
In theory, he said, a small inspector satellite could be sent toward a suspicious space object and, once alongside, illuminate it with a stream of particles or radiation to trigger a process that could reveal whether nuclear weapons are on board. “But this is a fairly hostile act. The other side might think you are trying to destroy their satellite,” Danagoulian said.
His solution is to let nature do the probing: detect the secondary particles that natural particles from space knock loose inside a nuclear device. In that case, the country that launched the suspicious object would have no grounds to accuse anyone of an attack.
Danagoulian proposed using natural protons with energies above 750 million electron volts as the source of that external particle stream.
These protons have a complex origin. High-energy cosmic particles from beyond the solar system — known as galactic cosmic rays — collide with atoms in the upper atmosphere and produce extensive air showers consisting of a wide variety of particles: mesons, muons, and neutrons. Most rain down toward Earth, but some go up into the radiation belts, where the neutrons decay into protons and electrons.
When such a proton with an energy of around a billion electron volts encounters the nucleus of a heavy element — uranium or plutonium, for example — a reaction known as spallation occurs: protons and neutrons are knocked out of the nucleus all at once. Protons, because they carry an electric charge, are slowed by matter, while neutrons continue traveling. Physicists use the same process to generate neutron beams in their own labs.
“One proton with the right energy can produce, depending on conditions, anywhere from 10 to 14 neutrons,” Danagoulian said.
To tell whether a suspicious spacecraft carries a nuclear warhead, then, you only have to detect the characteristic neutron flux coming off it.
Danagoulian’s proposal: how does a natural nuclear inspector work?
According to Danagoulian’s calculations, a detector satellite could be a small CubeSat in the 9U format — nine 10-centimeter (about 4-inch) cubes stacked together.
The detector itself consists of two panels measuring 30 by 30 centimeters (about 12 by 12 inches), separated by a 10-centimeter (about 4-inch) gap. The panels are made of plastic scintillator — any substance that gives off a flash of light when a high-energy particle hits it, charged or not. The scintillator is covered on both sides by plates of single-crystal diamond. Diamond, by contrast, responds almost exclusively to charged particles: they ionize carbon atoms, freeing electrons and leaving holes in the plate, and a current begins to flow.
This configuration makes it possible to distinguish charged particles from neutral ones automatically: if only the scintillator fires, a neutron has entered the detector; if both the scintillator and the diamond plate fire, the particle is charged, and those data are discarded.
The two panels allow the detector to work out which direction a neutron came from, separating particles that came off the “suspect” from stray ones.
To tell whether a nuclear weapon is aboard a satellite, Danagoulian’s inspector would need to remain within four kilometers (about 2.5 miles) of the “suspect” for about a week, analyzing its neutron flux. Close the gap to one kilometer (about half a mile), and the wait drops to an hour.
Ideally, the inspector would sit directly between the “suspect” and Earth — the easiest position from which to subtract the neutron flux coming off Earth’s atmosphere.
All of Danagoulian’s calculations about the radiation environment in orbit and the feasibility of neutron detection were based on data from the orbit of the Russian spacecraft Kosmos-2553 — the satellite U.S. officials say Russia is using to test components of a space-based nuclear weapon.
How would the ‘suspect’ spacecraft respond to such an inspection?
A nuclear inspector would need to maneuver precisely, catch up with potential targets, and hold a stable position at close range — by orbital standards — for long stretches.
Danagoulian cited a long list of cases in which satellites from one country have approached spacecraft from another, including the Russian Luch spacecraft that trailed European satellites, as Meduza has reported. He said such maneuvers have not led to serious political crises.
Inspector-satellite activity has, though, repeatedly provoked sharp statements and accusations from the United States that Russia is militarizing space.
And a “suspect” spacecraft is under no obligation to wait while the inspector accumulates enough neutrons — it might simply try to slip away. Chases like that have already happened in low Earth orbit.
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