What Happens if a Nuke Goes Off in Space?

Russia may be planning to put a nuclear weapon in orbit. We have known since the 1960s why that is a bad idea

The auroras over Hawaii on the night of July 8, 1962, were unlike any that humans had ever witnessed. “N-Blast Tonight May Be Dazzling; Good View Likely,” read a headline in the Honolulu Advertiser beforehand. Nine seconds after 11 P.M., a startling flash set the sky aglow like eerie daylight, slowly fading from green to yellow to orange before settling on a vivid, unsettling red.

The U.S. had just detonated a thermonuclear bomb 100 times more powerful than the one dropped on Hiroshima. Launched on a missile from Johnston Atoll, a U.S. unincorporated territory between the Marshall Islands and Hawaii, the bomb exploded at 250 miles above Earth’s surface—around the altitude in low-Earth orbit of most modern-day satellites. This event, called Starfish Prime, wasn’t the first or last time that the U.S. or Soviet Union tested nuclear weapons in space (there were more than a dozen tests between 1958 and 1962), but it was the most impactful. The blast generated a power surge over the Pacific Ocean that knocked out about 300 streetlights on the island of Oahu—and destroyed or damaged about a third of the roughly two dozen satellites then in orbit.

"The Starfish Prime shot is sort of the poster child for why we don’t like nukes blowing up in space,” says Jonathan McDowell, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. Indeed, only a few years later, in 1967, both the U.S. and the Soviet Union signed on to the Outer Space Treaty, which forbade putting weapons of mass destruction in orbit.

Now, some six decades after the last nuclear detonation in Earth orbit, the threat of another has emerged with the Department of Defense warning about a potential Russian program to place a nuke in space. When the United Nations Security Council recently put forward a resolution to reaffirm the ban on such weapons, Russia vetoed the measure. U.S. officials have said there is no “imminent danger” because no warheads are known to be in space. But they have deemed the prospect “deeply troubling” because a nuclear detonation there today would be far more destructive than even Starfish Prime.

On Earth, a nuclear explosion follows a well-documented catastrophic chronology. There’s the initial fireball itself, which can vaporize or burn everything in a wide radius. Then there’s the shock wave from the sudden change in air pressure, which can level buildings and ignite firestorms. And finally there’s the distinctive mushroom-shaped cloud formed from these effects—and the associated atmospheric fallout of deadly radioactive material, which can kill in a matter of minutes or decades.

In space, this explosion looks quite different. There’s no fireball, shockwave or mushroom cloud. Instead a bomb releases all its power as electromagnetic radiation, including gamma rays and x-rays. This unleashes three waves of destruction, explains Victoria Samson, chief director of space security and stability for the space sustainability organization Secure World Foundation. First, “there’s a big flash, and satellites within the line of sight are pretty much immediately taken out of commission by the radiation,” she says.

Then there’s the electromagnetic pulse, or EMP. X-rays from the explosion collide with atoms in the upper atmosphere and release electrons through a process called Compton scattering. These electrons, along with other charged particles from the explosion, run along the lines of Earth’s magnetic field, causing some of the spectacular auroras witnessed across the Pacific during the Starship Prime test.

Depending on a nuclear explosion’s size and altitude, its EMP can wreak havoc on the ground and in orbit, potentially damaging or disrupting unprotected electronics in spacecraft and in devices across a large swath of Earth’s surface.

But it’s the third and most lasting wave of space-based nuclear destruction that could be most devastating—a lingering, globe-girdling belt of high radiation. “It wasn’t so much the prompt EMP that got you. It was this extra radiation dose over months and years that killed a bunch of satellites,” McDowell says. Effectively, nuclear blasts in orbit create an artificial Van Allen belt, or a ring of charged particles that loops out from Earth along its magnetic field. Engineers try to keep satellites out of Van Allen belts if they can help it because the extra doses of radiation shorten spacecraft lifespans. Starfish Prime’s radiation belt lasted years—among its casualties was a satellite called Telstar, launched the day after the detonation, which ceased functioning following months of exposure to radiation levels 100 times higher than normal.

Today there are nearly 10,000 satellites in orbit. Some of the extremely important and expensive ones may be hardened against this kind of radiation. (It is possible but not certain that this includes GPS satellites; those details are kept secret.) Many spacecraft based in low-Earth orbit would likely be taken offline, Samson says, including a significant fraction of SpaceX’s 6,000-plus Starlink satellites in orbit, which, among other things, provide critical high-speed broadband to Ukrainian forces fighting against Russia’s invasion.

And depending on a blast’s location and intensity, people on the International Space Station (ISS), as well as on China’s Tiangong habitat, might be in danger. An EMP could knock out critical electronic systems on these orbital outposts, leaving their crews ill-equipped to navigate through a minefield of dead, drifting satellites. Even if no hardware failures occurred, the radiation exposure itself “could limit [crew] safety to a matter of hours or days,” Samson says, citing simulations from a 2010 Department of Defense report.

Most chillingly, although electronics can be shielded and Earth’s atmosphere would block most harmful radiation from reaching the ground, everyone on the planet would still be threatened by any orbital blast’s potential to escalate tensions and trigger a global thermonuclear war.

It is currently uncertain whether Russia’s pursuit of a space-based nuke is actually a serious project. “What’s not clear is [whether] this is [just some] PowerPoint by some general in the Strategic Rocket Forces or a seriously funded program,” McDowell says. And it may exist only as a scare tactic, without any real intention of being used, because of its apocalyptic implications. “I’m skeptical that [the Russian government has] a serious operational plan to fire nukes in a conflict in space,” he says.

Even so, Samson notes that the U.S. government seems to be taking this threat quite seriously. “I believe [that the] U.S. government honestly believes that there’s something that the Russians are working on,” she says.

Russia’s power in space has waned since the cold war, supplanted by countries such as China and the U.S., which now have strong commercial space programs. Its main foothold is as an enabling partner on the ISS, which is set to be decommissioned by 2031.

“[Russia doesn’t] have a strong civil space program anymore. And the one thing it has that’s tying it to the international community is going away in a few years,” Samson says. But the country is still a leader in counterspace weapons and operations that could damage other nations’ space capabilities. “That’s where it’s still maintaining its legacy from the cold war,” she says.

Still, while countries jam, temporarily disable and otherwise interfere with one another’s satellites all the time, destroying satellites would be “incredibly escalatory,” Samson says. “That’s never been done, and I think that’d be a huge red line to cross.”

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