Last month, several news outlets reported that Russia could be planning to deploy a space-based nuclear weapon, alarming, well, pretty much everyone.

US policy hawks, space environmentalists, and anyone with a lingering memory of Cold War-era fears over nuclear annihilation were all sounding the alarm about the threat posed by a Russian nuke in space. 

As scary as the prospects sound, the US government has assured people that the weapon doesn’t necessarily pose a threat to people on the ground. Instead, it would target other objects in space, like the satellites used by the US military for communications and other operations.

But that struck some as cold comfort, especially given Russian President Vladimir Putin’s unpredictability. And Putin has indicated that putting a nuclear power unit in space is a priority for the country.

In the long term, defense experts warn that having a nuclear weapon positioned in space could pose a threat to life on Earth by eroding international relations and space law. From clouds of space debris that could cut off access to space to the development of weapons that could launch from space to hit targets on the ground, space-based nukes have the potential to impact everything — and everyone. 

No country has ever used an anti-satellite weapon against another country, but several countries have destroyed their own satellites in demonstrations of their military capabilities — including the US, Russia, China, and India. 

These tests are not without controversy: a 2021 Russian test of an anti-satellite weapon, for example, drew condemnation from NASA for creating debris that threatened astronauts on the International Space Station (including Russian cosmonauts). Since then, a UN panel has called for a ban on the testing of such weapons and several European Union nations and the US have pledged not to perform destructive tests. 

A nuclear weapon in space would cause much more destruction than previous anti-satellite weapons tests, explained Andrew Reddie of the Berkeley Risk and Security Lab, as existing space-based weapons typically destroy just one satellite at a time. In the age of huge satellite constellations such as Starlink, knocking out a single satellite is more of an annoyance than a major threat.

To destroy satellites at scale, you need a different weapon, such as a directed energy weapon based on the ground. Or, you could use a nuclear weapon in space, which creates not only shock effects but also heat, radiation, and an electromagnetic pulse — giving it the ability to take out or impair entire networks. 

The best response the international community has had to date in restricting the stockpiling and use of nuclear weapons is international law. When it comes to space, the key piece of legislation is the Outer Space Treaty of 1967, of which Article IV prohibits placing nuclear weapons or other weapons of mass destruction in orbit.

Detonating a weapon in space would be unprecedented and could run afoul of international rules barring the use of indiscriminate weapons on civilians or civilian objects.

“It seems to be that any kind of destruction of something in space is an indiscriminate weapon, and indiscriminate weapons are prohibited, and the use of indiscriminate weapons are a war crime,” said Christopher Johnson, professor of law at Georgetown University.

However, this assumes that satellites are being destroyed by a kinetic impact. It might be possible to disable or jam satellites in another way, such as using an electromagnetic pulse, or EMP. Some reports have suggested that Russia is developing an EMP anti-satellite weapon rather than a nuclear one. If that could be done in a way that doesn’t create a debris field, that may not contravene the international law because it would no longer be a weapon of mass destruction or indiscriminate in its effects.

With the current situation, “We don’t know what is being threatened,” Johnson said and pointed out that the details matter a lot here and that Russia is capable of a very close reading of the relevant laws to stay within them. 

The reason that the use of weapons in space could be considered indiscriminate is because of the debris field they create. Destruction of objects in space creates large pieces of debris, which are hazardous but relatively easy to track. Where it gets dangerous is the increasing number of medium and small pieces of debris, which are too small to be trackable but are still traveling at high enough speeds to do tremendous damage to other objects or even people in space.

“A fleck of paint the size of your thumbnail can go through most spacecraft. Traveling at a very high velocity — 18,000 mph — it’ll go right through it,” said space debris expert Vishnu Reddy of the University of Arizona. 

A serious collision in orbit could create a field of small debris pieces that would quickly collide with other satellites, creating a cascade. At a critical mass, each collision creates more debris, which creates more collisions, which creates more debris, until an entire orbit becomes difficult or impossible to access. 

This scenario, known as the Kessler syndrome, could cut off access to space for generations: from making rocket launches more difficult, dangerous, and expensive to, at worst, making any kind of space travel completely impossible for decades and shutting humanity off from the stars.

This concept of the syndrome was first proposed in the late 1970s, when there were optimistic predictions that the Space Shuttle might fly as often as once per week. That never came to fruition, so in the intervening decades, there was less concern about the possibility of a cascading debris event.

But now, with the pace of both government and private launches ramping up to the highest levels ever, space debris is once again on everyone’s radar, Reddy said: “The old fear has come back.”

The most useful orbits around the planet are getting increasingly crowded, and even if humanity stopped launching things into space tomorrow, the debris already in orbit would continue to collide and make the problem worse. 

Over the long term, if this problem isn’t addressed, it could spiral into a Kessler syndrome, as the situation can go from bad to catastrophic quickly. “The timeline for the cascading collisional scenario is very short,” Reddy said. “We’re talking anywhere from hours to days to weeks, not months to years to decades.”

The use of a nuclear weapon in orbit, depending on its size and in which orbit it is detonated, could kick off such a cascading scenario. But this isn’t exclusive to nuclear weapons. It’s possible that a bad actor destroying a single, carefully chosen satellite could create a cascade, Reddy said, if they picked a vulnerable target. 

In geostationary orbit, for example, there are only so many slots available for satellites in the ring around the Earth’s equator. That makes the slots in high demand, as they are a limited resource. And this scarcity is compounded by the fact that it’s very difficult to remove debris from an orbit so distant, at over 20,000 miles from the Earth’s surface. If these slots are blocked by debris, it could cut off functionality for systems like communications satellites, weather satellites, and navigation satellites. 

“That would be really, really bad,” Reddy said. “One satellite explosion big enough would be enough to destroy a lot of assets in geostationary orbit.”

Although it’s unlikely that any actor would launch a nuclear weapon in space with the specific intention of kicking off a cascading debris effect, it might happen as a consequence of trying to destroy a particular military system. But the debris isn’t the only thing that has experts worried.

Security risk expert Andrew Reddie questioned what it would take to convert the technology for a nuclear anti-satellite weapon into a platform that could deploy nuclear weapons from space to targets on the ground. This would require a reentry vehicle, for example, which doesn’t exist yet but could theoretically be constructed based on existing technology. Nukes launched from space would give less warning time than those launched from the surface, threatening thousands or even millions of people.

It’s not that the deployment of nukes in space is necessarily likely, with no current indication that Russia is developing such a weapon. But it does show how nuclear weapons in space could shift the geopolitical landscape dramatically and why reports of potential space-based nuclear weapons have drawn such condemnation.

Russian President Vladimir Putin has denied any plans to develop a nuclear anti-satellite weapon and has said that Russia is against the deployment of nuclear weapons in space. And experts agree that Russia takes pride both in its space program and in its role in international governance as a permanent member of the United Nations, though the invasion of Ukraine has shaken the country’s international status and resulted in the suspension of joint space missions with other space agencies. 

For the Russians to develop or deploy such an anti-satellite weapon “would undermine their diplomatic efforts,” Johnson said. Russia has a global leadership role in space governance and was a key negotiator in the Outer Space Treaty, and going against that would be self-undermining. “They take their role seriously,” Johnson said.

There is also international pressure from beyond the US and Europe. Even China, which has a space program that is notably separate from other nation’s space programs and does not participate in international projects like the International Space Station, has emphasized that it is against the proliferation of weapons in space. US government representatives are trying to recruit China and India in discouraging Russia from pursuing nuclear anti-satellite technology. 

Deploying a weapon in space would be against Russia’s own self-interest, experts argue. Spreading a debris field across an entire orbit limits the ability of everyone to access space, including those who fired the weapon.

However, those effects are not necessarily symmetrical. “The Americans rely on space far more than both Russia and China, so in most domains, if you were to degrade it for everybody, that would be a problem,” Reddie said. “But if you’re degrading space, it’s going to asymmetrically affect the Americans. And the Russians know that.”

This raises the question of what the global consequences might be if — or when — any nation chooses to use a space-based weapon and whether the existing international legal structure could respond to that.

Space debris expert Reddy compared firing such a weapon to flipping a chess board when you’re losing a game: “It’s no longer about winning. It’s ‘I’m losing, so nobody wins.’”