Nuclear space-based ASAT weapons – A brief international legal perspective

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On 14th February 2024, US and UK media reported the emergence of serious national security concerns by senior American officials and lawmakers that the Russian state was pursuing the deployment of a nuclear-based weapon designed to eliminate (enemy) satellites. It was added that such a weapon had not yet been deployed, but had reached some stage of development.

The exact severity of the development remains hotly contested (also here) and presently unknown, but was described as a ‘destabilising foreign military capability’ to the US and its allies. There is speculation that it may be a political ploy to further pressure the US house to authorise further aid to Ukraine in its conflict against Russia. There has also been speculation regarding a critically-important ambiguity in the information released – is it a nuclear weapon, or merely a nuclear-powered weapon? The latter would be far less concerning, or destructive.

Yet, the White House NSC strategic communications coordinator, John Kerby, stated on February 15th, without any hesitation whatsoever, that the weapon, if deployed, would be space based, and a violation of the Outer Space Treaty’. Assuming this is true, and though not definitive evidence, the fact that no other weapons are prohibited by the Outer Space Treaty except for nuclear weapons/WMDs (see below) means that we have to consider the nuclear weapon scenario. The scenario for a weapon to be so definitively contrary to the treaty, and not nuclear, is small. Even if true in part, it would mark a frightening turning point in the decades-long consensus regarding nuclear-based weapons in space; and yet another compounding issue for an international legal order stretched to its limits.

As a result, this blogpost will divulge the basics of the legal framework governing nuclear weapons in space, contextualise it through a brief history of this framework, and address how this interacts with contemporary space-based (legal) challenges – including sustainability issues.

How such weapons may work

It seems highly likely that the intent of such a space-based nuclear explosion would be to knock out satellites through the emission of a powerful Electromagnetic Pulse (EMP) as a consequence of the nuclear detonation: rendering all electronics caught in the (potentially enormous) area – and thus also any satellites – damaged or outright inoperable. These effects can linger for years, forming an artificial radiation belt around earth.

One of the last nuclear tests in outer space, the American ‘starfish prime’, produced an EMP so (unexpectedly) powerful that it damaged all electronics within a ~900 mile / ~1450 kilometre radius – including on the ground in Hawaii. That explosion had a large yield of approximately 1.4 megatons. Its resulting electromagnetic effects carried by the planet’s magnetic field, flattening them into a radioactive belt, almost immediately rendered three satellites in Low Earth Orbit (LEO) inoperable, and ultimately destroyed eight in the months that followed due to lingering radiation in orbit. Not many in absolute terms – but roughly a third of those deployed in 1962; at time of writing, there are approximately 8,300. This was not even a weapon designed to produce as large an EMP as possible, nor was it calibrated to target specific orbits or regions – both things a new weapon might do. There is little doubt that deployment of such an EMP through a nuclear medium would be a highly destructive weapon.

Some observers have pointed out that use of such a weapon would damage Russia’s (or any nuclear-ASAT deploying nation’s) own interests; its own space-based infrastructure – with the implication being that they would thus never actually use it. Given the events of the past several years, however, can we really rely on the assumption that a state will not – even severely – damage itself to prevent another state from achieving/acquiring/joining something? No. Absolutely not.

Tearing up the nuclear-space consensus

It is pertinent to distinguish between the use/deployment of a nuclear weapon in space, and the mere placing of such a weapon into outer space. Both are prohibited through different instruments.

Use of such a weapon in space is, flatly, prohibited. The most obvious legal violation would be against the “1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water (otherwise known as the ‘Partial test-ban treaty’), to which Russia is (and for now remains) a founding party. It is a short treaty with one operable article, created immediately after the Cuban Missile Crisis of 1962. It simply states the following:

Article 1

1. Each of the Parties to this Treaty undertakes to prohibit, to prevent, and not to carry out any nuclear weapon test explosion, or any other nuclear explosion, at any place under its jurisdiction or control:
           a. in the atmosphere; beyond its limits, including outer space […]

The provision includes no further qualification. For now Russia remains a party to this treaty, though has pulled out of similar cold-war era treaties in recent years given the restrictions they place on military operations.

Further, the mere placement of such a weapon into space is clearly prohibited by international space law. The Outer Space Treaty is sometimes known as the ‘Space Constitution’, and is the most fundamental treaty governing outer space activities. All major space powers, including the US, Russia, China, Japan, and leading European states, are party to it. It plainly states:

Article IV

States Parties to the Treaty undertake not to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner.


These provisions are rather simple as international legal provisions go. They reflect an agreement between the Soviet Union and the US established after repeated nuclear crises in the 1960s that the use of nuclear weapons must be limited as far as possible – including in outer space.

Desecration of the space environment

In addition to the above, there is also (as is so common when talking about space-based issues) the issue of the debris it will cause.  Dead, inoperable satellites-turned-debris cannot manoeuvre to avoid orbital collisions with debris. They will remain in orbit, as debris, for potentially indefinitely depending on their altitude. While there is no binding international law or custom (yet) that prohibits the production of space debris, it could certainly be argued that the desecration of the space environment in this manner violates the Outer Space Treaty, Article IX of which prohibits the ‘contamination’ of the space environment and its celestial bodies.

We also know from the long-lasting consequences of Starfish Prime that, depending on the size and calibration of the weapon, entire regions of Low Earth Orbit could be rendered (near-) uninhabitable zones for satellites – or indeed any electronic systems – for potentially several years. This is strikingly similar, in terms of severity and also duration, with a Kessler Syndrome/Cascade; the worst case scenario following the continuous build-up of conventional space debris multiple decades from now. Entire regions of LEO could be rendered inoperable for satellites post-deployment of such a weapon, without considering the more immediate damage it would cause.

The Legality of the Threat or Use of Nuclear Weapons Advisory Opinion, though inconclusive in its final determination of the legality of a nuclear weapon’s use, does make clear that the long-term, and widespread nature of nuclear effects may well be in violation of international humanitarian law. The Court did not apply its reasoning to the realm of space specifically. However, it is worth noting that the effects of rendering regions of space uninhabitable is notably far more widespread than the effects of a typical nuclear weapon detonated on the ground (actually a little above) – whereas the latter may affect an area of hundreds of square kilometres, the former may affect an area of hundreds of thousands of cubic kilometres. Whether this changes the proportionality equation (preserved by the Court in relation to an extreme circumstance of self-defence) is perhaps worth further thought.

Further consequences

There has not been a known nuclear deployment in space since the 60s. Since then, the Partial Nuclear Test Ban and Outer Space Treaties have held firm, albeit with a couple of allegations of relatively minor primarily procedural breaches by various parties, from which no public consequence followed. The mere deployment of such weapons may therefore tear up over 55 years of legal and geopolitical consensus, and throw astropolitics into a new era of threat – one where, as is increasingly common in many areas, international rules give way to military and strategic interests.

It is far from clear whether or not the US or its allies have capabilities to stop such a weapon from being deployed should it reach space. In theory, conventional ASAT weapons (also heavily restricted) might be able to take them out before deployment, but this would of course amount to direct military engagement at a time where military intervention could have explosive consequences. Thus, once there, such  a weapon would hang in orbit like a Sword of Damocles ready to swing – potentially crippling parts of civil (or even military) infrastructure for a time if used – and causing significant military, economic and societal damage. Given that a nation could send a nuclear weapon into space atop a ballistic missile if it wanted to, the insecurity and fear of having a nuclear weapon orbiting above a country at any point might even be its primary destructive power. They could also be extremely difficult to detect/verify as there in the first place – unlike a ballistic launch.

Concluding remarks

If Russia, or any nation, were to do this, there is no doubt that the placement, or detonation, of nuclear-based weapons in outer space would amount to a flagrant violation of several enduring international treaties. A great deal remains unknown, and seemingly contradictory. It is nonetheless important (also for international lawyers) to prepare for all scenarios – including purportedly foolish, self-harming ones. Russia has already harmed its (and all of our) space environment via conventional ASAT testing.

This further raises the question about whether Russia may withdraw from further nuclear-restricting treaties to do this, as has been hypothesised. It already has with several. The departure of one of the most prominent space-faring nations from the Outer Space Treaty would be legally monumental at a critical juncture in the development of international space law. It would further reflect yet more dismantling of the most basic rules of the ‘rules-based international order’.

Finally, this threat – the ambiguity of its imminence and its true existence aside – highlights the importance of the defence of space-based infrastructure upon which so much of the modern world – and modern military – relies. The vulnerability of these systems is now plain for all to see, and may perhaps induce concrete action to mitigate their risks. The threat of an outer-space arms race remains firmly on the table as a result of these developments, to the detriment of global safety, and especially to outer-space activities.

Image: An artificial aurora in the sky above Hawaii – produced by high-energy electromagnetic radiation resulting from the detonation of the US ‘Starfish Prime’ nuclear test in 1962.

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