Outer space is becoming an arena for technological shows of force – whether by deployment of spy satellites or testing of weapons. What does international space law have to say on the militarization of space? In this three-post series, Pavle Kilibarda attempts a broader interpretation of the norms, one that would lead to a more pacifist reading of the law. The triad supplements Intercross’ series Why Outer Space Matters, which came out of the October 2016 Inaugural Plenary of the MILAMOS Project in Montreal, Canada.

The most widely ratified instrument of international space law, the Outer Space Treaty (OST), establishes a simple yet comprehensive regime limiting the use of weapons of mass destruction (WMDs) in or from outer space. Apart from the fact that nuclear weapons are mentioned specifically, space law does not define WMDs for its purposes, wherefore the term should be taken in the broadest possible sense.

Art. IV (1) of the OST states that States Parties “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 manner.” Before analyzing the substantive meaning of this provision, it is necessary to point out that, while there is no definition of WMDs under international law, it is generally taken to include nuclear, chemical, radiological and biological weapons. Bearing in mind that chemical and biological weapons are prohibited as a matter of international humanitarian law (IHL), and that the use of radiological weapons remains hypothetical for the time being, the content of the norm is principally relevant with regard to the use of nuclear weapons, for which no such prohibition currently exists.

An incomplete prohibition

The terms applied by Art. IV of the OST, “place, install or station”, seem to indicate both an element of permanence to the State Party’s activity involving WMDs, but also an element of purpose. The latter point means that this provision would not prohibit engaging in activities which may potentially have the same disastrous consequences as the deliberate use of WMDs, such as using nuclear-powered spacecraft (although there is a UN General Assembly resolution concerning these issues); with respect to the permanency requirement, the provision contains no term which would cover simple use in the broadest sense, which means that the OST neither prohibits the use of WMDs against objects located in outer space, nor indeed would it prevent, in any manner, the launch of an ICBM carrying such a weapon from one point on the surface of the Earth to another (bearing in mind that the trajectory of some of these missiles may cross what is usually considered the altitude at which outer space begins) as long as the ICBM path does not amount to “in orbit around the Earth”.

It should, however, be borne in mind that Art. IV (2) of the OST foresees that the Moon and other celestial bodies be used “exclusively for peaceful purposes” and prohibits any military activity there save for those where military personnel or facilities are used for peaceful purposes; provided that “peaceful” means more than just “lawful”, this further implies that any manner of attack conducted against persons or objects on them would run contrary to the OST – including, of course, by WMDs. The 1979 Moon Agreement further clarifies this prohibition, stating unambiguously that “States Parties shall not place in orbit around or other trajectory to or around the Moon objects carrying nuclear weapons or any other kinds of weapons of mass destruction or place or use such weapons on or in the Moon” (note that, as foreseen by Art. 1 (1), where the Moon Agreement uses the term “Moon”, it refers to all celestial bodies within the Solar System, other than the Earth). In spite of the fact that the Moon Agreement has not enjoyed widespread acceptance, particularly not among spacefaring nations, the provision detailing the WMD regime on celestial bodies, has never really been contested, with the Moon Agreement’s potentially limiting influence on development in space generally cited as the principal reason for States’ refusal to sign and ratify it.

Bearing in mind the above, a discussion of WMDs in space, for all practical intents and purposes, comes down to an analysis of the legal regime governing the use of nuclear weapons there. Space law is quite clear that they may not be positioned in outer space, nor that they may be installed upon celestial bodies; on the other hand, barring a general prohibition on the militarization or weaponization of space, the simple fact that an object carrying a WMD traverses outer space does not appear to be prohibited by the present state of the law. The remaining question is whether there is any norm in international law prohibiting the use of nuclear weapons against a space object?

The use of nuclear weapons against space objects

Although space law treaties as such remain silent on the matter, there is another treaty that could prove very relevant in this regard: the 1963 Partial Test Ban Treaty. The Treaty, inter alia, prohibits the carrying out of “any nuclear weapon test explosion, or any other nuclear explosion” in “the atmosphere; beyond its limits, including outer space; or under water, including territorial waters or high seas”; furthermore, the States Parties undertake to “refrain from causing, encouraging, or in any way participating in, the carrying out of any nuclear weapon test explosion, or any other nuclear explosion, anywhere which would take place in any of the environments described” (Art. 1).

There are two problems that may arise from said application of the Treaty, namely the fact that it may only have peacetime application, and then that it may only cover test explosions. In terms of the former, Art. IV of the Treaty states that it shall be of unlimited duration and that States Parties may withdraw from it with three months’ advance notice to the others, and only when they have judged that “extraordinary events (…) have jeopardized the supreme interests of its country.” This implies that the Treaty remains applicable in times of armed conflict; concerning the content of the prohibition, it is clear that the Treaty’s object is to prohibit “any nuclear weapon test explosion” in the aforementioned environments, but the addition of the term “or any other nuclear explosion” is particularly striking. While this could be taken as referring to non-test peacetime explosions such as their use for spacecraft propulsion, the term is sufficiently broad to allow the preceding argument. Likewise, the fact remains that a number of States invoked the Partial Test Ban Treaty as part of their arguments that the threat or use of nuclear weapons would be contrary to international law in their submissions to the International Court of Justice at the time of its advisory opinion on the matter (the ICJ did not pronounce its opinion regarding such an interpretation of the Treaty).


To summarize, space law explicitly prohibits installing WMDs in outer space and on celestial bodies, and an argument can be made that it likewise prevents their use against objects in space given language contained in the Partial Test Ban Treaty. For all practical purposes, nuclear weapons are the only type of weapon that are relevant to this discussion at present, and if the above reading of the Partial Test Ban Treaty stands, then their use in space would be prohibited altogether.

At any rate, it is difficult to imagine a realistic scenario where a State would ever choose to resort to using nuclear weapons in space at all, but the possibility cannot be excluded, and there is little doubt that such use would not only be environmentally devastating, but would put an extra crack in the psychological barrier of never again resorting to the use of such weapons in the first place. Certainly, a hypothetical scenario wherein nuclear weapons would have to be used to avert an apocalyptic meteor headed for the Earth would warrant their use – but such a possibility remains too remote to merit debate at present, and there is little doubt that universal consensus would swiftly be achieved should something similar come to pass, thereby legalizing their use for a good cause.


Related content

  • Space law revisited (1/3): The notion of ‘peaceful uses’ and the Outer Space Treaty – Pavle Kilibarda, 7 November 2016.