NATO in Outer Space: A Domain Too Far?

by | Oct 1, 2020

Outer space. NATO Opening remarks by NATO Secretary General Jens Stoltenaberg 2019


This post examines whether Article 5 of the North Atlantic Treaty is applicable to armed attacks in outer space. NATO nations have recently recognized outer space as a new operational domain for the Alliance. Although the drafters of the North Atlantic Treaty did not envisage that the agreement would apply in space, the Treaty’s actual text does not preclude this.

The militarization of space is advancing at a rapid pace. Armed forces around the world increasingly depend on space assets—in particular for command and control, intelligence, and communication. More States are investing in military space systems than ever before. Meanwhile, the leading space powers continue to enhance their capabilities, with an increased emphasis on counter-space and antisatellite operations.

In line with these trends, NATO has taken a renewed interest in space. In June 2019, NATO defense ministers adopted an overarching space policy designed to guide the Alliance’s approach to space and the opportunities and challenges it presents. In November 2019, NATO officially recognized space as an operational domain alongside air, land, sea, and cyber. As Secretary General Jens Stoltenberg noted at the time, space assets operated by allied nations are vulnerable to hostile interference, with potentially severe consequences to communication and other vital services on earth. Space is also essential to NATO’s deterrence and defense mission, including its ability to navigate, gather intelligence, and detect missile launches.

None of the NATO-issued press releases and statements clarify whether Article 5 of the North Atlantic Treaty—the mutual assistance clause at the heart of the Alliance—may apply in outer space. This stands in contrast with recent pronouncements on cyber space and hybrid warfare. In their Wales Summit Communiqué of 2014, NATO leaders affirmed that cyber defense is part of NATO’s core task of collective defense and that a cyber attack could lead to the invocation of Article 5 of the North Atlantic Treaty. In the Warsaw Summit Communiqué of 2016, the allies declared themselves ready to counter hybrid warfare as part of collective defense, including by invoking Article 5.

Declaring Article 5 to be applicable in outer space would send a strong signal that the allies are prepared to defend their interests in this domain, including through the use of force, if necessary. However, it is one thing for NATO to adopt a doctrine for space operations and to formally declare space as an operational domain, but quite another to extend its collective self-defense remit into space. Without access to organic or shared capabilities to defend allied space objects, such a move would lack credibility. Recognizing the applicability of Article 5 in outer space also runs into legal obstacles. The purpose of this post is to take a closer look at the latter.

Armed Attacks and Space 

Whether or not Article 5 of the North Atlantic Treaty may extend to outer space depends first and foremost on the terms of the treaty itself. Article 5 is concerned with the exercise of the right of individual or collective self-defense in response to an armed attack:

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all; and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defense recognized by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security.

It is clear from the Treaty’s drafting history that the phrase “armed attack” derives from Article 51 of the UN Charter. Its meaning therefore coincides with the meaning of the term under the Charter. The upshot of this is that Article 5 incorporates the key concepts and standards that flow from the UN Charter with regard to the use of force.

Two points may be drawn from this. There can be little doubt that incidents amounting to an “armed attack” may occur in, from, or through outer space. Leaving aside the debate as to where exactly this gravity threshold lies, it is not difficult to envisage scenarios where even a high threshold could be met. One example would be a deliberate kinetic attack carried out by one State—either from earth or from space—against the sovereign space object of another State, for instance a satellite, leading to its physical destruction. Another example might involve one State using its space capabilities as a platform to launch a kinetic attack on the territory of a different State that results in the loss of life and destruction of infrastructure on the ground.

If such an attack were to occur, it would give rise, in principle, to the right of self-defense under Article 51 of the Charter. This would, in turn, open the door to the invocation of Article 5 of the North Atlantic Treaty. The principle that outer space is reserved for peaceful purposes does not preclude this. Article III of the Outer Space Treaty declares that activities in the use of outer space shall be carried out in accordance with the UN Charter, and thus the right of individual or collective self-defense under Article 51.

The second point is that recognizing the applicability of Article 5 in outer space would not, without more, resolve any of the uncertainties associated with the notion of armed attack and the use of force in self-defense. For example, is anticipatory self-defense to protect critical space assets permissible? If so, what standard of imminence applies in space? Could non-kinetic interference, such as signal jamming, rise to the level of an armed attack? If so, should we apply a loss of functionality test? Could indirect action—for example the use of space debris to disable a satellite—amount to an armed attack? Does the accumulation of events theory translate into space? Is self-defense available if an attack were to emanate from a non-State actor? On its own, the mere recognition of the applicability of Article 5 to space would leave these and other questions unanswered. It would also not remove the need to actually invoke Article 5. As the Wales Summit Communiqué makes clear in relation to cyber attacks, any decision to invoke Article 5 would be taken on a case-by-case basis. There is nothing to suggest that different principles would apply in relation to space attacks. Whether or not an incident in space amounts to an armed attack for the purposes of Article 5 is subject to the individual determination of each ally.

Accordingly, if the formal extension of Article 5 into space is meant to serve as a credible deterrent, a broad range of legal and policy questions need to be considered and answered in some detail. An extension of Article 5 is therefore best seen as a catalyst for detailed strategic planning, rather than as a substitute.

Geographical Limitations 

NATO is a regional, not global, alliance. Even if an armed attack against one of the allies were to occur in or from space, the duty of mutual assistance set out in Article 5 of the North Atlantic Treaty would be engaged only if the attack took place “in Europe or North America.” The scope of application of Article 5 is defined in greater detail by Article 6 of the North Atlantic Treaty:

For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:

on the territory of any of the Parties in Europe or North America, on the Algerian Departments of France, on the territory of Turkey or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;

on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea or the North Atlantic area north of the Tropic of Cancer.

The first sub-paragraph of Article 6 deals with attacks on allied territory. The actual location and extent of these territories and islands is of limited relevance in the present context. What matters is that they are geographical features on earth. Since space and celestial bodies are not capable of sovereign appropriation pursuant to Article II of the Outer Space Treaty—a principle which may be taken to reflect customary international law—they cannot qualify as the “territory of any of the Parties.” Nor would they qualify as islands within the meaning of the law of the sea. Consequently, while armed attacks launched from or through space into the territory or islands of NATO members fall within the ambit of Article 5, the first sub-paragraph of Article 6 clearly excludes armed attacks launched against their assets in space, including anti-satellite attacks.

The second sub-paragraph deals with attacks against “forces, vessels or aircraft of any of the Parties,” when in or over the relevant territories. It is tempting to assume that the “forces, vessels or aircraft” triad corresponds to the three armed services: land, naval, and air. However, this is not the case.

Turning to vessels first, in its ordinary usage, a “vessel” refers to some kind of watercraft. This is reflected in the International Convention for the Prevention of Pollution from Ships, which defines a ship to mean “vessel of any type whatsoever operating in the marine environment and includes hydrofoil boats, air-cushion vehicles, submersibles, floating craft and fixed or floating platforms.” The term therefore excludes spacecraft or other space vehicles, since these do not operate in the marine environment. While some spacecraft or their components are designed for landing on water, they do not come within the meaning of a “vessel” while operating in space.

The ordinary meaning of an “aircraft” is a vehicle that can fly. Annex 7 of the Chicago ​Convention on International Civil Aviation defines aircraft as “[a]ny machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the air against the earth’s surface.” While there is no agreed-upon method for delimiting airspace from outer space, prominent approaches put space above the earth’s atmosphere, beyond the maximum altitude of flight that can be attained by an aircraft or at the point where aerodynamic lift is exceeded by centrifugal forces. What follows from these definitions is that a vehicle exceeding these limits can no longer be considered as an “aircraft.” Accordingly, space vehicles are not covered by this term. While some space vehicles may leave or enter the earth’s atmosphere by relying on aerodynamic lift just like conventional aircraft, they would no longer qualify as such once they operate in outer space.

This leaves the concept of “forces.” That notion must encompass not only land formations, but also naval and air forces; otherwise, attacks on the crew of “vessels” or “aircraft” could engage Article 5 only if the crew members were attacked inside their vehicles. That could not have been the intention of the drafters. In fact, the term “forces” is normally used to cover all three of the traditional service branches—a point confirmed by the NATO Status of Forces Agreement of 1951.

Consequently, any land, naval, or air forces assigned to or engaged in space operations fall within the definition of a “force” for the purposes of Article 6 of the North Atlantic Treaty. Adopting an evolutionary interpretation, a compelling case can be made that the term should also include space forces organized as a distinct branch of the armed forces. Moreover, since the notion of “vessels” and “aircraft” excludes land vehicles, the latter must be part of the concept of “force.” Otherwise attacks on land combat and transport vehicles would not engage Article 5. Such an interpretation would be absurd. Consequently, if the notion of “forces” includes space personnel and vehicles other than ships and aircraft, there is no reason why it should not also extend to space vehicles.

While the second sub-paragraph of Article 6 therefore covers attacks on space forces, including their personnel and vehicles, such attacks must take place “in or over” the territories defined in that provision. Evidently, personnel and vehicles operating in space are not present “in” any territory. However, they may be described as being “over” allied territory when travelling through a sector of outer space that puts them above any of the territories described in Article 6 of the North Atlantic Treaty. Much in the same way, satellites in geosynchronous or geostationary orbit are commonly described as being “over” a particular territory or geographical feature on earth when orbiting above it.

The point to take away from this is that Article 6 includes within the scope of Article 5 attacks carried out in outer space against the forces belonging to a NATO ally, provided those forces and their assets are subject to attack when located over any of the territories defined in Article 6. We may call this the “North Atlantic space area.”

There is a catch, however. The text of Article 5 itself refers to armed attacks “in Europe or North America,” without mentioning attacks taking place “over” those territories. Armed attacks occurring inside the national airspace of NATO members may be described as occurring “in Europe or North America,” but attacks against allied assets in outer space cannot be described in that way. The narrower wording of Article 5 thus seems to exclude attacks in space, after all.

Yet this restrictive interpretation overlooks the fact that the second sub-paragraph of Article 6 also mentions attacks on forces, vessels or aircraft in or over “the Mediterranean Sea.” The Mediterranean Sea includes the territorial waters and airspace of North African countries. These waters and airspaces are not “in Europe or North America,” which means that geographical scope of Article 6 exceeds the narrower wording of Article 5.

This opens the way for a different interpretation of the two provisions. It seems that Article 6 was meant to qualify Article 5, and not the other way around. Consequently, if Article 6 covers attacks against allied forces in space—as we have found it to do—then Article 5 must be interpreted accordingly.

Beyond the North Atlantic Space Area: Attacks in Space?

The adoption of the North Atlantic Treaty preceded the onset of the Space Age by more than eight years. Its drafters may be forgiven for not having envisaged its application to outer space. However, nothing in the text actually precludes it from doing so. Armed attacks emanating from space are covered by Article 5 without a shadow of a doubt. This is important, but attacks on assets located in space are of equal, if not greater, concern. Although the matter is not free from ambiguity, a credible argument can be made that certain attacks in space are also covered by Article 5.

The allies could remove any lingering doubts by declaring themselves ready to counter attacks on allied space objects as part of NATO’s collective defense mission, much in the same way as they have done in relation to cyber attacks and acts of hybrid warfare.

Alternatively, they could issue a formal interpretive declaration to the same effect. The North Atlantic Council took a similar step when Algeria gained independence in 1962. There, NATO noted that the clauses of the North Atlantic Treaty concerning the Algerian Departments of France had become inapplicable. Compared to a summit communiqué, such an interpretative declaration would offer an opportunity to clarify how Article 5 applies in outer space in greater detail.

The allies could decide to go further. As we have seen, under the current wording of the North Atlantic Treaty, an armed attack on allied space objects would engage Article 5 only if the object comes within the definition of a “force” and the attack took place “over” the territories defined in Article 6. This leaves non-military space assets and those orbiting over different parts of the globe vulnerable to hostile acts. NATO nations could amend Article 6 to expressly include armed attacks against their space objects anywhere in outer space. Indeed, the idea of modifying the geographical scope of application of the North Atlantic Treaty is not entirely fanciful. The allies did precisely this when Turkey acceded to the Alliance in 1952.

However, a formal treaty modification to extend Article 5 to all space attacks would amount to a significant expansion of NATO’s collective defense responsibilities.

This leads us back to the question of credibility: an intent to defend allied space objects from attack is credible only if matched by appropriate capabilities. Delivering such capabilities would seem more demanding than revising the North Atlantic Treaty.


Dr. Aurel Sari is an Associate Professor of Public International Law at the University of Exeter, the Director of the Exeter Centre for International Law, a Fellow of Supreme Headquarters Allied Powers Europe, and a Fellow of the Allied Rapid Reaction Corps.