The Customary Status of the Outer Space Treaty is not an All or Nothing Proposition, by Ian Perry

The following is a guest post by Ian Perry, an attorney who received a J.D. and an LL.M. in air and space law from the University of Mississippi School of Law and is pursuing a J.S.D. in space law with the University of Nebraska College of Law.  LM


When different nations enter into agreement on a topic they may do so by a treaty in which they explicitly spell out what they have agreed to. Customary international law on the other hand, involves an implicit agreement between states, is traditionally understood to gradually form over time, and is not always codified in a treaty. Customary international law is relevant to the Outer Space Treaty because of claims that the treaty has become customary international law, which could mean that it might apply to states that have never ratified it or have withdrawn from it.[i] As the following shows, existing state practice is insufficient to make the entire Outer Space Treaty customary international law.

What is meant by customary international law? Customary international law is a kind of international common law; it is a body of rules and principles said to arise informally from the general and consistent practice of nations. See RESTATEMENT (THIRD) OF FOREIGN RELATIONS LAW OF THE UNITED STATES § 102(2) (1987). Evidence of customary international law includes judgments and opinions of international tribunals, such as the International Court of Justice (whose judges are approved by the U.N. General Assembly and Security Council); judgments and opinions of other nations’ judicial tribunals; and scholarly writings. Id. § 103.[ii]

Customary international law may apply to a local region, to all countries, or to all countries that have not persistently objected. The amount of state practice required for a custom to form is not consistently defined in the relevant literature. Customary international law can exist in the absence of a treaty or alongside a treaty, and there are many scholars who find at least parts of the Outer Space Treaty customary international law.[iii] The fact that part of a treaty overlaps with customary international law does not necessarily mean the whole treaty is custom. Customary international law is generally described as arising out of state practice done out of a sense of legal obligation—a rule in one part of a treaty might meet this test in some situations without all parts and all applications of the treaty doing so. For example, in North Sea Continental Shelf, the International Court of Justice distinguished different types of precedent and indicated that state practice in the form of maritime delimitations between geographically opposite states provided insufficient precedent to establish a customary rule regarding lateral delimitations between adjacent states.[iv] Practice in earth orbit is not necessarily precedent for celestial bodies. One need not say that the whole Outer Space Treaty has the same status under international law. A particular provision of the Outer Space Treaty might embody customary international law with regard to activities in an area where there is more state practice (such as with regard to free movement in earth orbit), and not embody it with regard to all activities in a physically different area where there is less state practice. By way of illustration, ask the question: Do states know enough about operations on the surface of Mars to intend by compliance with the Outer Space Treaty now to give up a future right to withdraw from some of its provisions if they are found impractical there? The ICJ has found actions in coastal waters up to about 60 miles from shore to be less than sufficient to establish a norm for waters 120-200 miles from the coast.[v] Manned stations on other planets appear to be at least as distinct from earth-orbiting satellites as these different areas of the ocean are from each other. Accordingly, current state practice is generally insufficient to make even relatively long-standing implementations of the Outer Space Treaty establish customary law for all celestial bodies. Continue reading