Between vertical frontiers : exploring CIL for a common legal delimitation of outer space

Okoli, Dominic (2025) Between vertical frontiers : exploring CIL for a common legal delimitation of outer space. (PhD thesis), Kingston University, .

Abstract

The boundary between airspace and outer space has not been commonly delimited for legal purposes, though different laws- including domestic and international - regulate space activities. Efforts to delimit them through multilateral treaties have been stunted primarily due to a general lack of political will. This phenomenon can be attributed to the dominant influence of certain states' national interests, which lean towards supporting an undelimited outer space. Research has shown that as this situation persists, the development of customary international law (CIL) on the matter did not halt. Several states and international organisations have made unilateral efforts to delimit airspace and outer space. This, however, resulted in varying altitudinal thresholds. This study aims to investigate them to reveal any evolving or evolved common CIL rule. Drawing from an extensive literature review, domestic legislations, policies, official reports, and correspondences of states and international organisations, this study examines numerous opinio juris-backed practices of states and international organisations relevant to the delimitation of the vertical frontiers. The states and international organisations examined were those that have responded officially on the matter before the United Nations Committee on Peaceful Uses of Outer Space (UNCOPUOS) and those that have submitted their relevant legislations to the United Nations Office for Outer Space Affairs (UNOOSA). Analysis of these opinio juris – backed unilateral practices reveals a common altitudinal height in the vicinity of 99.5 km (approximately 61.83 miles) above Mean Sea Level (MSL), with a 10% trimmed mean of 99.2 km (approximately 61.64 miles) above MSL. The study concludes that since international law prohibits states from exercising sovereignty in outer space, the identified common altitudinal height derived from their opinio juris-backed unilateral practices should serve as a common delimitation for airspace over which they have exclusive sovereignty. The study consequently reviewed this discovery of common altitudinal points upward to the nearest whole number and recommended 100 km above MSL for adoption as the common nadir of outer space.

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