Debate over the delimitation of airspace and outer space has persisted since the dawn of the space age, without resolution. With the development of hybrid aerospace vehicles that can operate in and transition between the two zones, the line between their disparate legal regimes will be tested. And this test may not come with an after-the-fact courtroom debate over applicable liability regimes, but rather it may come with a real-time decision made in a military operations center as to whether an aerospace vehicle has violated sovereign airspace and should be shot down. The recent shoot-down of a Russian bomber by Turkish fighters is just one example of the sensitivity over violations of territorial integrity.

This article explores the controlling law, in the absence of a clear resolution from states, as to the legal regime governing the transition from air to space. While others have examined delimitation from the standpoint of liability, air and space traffic management, or other perspectives, this work addresses delimitation through the lens of the potential use of force and state security interests. It first breaks down air and space into their core, or black-and-white, elements, with particular attention to their rules of sovereignty and the distinction of state and civilian vehicles. It then examines the gray area in between the regimes in two ways, using a conservative (or positivist) approach and a commensurability analysis. These modes of analysis reveal that under the current legal construct, the lowest satellite orbit should be regarded as the cap on state assertions of sovereignty, and therefore, the legal line between the airspace and outer space regimes from a use of force and security perspective.