Decades of international debates have been insufficient to build consensus over a universal, multilateral delimitation of outer space. A legal frontier, still undelimited, exists between the territorial air space (under national control and jurisdiction) and outer space (where no claim of sovereignty is authorized). As technology advances, blurring the differences between aeronautics and astronautics, this question gains renewed relevance for practical purposes. The long-term sustainability (LTS) of outer space depends upon recognizing outer space as a specific and unique domain, distinguished from any other on Earth, and subject to a specific body of rules. After considering different proposals for the delimitation of outer space, as well as domestic regulation, an alternative is suggested, acknowledging different perspectives while including conciliatory features.