Humanity's historical trajectory has been defined by an unbroken expansion of agency—our increasing ability to control our environment, movement, and destiny. This paper argues that the logical culmination of this trend is the development of mobile world-systems—self-sustaining, navigable civilizations capable of steering their own cosmic trajectory.

Current space expansion strategies remain base-centric, relying on fixed planetary outposts that are constrained by resource dependence, static positioning, and vulnerability to external threats. In contrast, mobile world-systems eliminate these limitations by combining the resilience of natural ecosystems with the adaptability of engineered environments.

This framework introduces the Cosmic Agency Ratio, a quantitative model demonstrating that human survival depends on the continuous increase of control over our environment. We argue that intermediate states—partial agency without full cosmic mobility—are inherently unstable over long timescales, leaving civilizations vulnerable to existential threats. The only sustainable outcome is full agency: the ability to dynamically navigate space rather than being bound to planetary fate.

By synthesizing technological, societal, and philosophical imperatives, this paper presents mobile world-systems as not just a possibility, but an evolutionary necessity. The choice is clear: humanity can either remain passive passengers on a drifting planet, or seize control of its cosmic trajectory and take the helm of its own future.