The Anthropocene may entail at least two major civilizational and planetary events; the environmental crisis and the project of spreading humans and Earth-life to other planets. This thesis operates in the span between these two events. Its objective is to explore what ethics may secure and promote long-term sustainability of Earth-life in space, and how these moral values may be promoted and adopted. With an environmental ethics approach to the young interdisciplinary field of space ethics, this thesis thus aims to tentatively and exploratively draft a viable ethic for long-term sustainability of Earth-life in space. At that, the study focuses on the challenge of human moral adaptation in the scenario of human migration to outer space. By using the methodology of macrostrategy, this study applies a wide scope in terms of time and space, in order to analyse trends, risks and correlations between ethics on the one hand, and political, social and cultural, as well as evolutionary and planetary aspects of human civilization, planet Earth and the cosmos. It moves between fields such as evolutionary- and astrobiology, Russian cosmism and posthumanism. The conclusions of the study suggest a need for an updated epistemology which gives justice to the factual holism and relationalism of the world and the universe. This epistemology serves as the basis for a proposed ethic for the project of spreading humankind and Earth-life to other planets. This proposed ethic considers certain aspects of altruism and ecocentrism placed in a cosmic context, while staying grounded in the realism that is human nature. Further, the research suggests that a viable space ethic must with time expand its scope in time and space in order to reach an appropriate size and scale. This is proposed on the basis of a need to navigate through existential risks to humankind and Earth-life. The study simultaneously identifies significant obstacles for such a moral development in the current evolutionary trajectory of the human being. In order to overcome these obstacles, and by the rationale of the severity of the current crisis, the study concludes that a combination of extensive moral education together with applied moral enhancement by the aid of our technology, may be appropriate and necessary means to promote and adopt this expanded ethic. As such, the study implies that a moral development of humankind may be relevant to the success of the project of becoming multiplanetary.