This dissertation is a coordinated ethnographic case study of environmental science, medicine, technology, and design in an American human spaceflight program. Its goal is to investigate how astronautics contributes to shaping "the environment" as an extensive contemporary category of knowledge, politics, and social action. Based on fieldwork conducted primarily at the National Aeronautics and Space Administration (NASA)'s Johnson Space Center in Houston Texas from 2005 — 2008, the study argues that, in practical and meaningful ways, ecology and cosmology are co-constituting in American astronautics. Using participant observation and archival data, the study evaluates how astronautics practitioners know and work with "the human environment" on a scope that includes vehicle habitats and the heliosphere and on scales ranging from the molecular to the cosmic. In this work, people shore up and break down unusual human/environment boundaries, making sense of what it means to do so in technoscientific as well as sociopolitical, symbolic, and transcendental terms. The four cases analyzed are: (1) how space analogue missions operate as simulations but also make arguments that extreme environments foster progress through confrontation with adversity, (2) how space biomedical subjecthood is fundamentally environmental rather than biological, (3) how "habitability" works as a key elaborating concept among space architects so that they can connect extraterrestrial and terrestrial habitation problems and solutions, and (4) how Near Earth comets and asteroids have moved from being obscure astronomical objects to objects of environmental policymaking that extends into the heliosphere and into the far future. The study's analysis brings social theory about the spatial politics of knowledge into dialogue with conceptual frameworks from the social studies of science, technology, and environment. As an ethnography of outer space as extreme environment rather than territorial frontier, the study highlights astronautics' connections to broader domains of environmental science and technology, and by discursive and practical extension, to a spectrum of American environmentalisms and engagements with extremity. In doing so, the study elaborates astronautics' role in making ecological knowledge, and attendant concepts like adaptation and evolution, cosmologically scalable.