What is space junk, and who defines pollution in an environment seemingly devoid of nature as we know it? Beginning with the launch of Sputnik in 1957, spacefaring nations transformed the region between the upper atmosphere and the moon from a wilderness into a landscape. Like any terrestrial industry, the construction of a satellite infrasctructure in orbit also yielded a system of byproducts—human-made waste colloquially known as “space junk.” Although remote and largely invisible to the majority of space technology users, the orbital environment nonetheless played a critical role in Cold War geopolitics. Contrary to current space policy literature that portrays space junk and awareness of space junk as recent phenomena, communities around the world were both aware and concerned about space junk from the very first moments of the Space Age. By tracing convergent changes in the orbital landscape and in the political landscape below during the Cold War, concurrent with the rise of mainstream environmentalism, this dissertation reveals the roots of an international understanding of the borderlands between Earth and outer space as a natural environment at risk. Focusing on highly mobile, unruly space junk artifacts illuminates the many ways that humankind mutually shaped and was shaped by the global ecosystem surrounding our planet during the Cold War. Situated at the intersection of the histories of science, technology, and the environment, this dissertation illustrates how space junk in orbit and falling to Earth brought geographically and politically disparate states into dangerous proximity during the Cold War. An international consciousness of outer space as a fragile environment emerged early in the Space Age, and influenced the negotiation of new modes of international scientific and environmental governance in near-Earth space.