The visions of the primary protagonists of the development of outer space diverge in terms of both their motives for and means of extending life into the cosmos. The present article analyses the implications for sustainability associated with the visions of three prominent entities - the National Aeronautics and Space Administration (NASA), the 'space entrepreneurs' who lead the private spaceflight industry (such as Elon Musk, Richard Branson, and Jeff Bezos), and transhumanists. This latter category aims to accelerate evolutionary processes to transform humans into a new 'posthuman' species which will be imbued with a greatly extended lifespan and other capabilities that enhance survivability in outer space. Due to the inchoate nature of safer and more affordable spaceflight, it is currently unclear which vision of space development will come to fruition. As explored in this article, the proposals advocated by NASA, space entrepreneurs, and transhumanists are associated with divergent implications for sustainability. Trade-off decisions must be made in terms of whether to minimise impacts on Earth, other celestial bodies, or the human form. While it is currently unclear which vision will eventuate, the process of space exploration and settlement is poised to considerably alter current conceptualisations of sustainability.