This talk explores the careers of two scientifically trained professionals who designed, developed and popularized radical technological solutions to the limits imposed by Earth's finite resources. Starting in 1969, Princeton physicist Gerard O'Neill looked outward, developing elaborate plans for space-based settlements and factories as one path to alleviate pollution and overpopulation. A decade later, M.I.T-trained K. Eric Drexler turned to the nanoscale and imagined self-replicating nano-machines that could assemble things with atomic precision. Both men were "visioneers" – a category of historical actor who combines futurism with engineering, advocacy, and community building. As they promoted their solutions to the broader public, they often saw their ideas co-opted and transformed. This talk explores the ways in which O'Neill and Drexler's "visioneering" influenced diverse audiences as well as the dangers – oversimplification, misuse, and misunderstanding – that challenged their attempts to design the technological future.