A new movie brings to life the legacy of a physicist who has played an influential—but largely unheralded—role in shaping the vision of space settlement.

The documentary The High Frontier: The Untold Story of Gerard K. O’Neill takes its name from the 1977 book The High Frontier: Human Colonies in Space authored by Princeton University physicist Gerard K. O’Neill. The movie will be released on April 17, and it’s an excellent film for those seeking to better understand the future humans could have in outer space.

O’Neill popularized the idea of not just settling space, but of doing so in free space rather than on the surface of other planets or moons. His ideas spread through the space-enthusiast community at a time when NASA was about to debut its space shuttle, which first flew in 1981. NASA had sold the vehicle as offering frequent, low-cost access to space. It was the kind of transportation system that allowed visionaries like O’Neill to think about what humans could do in space if getting there were cheaper.