The plans to set up human colonies in space and spaceships that will take us to the stars. Richard Hollingham travels to the Tennessee Valley Interstellar Workshop in Chattanooga, Tennessee to meet scientists, engineers, doctors and anthropologists who are working on it.
These are not dreamers - although they all have an ambitious dream - but well qualified experts. Several work at Nasa, others have day jobs at universities and research institutes.
Richard hears of proposals to build giant space stations and worldships - vessels packed with the best of humanity. These caravans in space might be lifeboats to escape an approaching asteroid or perhaps the first step to colonising the galaxy.
With contributions from Technical Adviser to Nasa's Advanced Concepts Office Les Johnson, Director of the Space Engineering Centre at the University of Arizona John Lewis, architect Rachel Armstrong and anthropologist Cameron Smith.
This programme first aired in November 2016.