 That was apparently the only star at which settlement was possible on the "Vanguard" path.The conversation takes place in 4291, and it is mentioned that the settlers have been there for 800 years.Another reference to Heinlein's Future History is a passage describing Joe-Jim's enthusiasm for the works of "Rhysling, the blind singer of the spaceways," a poet and the central character of the Heinlein story "The Green Hills of Earth".