It is worth remembering that Heinlein and KSR are writers of fiction, which tends to gloss-over difficult problems in favour of pleasing stories. Neither were/a

It is worth remembering that Heinlein and KSR are writers of fiction, which tends to gloss-over difficult problems in favour of pleasing stories. Neither were/are scientists, although Heinlein had an engineering degree, and it would be inadvisable to rely on their work. In 1966 the space endurance record was just under 14 days (Gemini 7) and very little was known about the long-term effects of low gravity environments at that time.