Editing Orphans of the Sky (section)

==Scientific plausibility==
The production of “artificial gravity” by way of the [[centrifugal force]] of a spinning spacecraft is a common science fiction trope. The depiction of the physics of the Ship's [[artificial gravity|artificial (rotational) gravity]] is accurate: It spins to simulate gravity, which increases as one moves outward from [[zero-g]] at the center. However, such a spacecraft would have to be much larger than, for example, the [[International Space Station]].  According to aerospace engineer John Page, "It would have to very large—much larger than a football field."<ref>[http://www.popularmechanics.com/space/rockets/a8965/why-dont-we-have-artificial-gravity-15425569/ Why Don't We Have Artificial Gravity?]</ref> Heinlein’s Starship ''Vanguard'' qualifies as large enough. Although the precise dimensions of the Ship are never given in the main text of the book, the original 1941 magazine appearance of “Common Sense” — the second half of the novel — begins with a brief precis of the first half and describes the Ship as “a giant cylinder, five miles long, two thousand feet thick, spinning slowly on its axis.”<ref>[https://archive.org/details/Astounding_v28n02_1941-10/page/n101/mode/2up "Common Sense" (original publication)]</ref> (This precis, being unnecessary, was not included in the book version, but these same dimensions found their way into the blurbs on the backs of some of the subsequent paperback versions.) Heinlein makes nothing of the fact that the "muties" (denizens of the "low-weight" upper decks) would be at a distinct disadvantage maneuvering in the "heavy-weight", lower level decks.

Two-headed humans such as Joe-Jim — known as [[dicephalic parapagus twins]] — do exist, and are one variation of [[conjoined twins]].<ref>[https://www.bbc.co.uk/programmes/b01s5b2d/episodes/guide BBC program on]</ref> Their dual brains, however, do not alternately share control of the shared body's limbs and other organs (as in the book), but are wired (innervated) separately to the right and left sides of the body respectively.

The Ship's "Converters" are a fictional extrapolation of [[waste-to-energy]] technology. They reflect an early 1940s viewpoint of atomic power, with atoms of any element "ripped apart" in an unspecified manner.

The notion of a giant planet with a habitable moon went against theories of planetary formation as they stood before the discovery of [[Hot Jupiter|"hot Jupiter" planets]]. It was thought that planets large enough to have an Earth-sized moon would form only above the "snowline," too far from the star for life. It is now believed that such worlds can migrate inwards, and habitable moons seem possible. The existence of [[exomoon]]s has not been confirmed, but there are candidates.<ref>[https://www.nasa.gov/ames/nasa-supercomputer-assists-the-hunt-for-exomoons NASA Supercomputer Assists the Hunt for Exomoons].</ref>