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"Ralph 124C 41+" is awful. After that, Gernsback published SF of others, rather than his own, and as a publisher did much to launch SF as a genre.

Before SF, there were "Edisonades", the classic being "Edison's Conquest of Mars". That's a milestone in science fiction. It's one of the first space travel stories, and it's reasonably plausible given scientific knowledge at the time.

Heinlein is not from the "beginnings" era. Heinlein is from the "golden age" of SF, when a space-oriented future looked not only technically possible but close. Watch "Destination Moon". Most of Heinlein's better works predate the discovery that Mars barely has an atmosphere and Venus is well above the boiling point of water. Planetary colonization within the solar system looked possible back then.

[Animats]

(Sorry about posting that twice. Hacker News was reporting gateway timeouts.)