There was also another problem: which period of these two genres are we talking about? If we're looking at the early years of space opera and epic fantasy, then the connection is apparent, but diffuse. Both epic fantasy (what might have been better termed as heroic fantasy in its "root" period) and space opera in the first half of the 20th century shared roots with the adventure fictions that preceded them. Space opera arose, more or less, out of the planetary romances of writers like Edgar Rice Burroughs, proto-space opera writers like E.E. "Doc" Smith (though some might disagree with that assessment) and late-19th century "future war fiction" (see I.F. Clarke; I would argue that space opera gets its political undercurrents from this movement). Both forms (space opera and planetary romance) are hard to distinguish, since they often share in the same melodrama, with "space opera" typically playing within a much wider canvas (though not always), and both forms share a common root in the late 19th century adventure stories and the pulps that followed.
These are simplistic explanations, of course. If you want to know the full history of either field -- science fiction or fantasy -- you need to read a few books on the subject, and that's certainly true of me, since fantasy is not my academic field. Regardless, even if we start from such fundamentals as the roots, it becomes clear that though the genres start from very similar places (divergent in parts, of course), their paths to the present were drastically different.
Space opera began to move away from its planetary romance roots by the time the Golden Age rolled around, embroiling its future narratives in complicated allegories of contemporary politics and economics and developing grandiose "universes" in which larger questions about humanity and its culture could be asked and sometimes answered. This is not to suggest that space opera discarded the melodramatic space adventures of its roots. Rather, space opera gained three faces best exemplified by Star Wars (1977; dir. George Lucas), Star Trek (1966-1969; created by Gene Roddenberry), and 2001: A Space Odyssey (1968; dir. Stanley Kubrick): it became the high-flying epic adventure of its roots (similar in style to early epic fantasy), the wide-canvas exploration of human ingenuity and identity, and the philosophical interrogation of the human self.
I hope this is a real trend and not some figment of my imagination. Science fiction and fantasy are my great literary loves, but I think it's the perception of fantasy as the "less serious" genre which has left it out of academic conversations, except in those rare cases where one must talk about a fantasy without talking about it being a fantasy. I've certainly seen things changing within academia overall, though science fiction still remains the critical focus. But if I'm right that fantasy is pushing back against its roots, not to "diss" those roots, but to advance the genre as a whole towards more original (or at least less derivative) narrative practices, then fantasy will become as much a part of the academic discussion as science fiction. For me, that's a good thing, because it makes talking about fantasy less like talking with a push and more like having a conversation with a forest.
Of course, I could be very very wrong here. If so, please put forth your own argument in the comments!
: I would also suggest that most early space opera is decidedly not political in nature in the same way as "future war fiction." Its purpose was entertainment, if not explicitly, then certainly in form.
: I would distinguish planetary romance from space opera by arguing that the former typically focuses on adventures contained to a single world (i.e., Burroughs) while the latter typically explores multiple worlds and assumes interstellar space travel is "easy." These definitions do not hold as the New Wave or the New Space Opera movement significantly shift how space opera is perceived.
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