I found this definition: "henceforth, space opera meant, and still generally means, colorful, dramatic, large scale science fiction adventure, competently and sometimes beautifully written, usually focussed on a sympathetic, heroic central character, and plot action [this bit is what separates it from other literary postmodernisms] and usually set in the relatively distant future and in space or on other worlds, characteristically optimistic in tone. What is centrally important is that this permits a writer to embark on a science fiction project that is ambitious in both commercial and literary terms."