This is an interesting question and one that will often cause a great flaming row among, mostly, science fiction geeks.
To some, it’s a polite way to say pulp garbage that is formulaic, cliché as hell, and stupid. To others it’s epic battles, and damsels in distress. For some it’s vast armies squaring off to decide the face of a world.
Maybe it’s all of those, and none of them. Maybe it’s something both simpler and more complex. Have I gone all Dickens-ish to sound witty or do I have a point? Well, I do. I think each of those definitions looks at one facet of the truth, but by claiming to be the whole of the definition, while only seeing a piece they become flawed and distorted. They are both valid and wrong. The answer to what the genre is, I believe, can be given simply – but the simple definition extrapolates to express a very complex, rich, and varied genre. So it is, both simpler and more complicated than those definitions.
Let’s start with what (at the moment) Wikipedia has to say, because this is a good starting point.
Space opera is a subgenre of science fiction that emphasizes romantic, often melodramatic adventure, set mainly or entirely in outer space, generally involving conflict between opponents possessing advanced technologies and abilities. The term has no relation to music and it is analogous to “soap opera” (see below). Perhaps the most significant trait of space opera is that settings, characters, battles, powers, and themes tend to be very large-scale.
Sometimes the term space opera is used pejoratively to denote bad quality science fiction, but its meaning can differ, often describing a particular science fiction genre without any value judgement.
Let’s now look at the origin of Space Opera. It was the rebirth of an old westerns genre called the Horse Opera.
I could go on forever with examples and details and a load of over complicated crap, instead I’ll say it this way: Space Opera is heroic fiction. That’s all. It’s ancient myth and epic legend in space or in the far flung future. Just as Horse Opera was the epic legends and myths of the wild west. Or how Sword and Sorcery fantasy is the epic myth and legend of a land of myth and legends.
It’s epic scale. Worlds in the balance, fate of the universe at stake, etc. Just as was the story of the Trojan war. The world of those in Troy were at stake. Great armies were amassed, and heroes did battle. Heroes did battle. Soldiers fought and died, Odysseus had an adventure. There’s Helen, and I believe Aeneas makes an appearance, and so on. Space Opera is Star wars with Luke fighting Vader and Palpatine; of Han and Chewie, Leia and Lando, C3PO and R2, how that unlikely group of unlikely friends defeat the mighty Galactic empire! With, yes, a little help from the rebel alliance and Wedge and such, but that’s beside the point.
It’s good versus evil. It’s black hat and white hat. Now it CAN, but needn’t be white hat lawful stupid Superman as the overgrown Eagle Scout nature. It can be wolverine and his heart of gold and solid moral compass inside a gruff exterior – white hat doesn’t HAVE to be Thou Shalt Not Kill, it can simply be Thou Shalt Not Murder, or Death Before Dishonour. Just as black hat villains need not be Satan or the like. Magneto, or Darth Vader, the clearly evil men with noble intent – the end justifies the means.
Space Opera is people doing extraordinary things in an extraordinary setting. It’s heroes and villains that fire the imagination with weapons, vehicles, friends, minions and allies that spark a sense of wonder. A Space Opera is Sword and Sorcery with ray guns. Sword and Sorcery is Space Operas with wands. Just as a Horse Opera is those with six shooters. Support You Local Sheriff would be a Horse Opera, an old west Space Opera, the sense of wonder born when you see him shoot twice through the hole in the middle of that washer, him clearly noble and law abiding, brave and such … even if he isn’t saintly. He saves the whole town, hero of the territory. And so on, and so forth.
That is what Space Opera is. It isn’t vast armies, but it can be. It isn’t great men, but it should star them. It isn’t damsels in distress, but they help. It is a tale of how the good guys beat the bad guys through death defying adventure equipped with shiny things that whir, blip, bling, and beep. Space ships make banking turns, go whoosh, and shit blows the hell up damn it! Space Opera is the Ewoks defeating the Imperial forces, and it’s that tiny band of rebels who hlp them do it. It is John Carter leading a horde of green Martians into Battle, leading mind ye, and those guys fight and provide background action while he, Woola, and Tars Tarkas battle to the death against the chief of the yellow Martians to defeat him and end the battle.
Melodrama, as Wikipedia says. It doesn’t have to be bad; it just means a bit exaggerated or larger than life. Everything is big, metaphorically or literally.
It, like any genre has tropes. Many of these tropes are disparaged as clichés, which is quite unfair. That the butler did it, is a cliché. The dashing, noble, honest, handsome figure fighting his way though seas of foemen to win to the side of his lady love, and saving the world on the way or after some romantic speeches and kissing, that’s what makes it this instead of that. You can tinker with a trope, but if you take it away completely the story ceases to be recognisable.
See? Simple, but complicated. A bit of each of those, but not quite any of them. Want to define the genre perfectly? Find the words that are the things Star Wars and Fifth Element have in common, then compare them to the genesis of Space Opera, Lensman and Triplanetary. It’s all there, trust me.