Like D&D and other fantasy games, science fiction games have their own hierarchy as well. Unlike fantasy, there is no single SF game that casts a long shadow over the rest of the genre, and for purposes of this overview we are only looking at actual SF games which encompass space travel, visiting other worlds, and exploration of the future of humanity and/or it's dominion of space. Sorry post-apoc games, you'll get your own post soon!
The hierarchy of SF games doesn't suffer under the presence of Traveller, despite the ubiquity of the first truly authentic SF game on the market. Rather, SF RPGs tend to reflect a style focus. Rather than try to identify the style in context of subgenres (space opera, hard SF, soft SF, exploration, war, etc.) I wanted to identify how game systems tend to focus their rules on types of play, creating distinctly different narrative/game experiences through the discovery and exploration mechanics.
For example: you are actively encouraged to role up new worlds and explore new hexes on the subsector map in a Traveller game, often with very little prep from the GM, but a game with any of the many Star Wars RPGs tends to focus on specific settings and locales defined by the Star Wars universe in its various incarnations, and new worlds operate on rule of cool rather than any sort of logical design system. So with that said....here's how SF games tend to fall in the catalog of Discovery and Exploration:
1. Procedural Exploration Games
Traveller is clearly a fine example of this. You're game is focused on exploration, and the rules support a mechanism to facilitate going to places and locales without necessarily having to know what is around the corner in advance. Such systems are often very old school in feel, and they focus heavily on procedurally generated content, and may even offer settings that still nonetheless require more rolls and design to flesh out as they are explored.
Traveller is the poster child, but other OSR games such as White Star offer soft science variants on generating worlds and systems to explore. Other games which offer this approach include GURPS Space (though it definitely works better as a type 4 below), and just about any system or sourcebook with a dedicated set of charts and mechanisms to create explorable content (often even on the fly).
2. Defined Setting Games
A large number of SF RPGs fall in this category. Star Trek, Coriolis, Infinity, Cold & Dark, Fading Suns, Alternity's Star Drive, Shatterzone, Burning Empires, The Last Parsec and many more all have tightly defined universes and even if they offer some rules on procedural content generation, it's usually eclipsed by the tightly defined worlds of the campaign that are often elaborated on in great depth and design. These are games not so much about SF for its own sake, but SF of the type and flavor of the specific universes under scrutiny in the game's setting itself.
3. Rule of Cool "Experience" Games
Most Rule of cool games actually work as defined setting games, too. The difference is that these games tend to eschew consistency of setting or scientific principles in favor of "worlds that are cool to explore." A common trait is that space exploration is easy, and most worlds you visit tend to be the kind you can survive because it's really about blasting your way in to the evil empire's secret base. Soft SF or space fantasy fits here really well. The many Star Wars RPGs, Starfinder, possibly Warhammer 40K RPGs, Hard Nova, Star Frontiers, most Savage Worlds settings (Flash Gordon and Slipstream in particular) and Firefly tend to fit this style of experience. A common trait of these games is that the world design often favors planets that may improbably have earthlike atmosphere despite being volcano worlds, or ice worlds, or "insert single terrain type here" planets. These games could be called "Pulp" Games as much as "Rule of Cool."
4. Scientifically Accurate Games
This is the most neglected corner of the experience mostly because it requires the most effort to work with. You can run games with a large enough toolbox from other categories here if they are designed to support a wide array of experiences....Traveller, for example, can be played fairly straight as a scientifically accurate game with minimal tweaking. GURPS Space is the poster child for this sort of game experience. Some games focused on near future cyberpunk and transhumanism fit here nicely, including Transhuman Space, Mindjammer, and Eclpse Phase. An irony of post-singularity games like the aforementioned titles is that they feel really "out there" yet tend to be tightly focused on logical extrapolations of a near future technological explosion in AI, transgenics and other technology that could fundamentallyc change the landscape of the future. And of course, a common trope of the scientifically accurate game is a tendency to eschew FTL drives, or to provide some sort of semi-realistic explanation for getting around it.