Soft Science Fiction or Science Fantasy
Generally, when differentiating fantasy from science fiction, a very simple key is used: if it is pre-modern, it is fantasy, and if it is futuristic, it is science fiction. Thus a story set in ages before industrial revolution is automatically considered fantasy as long as it includes magic and supernatural. Likewise, a story set in the far future is automatically considered science fiction, regardless of whether it is based around science or around magic.
Personally, I cannot help but disagree with this. Time period alone does not tell us what is the fundamental nature of the work. What matters is the nature of the work and relation to the society it is written in. Work set in Middle Ages can be science fiction if it has technology that was not available at the time it was written. In general, science fiction explores possible inventions of the future and their impact on the society. Fantasy however is based around supernatural elements such as magic, dragons and so on.
Hard science fiction would thus be based around things which we know are scientifically and technically possible, but may not be practical – or will not have been practical at the time. Examples would be works including space elevators, generational or nuclear pulse propulsion ships. 20 000 Leagues Under the Sea is also science fiction – submarines may be extant today, and Nautilus would be positively primitive compared to modern AIP or nuclear submarines, but at the time when the story was written it was not something that existed.
Soft science fiction takes scientific concepts and theories and bases itelf on their practical applications. Things such as warp drive, hyperspace / subspace and jump drive are all based in Einstein’s timespace and related concepts. Specifically, warp drive and jump drive are all based around gravitational bending of timespace being used to move the space ship is part of instead of the ship itself – the ship thus travels faster than light without moving faster than light. Hyperspace is based on moving from realspace into theoretical dimension where distances are significantly reduced, and then popping back up.
Softer end of soft science fiction largely makes up its science. What differentiates science fantasy and really soft science fiction is thus the fact that latter tries to maintain an illusion of scientific background. However, (some) stories from this area might be classed as science fantasy.
Science fantasy mixes magical or supernatural elements with (pseudo)scientific ones. Science may be present, but it will typically not be used in an accurate or realistic way. Religious science fiction, involving miracles, angels, demons etc. is also science fantasy. What differs it from high fantasy and urban fantasy is that it also includes speculative science, such as lasers and starships. If a sci-fi story includes supernatural elements, especially if these are its core concepts – such as the Force in Star Wars or Warp in Warhammer 40 000 – then the story is science fantasy.
Thematically, science fiction is about futurism. It explores technology and its social implications. Science fantasy stories tend to be space operas: focus on characters and their adventures rather than technology.
One should also keep in mind a difference between settings and stories. While Warhammer 40 000 (as explained below) is definitely science fantasy, individual stories set into the setting can vary from pure fantasy to pure science fiction. Forges of Mars trilogy may be example of the latter.
20 000 Leagues Under the Sea is science fiction. While technology discussed there is plentiful today, it will have been revolutionary when the book was written. Nevertheless, it is fundamentally based in known scientific principles of the time, and so may be classified as hard science fiction.
Star Wars are science fantasy. It has the Force, swords, wizards, princesses and so on. Attempts to make it more “scientific” such as midichlorians were not well received by its fanbase. However, it is also not pure fantasy as it also includes advanced technology, space travel, aliens and robots. If this advanced technology is removed, we end up with a pure traditional fantasy story; but at the same time, advanced technology is fundamental to nature of Star Wars, which means it cannot be classified as traditional fantasy.
Star Trek is a soft science fiction with significant elements of science fantasy. Ignoring the wholly fantastical premise of the Star Trek itself (humanity working together flawlessly), it has a lot of traditional fantasy elements: talking to the souls of the dead, godlike beings, human ascension and so on. Where it differs from Star Wars is that it attempts to explain all of these in scientific (or pseudo-scientific) terms. More importantly, it is also fundamentally rooted in technology and exploration of its social implications.
Warhammer 40 000 is science fantasy (though with a basic premise much more realistic than that of Star Trek). It has a lot of technology which is possible, much which is plausible and a lot which is either impossible or impossible to predict. This technology is freely mixed with magic and supernatural. Hyperdrive used would be standard sci-fi, if it weren’t for the fact that it takes ships into hell. All alien species are based on standard fantasy races, with even the names stolen from Lord of the Rings (the Aeldari, for example). In fact, it is basically Warhammer Fantasy transplanted into space: space elves, space orcs, space undead, space wizards, space knights, space demons, et cetera. Movie Event Horizon also counts, and it was adopted by the community as an unofficial Warhammer 40 000 prequel.
The Martian is hard science fiction. It includes basically current-day technology, and absolutely no magic or supernatural. The only fictional things there are the mission(s) to Mars and the spaceships which take them there, but these too are based on things which are known to be possible with modern-day science.
The Colony likewise is hard science fiction, detailling the survival of a community through the new Ice Age. None of the technology included is something that wouldn’t be recognized by the people living today, and there are no fantastical elements either.
Doom is a series of science fantasy games. It tells of an invasion of demons which is pushed back by a plasma-gun-totting Space Marine. The latest installment and reboot of the series goes even further into fantasy territory.
Dragonriders of Pern are one of works from which Paolini stole ideas for the Inheritance Cycle. It has dragons and humans riding them on an extraterrestial planet colonized long time ago. Due to a combination of fantastical and scientific elements, they fall into science fantasy category.
Evacuate Earth: Death by Neutron Star is hard science fiction. It details a scenario which is eminently possible – a rogue neutron star on its way to eat Earth and the rest of the Solar system. Generational spaceship which is evacuating selected humans from Earth is likewise eminently possible and based in known technology and physics.