The title says it all. How would one go about creating a planet with aliens that have medieval grade technology (windmills and carts and stuff.) No high level technology. However, I want the aliens to be capable of space travel. I feel like a portal of some sort might work but I really don't want anything magical; all science please.. Thanks :)
It's called Stargate SG-1.
A long forgotten race of Ancients built a network of Stargates. They eventually evolved to a higher plane, but left behind many of their working portals to other planets. Subsequent species discovered the network, some of whom trafficked in less-developed species as slave labor.
Over the centuries the political landscape shifted and the parasitic species abandoned the planet leaving the primitive worker species to fend on their own. Eventually the primitive species discover the Stargate and conquer the galaxy with gumption, the spirit of friendship, and a series of unlikely scenarios where far more advanced societies are easily befuddled by aliens with only medieval technology and a simplistic understanding of science.
Wetcircuit has given a very good answer on the portal front, but a non-portal alternative might be that the alien world is in fact a very low gravity planet.
Imagine (if you will) that life formed on a moon that was around half the mass of our moon, orbiting a brown dwarf which in turn is orbiting a main sequence star. The brown dwarf still gives off some heat, which in turn is supplemented by the main sequence star for at least some of the orbit of the moon, meaning that it's possible that you could have liquid water, and therefore life, even complex life.
I say possible because many things would have to be in place to support this life, not the least of which is protection from the radiation from both the brown dwarf and the primary star; this is a very unlikely scenario but mathematically possible. To continue:
You only have to look at the Apollo lander launches to tell that the gravity well of such a moon requires FAR less energy to escape. Getting off Earth took a full 3-stage Saturn V rocket. Getting off the moon in the lander took an engine bolted to the bottom of the lander. On a moon with half the mass, the energy requirement would be even lower and could be possible with gunpowder technology. Is that medieval? Perhaps not European, but certainly Chinese.
You could then at least escape the moon's gravity well and orbit the brown dwarf, possibly explore a little more of this increasingly interesting solar system with probes and the like.
As for actual 'manned' space travel, this would also likely be possible because such a low gravity planet likely has a low atmospheric density, meaning you don't need a particularly strong pressure vessel to carry your crew. Blacksmiths were definitely a part of medieval society, and it's possible (although not perhaps realistic) that they could build a metal 'carriage' that could hold sufficient atmosphere for a limited journey, say a few orbits of the moon. That would still count as space travel, albeit limited.
I haven't run any numbers on atmospheric density, minimum mass of a pressure vessel, or even energy requirements for escape velocity, so I can't give you a specific boundary of technology or gravity well at which this all becomes possible, but it's certainly another option to consider.
The planet has been either visited or inhabited by a spacefaring civilization at some point in the past. Now only their machines remain, and at least some of those are still functional.
This is seen in:
- Steven Universe;
- Stargate (as seen in wetcircuit's answer);
- Starcraft, at some point in the Protoss history;
- DC Comics - in some stories (War of the Light, if I recall correctly), the Daxamites gave up on spaceflight and destroyed all the related tech on their planet. This was not enough to keep some of them from eventually leaving the planet, though.
A gift from the aliens
Aliens in a low-tech scenario are given the means to travel to space by a more advanced power.
- The Green Lanterns do it all the time. It is kinda their thing.
- In the Fallout series of videogames, a cowboy, a samurai and a 21st century soldier live in a UFO.
- Some crackpots imply here and there that some humans have been taken to space by the Anunnaki.
Simple, raw oppression
Humanity is kept at a low technological level by a higher power. People may go to space if they have permission - or if they steal a ship.
- In the Old Man's War, the supranational, spacefaring government of Earth separates humankind in two groups: those who leave the Earth, and those who don't. The latter are kept in the dark about interplanetary affairs and are not allowed to build their own ships, nor communicate with aliens.
- In the Marsbound trilogy, a post-singularity civilization renders electric and electronic machines powerless on Earth. To add insult to injury, they disintegrate the Moon into dust and put its remains in various LEO's, so that anything that attempts to go into space will disintegrate upon hitting that powder. A martian ship is allowed to land on Earth and later leave it, though.
And if none of those suit you...
The Discworld is a flat, discoidal planet. The various civilizations of this world enjoy technologies ranging from fire & wheel to victorian, averaging on medieval. In the second book, the astronomer-priests of one of the rim islands send a mission to space, to find out some important things about the underside of the disc:
Tumbling past, totally out of control, is the bronze shell of the Potent Voyager, a sort of neolithic spaceship built and pushed over the edge by the astronomer-priests of Krull, which is conveniently situated on the very rim of the world and proves, whatever people say, that there is such a thing as a free launch. (...) Plunging along some two miles above (...) is Rincewind the wizard, in what on the Disc passes for a spacesuit. Picture it as a diving suit designed by men who have never seen the sea.
If your planet is not flat, then you need at least two catapults. The larger one will shoot the smaller one outta the atmosphere. The smaller one then acts as a launch-stage and shoots the spacefaring vessel proper, giving it the delta-v it needs for a stable orbit or escape trajectory.
Building from the low gravity answer...
Imagine a bunch of rickety, wood and iron barrels sealed with pitch, exploding through the sky using Orion-project style propulsion that uses black powder instead of nukes. The barrels join up in orbit to form a shanty town of a space station. In the center are great brass chambers with glass skylights, some ornately stained with religious imagery, letting light shine down on farm plots. Many of these stations are destroyed by careless docking accidents, mishandled propellents, spontaneous decompression, and wood worms, but fortunately this race is as ambitious as it is prolific, and there are always more astronauts to send up!
Navigation would typically be a real challenge for such a primitive folk, but you don't need driving directions if you don't have a destination. These are explorers! Like the spores of a mushroom, they spread out in their rickety death traps across the cosmos, hoping to find a rock to land on. Many of these vessels wander the vacuum of space for hundreds of years, with generations of passengers living out their days in the cramped vehicles.
Would help if aliens had slow metabolism, high radiation tolerance, and healthy dose of crazy.
solar sailing and gravity currents
This is based on those little solar windmills that you can make (the ones that spin when they hit sunlight) it takes that same idea that makes those things spin and you make a sail out of it. idk how they get into orbit but once in orbit these sails can catch the sunlight like a sail boat catches the wind. the crews are exports at using the gravity of interstellar objects to speed around, but the craft itself would be carved out of a solid piece of obsidian (can't have air inside the walls because then it would explode)