There is a double planet system. Both planets are habitable. One planet has humans and the other one animals. So instead of seeing a grey Moon in the sky, the humans living on the first planet see another world with forests, oceans, rivers, etc. So it is widespread knowledge that the "Terra" is just another planet, that humans live on a sphere. I want these double planets to be closer to each other than our Moon is to our Earth, but I want to have a scientific facts whether that is possible or not. For long time, humans on planet 1 (the "Terra") have looked at the habitable planet 2 (the "Luna"), but they haven't been able to reach it yet, until now.

On the "Earth" there is a civilization with pre-modern technology. By that I mean that they haven't had the industrial revolution yet. But they are slowly going into an industrial evolution. I imagine this civilization being similar to medieval China. Medieval China was known for having a strong totalitarian government, and many advanced mechanical inventions. The Roman empire would also be a good example describing this one empire on the planet "Terra".

The most advanced civilization on "Terra" is an empire with a strong government, prosperous economy, and a current level of technology similar to Leonardo DaVinci, medieval China, or Hellenistic Egypt. This empire is starting to undergo an industrial evolution, with the steam engine being just invented. The government heavily finances science and technology, employing geniuses similar to Archimedes or Newton. So they have access to both mechanical engineering knowledge, and mathematics physics such as calculus. Although cars and trains have not been invented yet and all travel and work has to be done by beasts of burden. The empire is about the size of maybe 2/3 the United States, with lots of fertile land, natural and man made waterways, coal and metal deposits near the surface. It is ruled by an emperor as a supreme monarch with a centralized bureaucracy similar to medieval China. The empire is now currently stronger than it has been ever before, with it's major rivals defeated, and no internal or external threats.

The totalitarian government of this empire can make it's citizens do whatever it pleases. The empire has built pyramids, great walls, canals, bridges, aqueducts, and roads without too much difficulty, and zero social protests. Now they choose to go the the double planet "Luna". The emperor gave his word, to employ all the resources of the entire empire for achieving the singular goal of landing people on the "Luna" as representatives of the empire.

Assuming that the atmosphere and the biosphere of the "Luna" is compatible with "Terran" life, including humans, who do not need a special suit or anything to live there. "Terran" animals and plants can survive in "Lunan" environment, and vice versa. Sure the animals and plants of both these double planets have different evolutionary histories, but they still have the same DNA cell based biological structure. Perhaps they were descended from the same microbes which seeded both these double planets.

Just like medieval China, the empire has knowledge of rockets, having used them successfully in warfare to conquer less advanced civilizations. The plan is to somehow be able to get people, animals, and plants from the empire up to the "Luna", have them survive the landing (or use parachutes which are known by then), then establish a colony for the empire, and after some period of time march to a certain region, and light up a huge fire that could be seen from the "Terra" as a sign of success. Because the "Luna" is easily seen from the "Terra", maps of the landscape have been made in advance. It is a one way trip only, and other than lighting up large fires there is no method of communication. The empire doesn't have electromagnetic communications technologies yet.

Assume that the government is determined to get humans up to the other planet, using all the economic, human, and technological resources that they have available, and can try to extend their resources by encouraging progress in the related fields. Assume that there are little to no inefficiencies in the government, no social unrest, and the people can be brainwashed to do almost absolutely anything. Also assume that this is a long term goal. It's not a space race. The empire can work on this for hundreds of years, gradually advancing their technology through the industrial evolution. But they will not give up until they can see the outlines of towns on the other planet. Either the emperor is extremely long lived, or the entire dynasty has the same mindset.

Actual Question

Could such an empire realistically achieve such a task? If so, how would they do this? I mean, what would be the absolute minimum level of technology to establish a colony on the "Luna". If they need to wait hundreds of years until they develop cold war era rockets, they will do just that. But can they get to the "Luna" before reaching that level of technology though? Can they do it using (medieval China style) solid fuel rockets alone? Can we think of any other alternative launching systems that can be made with pre-industrial or early industrial technology? And what about landing on that other planet? And they also need to lift not just two men, but a whole population of men, women, farm animals, and plants in order to establish a colony on the first try. There is no way of going back for the colonists.

If there is a way to modify the planets to make this story more plausible, such as decrease the gravity, or make the planets closer together, that is acceptable, but it has to be science physics based.

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    $\begingroup$ Your scenario is rather similar to the novel The Ragged Astronauts, by Bob Shaw. I'd strongly recommend reading that before putting much effort into this concept. $\endgroup$ Oct 24, 2020 at 21:16
  • $\begingroup$ @DKNguyen I think that life could arise on an Earth like planet having a smaller gravity, maybe like the Moon or Mars. I am not sure by how much the gravity can be decreased and still keep the double planets arrangement. $\endgroup$
    – Galaxy
    Oct 24, 2020 at 21:36
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    $\begingroup$ I was thinking on From the Earth to the Moon from Jules Verne. It uses the same premise of the OP for getting a rocket based on powder. Although they would need quite accurate calculations and physics knowledge. I would suggest them to first trying to get a cannon ball on those forests (or at least not to fall on them!) before attempting to place people inside. $\endgroup$
    – Ángel
    Oct 24, 2020 at 22:02
  • $\begingroup$ What sorts of tidal effects would a mass this large have? That could be an evolutionary factor in life on this world when compared to ours. $\endgroup$ Oct 25, 2020 at 3:50
  • $\begingroup$ ObSMBC: smbc-comics.com/comic/2013-01-12 $\endgroup$ Oct 25, 2020 at 6:00

8 Answers 8


No. It can't be done.

A medieval civilisation, let alone an imperial system, lacks the materials technology to build the large rockets needed to travel the distance from Earth to the Moon (as is the case in your scenario). They also lack the resources and the knowledge to understand the problems they need to circumvent for translunar travel.

The USA had to devote roughly 5% of its GDP in the 1960s to fund, service and complete the Apollo Project. Now it might be possible to imagine the empire on Terra developing industrial technology and eventually mechanical forms of transportation such as railways, steamships, aviation and jet aircraft. This will transform the empire into a twentieth century industrial society. Such a society could devote the intellectual capital, the scientific and engineering innovation, and material resources necessary for travel to Luna. In our world we cannot contemplate dispatching a colonizing expedition to our Moon. While your Luna has the advantages of being a habitable planet, the same physical and technical obstacles exist.

Basically travel to and dispatching a colonial expedition to Luna by even a global medieval empire would be far beyond their capability.

Would moving the planets in the binary planet system closer together make it easier? Also, would reducing the planets' gravity make it easier?

In the principle, yes. Although you don't want to move both planets too close. This might put them in each other's Roche Limit and tidal forces would tear the planets apart. Rocket propelled vehicles would only need lower values of their top velocities to travel between the double planets. Planets with lower gravities will require rockets with lower escape velocities.

This all looks good, but unfortunately it's not. All the science, technology, industrial capacity, engineering and resources will still be far beyond any medieval civilization. A global-spanning empire on Terra may command greater resources and knowledge than any medieval civilization in our history, but it will still far short of what is required to travel to the other world of a binary planet system.

Once such a civilization has progressed beyond the medieval and developed modern technology and science it would be capable of such travel. Possibly their version of early twentieth century could begin travel to Luna, at least, experimentally. Having a habitable planet beckoning in your sky for all of their history will provide a ready impetus to develop space travel and its technologies much earlier than in our history.

Is there another science-based solution? Strictly, no. Since this is a work of fiction it is possible to imagine a medieval version of antigravity, Say, the equivalent of Chinese alchemists inventing something like Cavorite. Then using for manned flight and eventually space travel. But this is a science-fictional solution to what is otherwise an impossible problem.

CONCLUSION: Spaceflight between planets separated by the equivalent distance from Earth to the Moon is unobtainable by medieval societies. The technology is beyond their capacity. It is a practical impossibility. The alternative solution is to bypass the science and employ an impossibility (antigravity or a "magic" space drive) to permit medieval space travel.

  • $\begingroup$ A habitable moon also has the benefit of a thick atmosphere, meaning you'd be able to aerobrake and significantly reduce the delta V costs. Together with the lack of need to haul nearly as much equipment - you can gather many resources in situ - it would be a one-way trip, but we should be able to set up a colony there with today's tech. Pretty much Apollo except that the astronauts decide to start taste-testing the local flora instead of bagging up and going home. $\endgroup$ Oct 25, 2020 at 16:45
  • $\begingroup$ @JohnDvorak Aerobraking would help reduce delta V & this could be feasible with current tech. But the question was about medieval tech. Lower mass planets closer together would help. Plus aerobraking. Actually I felt sad explaining why it wouldn't work. A lovely idea of medieval astronauts travelling to the other component of a double planet. Taste-tasting the local flora would be inadvisable. Many plants can be poisonous. Better to bag them to bring home, then find out if they're safe to eat. Thanks for commenting. $\endgroup$
    – a4android
    Oct 26, 2020 at 1:11
  • $\begingroup$ reducing the surface gravity signifcantly presumbly has major impacts on the atmosphere as well $\endgroup$
    – jk.
    Oct 26, 2020 at 9:50

The Basics of a Binary world

As it just so happens to happen, i too work on a Visual Story about a Binary System. Only in my case it´s the Sisters Hela and Vesna. But the General idea for both of us is the same. Two VERY close Planets. Like this:enter image description here.

The only really important information about such a world for this question is the lack of Movment between the worlds. This means you can, in theory, just go up and will land on the other plant.

The Problems

Now for a Modern Civilisation, this sort of set up is a dream. Going between words takes less than a day and the Math is super simple. Not to mention of the stripped down Life Support and so on.


For a Civilisation such as your´s the Planet might as well be on the other side of the Universe. Sure, IF you can Build a Rocket with 8km/s of Delta V you will reach the other Planet. But then what ? You fall on this world. Which means you might enter the Atmosphere at 8-9km/s. Even modern Spaceships cant do that.

IN addition to that, your people have no way of knowing if a Mission worked or what went wrong. So there is no way to improve anything. And this does not even mention that they cannot build a Deep Space ready craft. Anything they send up will either be Destroyed by the heat of the Rocket and lack of Radiators, Explode or Freez.

And again, when something goes wrong, they have no way of knowing WHAT happened.

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    $\begingroup$ Since most of today's spacecraft only go to low orbit, they are only designed to re-enter from orbital speeds. Apollo re-entered at 11 km/sec and did just fine, and the Stardust capsule re-entered at over 12 km/sec (although it crashed, it didn't burn up in the atmosphere). Heat shields are a materials problem, at least theoretically possible at this tech level. $\endgroup$ Oct 25, 2020 at 5:53
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    $\begingroup$ I would disagree. All of these Crafts more or less entered Horizontaly, while any low tech space craft will just enter vertical. This will destroy the craft as it enters dense part of the Atmosphere. $\endgroup$
    – Erik Hall
    Oct 25, 2020 at 11:46
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    $\begingroup$ To add to fluffysheap’s comment: Cork is a common heat shield material. So this would probably be the easiest part for a medieval society. $\endgroup$
    – Michael
    Oct 25, 2020 at 15:02

If your civilization has gunpowder, then this discussion at Space SE might be of interest. Using our Earth-Moon as reference, it would take a lot of gunpowder. And that assumes you fire the ship from a cannon, which would kill the astronauts. To have a ship that accelerates at a tolerable rate, and also has extra gunpowder to allow landing on the other planet, would take a lot more.

  • $\begingroup$ OP explicitly talks about landing, while the referred question makes clear that at most it would be a crashing. $\endgroup$
    – L.Dutch
    Oct 25, 2020 at 15:39
  • $\begingroup$ @L.Dutch-ReinstateMonica even with the ability to aerobrake at Luna? $\endgroup$ Oct 25, 2020 at 16:52
  • $\begingroup$ @JohnDvorak aerobrake with which atmosphere? $\endgroup$
    – L.Dutch
    Oct 25, 2020 at 17:07
  • $\begingroup$ @L.Dutch-ReinstateMonica Galaxy's Luna is stated to have atmosphere compatible with Terran life. That implies roughly standard gravity, temperature and pressure. Humans are picky. $\endgroup$ Oct 25, 2020 at 17:10
  • $\begingroup$ For those more unfamiliar with orbital mechanics, this answer should explain how crashing on the Moon is rougly equivalent to aerobraking at the "Moon". $\endgroup$ Oct 25, 2020 at 18:10

Unless you are willing to MASSIVELY reduce the size of your "Earth", thus greatly reducing the velocity required to orbit it, no deal.

Using our Earth and history as a template, the absolute earliest we could aspire to get to space would be in the 1910-1920's. This is on the assumption of infinite goodwill and cooperation being available. Think something like a massively scaled up and complexified German A4 rocket.

You need a pretty good metallurgy, moderate chemistry, and a stonking huge infrastructure of manufacturing and supply chain.

No, a gunpowder cannon, even of city size, simply cannot the velocities needed. Not to mention it turns any passenger into strawberry puree at launch.

Sorry, but mere motivation, even at global and religious-fanatic level of support, cannot facilitate space access without the required technologies.

P.S. I refer to "getting to space", as that is a necessary step to reaching another planet, AND is by far the hardest and most dangerous part of the job. Once you are in orbit, you can use much more leisurely ways to get to the other planet. And landing on a planet with atmosphere is as simple as wrapping your lander in a nice thick layer of cork, then once slowed down dumping the cork heatshield and deploying parachutes. (no return option!!!)


The problem you have is the gravitational pull. You have to break out of Earth's (thier earth not ours ) Gravity well. Doing this requires a lot of energy. Much more then what a primitive civilization probably couldn't accomplish on our Earth. However we don't know enough information about your planet there are theoretical ways For planet to be formed that make escaping the gravity well easy I recommend the you go to YouTube and look up Isaac author as he has a whole video on this I will summarize one of his propositions here

Mountains it's hard to imagine a mountain large enough to get you into low orbit but we must remember that even within our own Solar System There are mountains that make mount Everest look like it tiny pile of pebbles. So it's possible that a planet could have amounted tall enough that it counts the energy cost of breaking out of orbit

Spaceship The complexity of your spaceship is dependent on how long you need it to stay in space by Lowering the cost of getting out of orbit You have already minimize the complexity considerably. At the very least You'll need something that is airtight everything beyond that depends on how long you gonna be there (in space) and how many people you're bring. I would look at some of the very old subs designs I had to make a guess I would say you'd need at least that Tech level.

I strongly however recommend you watch the video as It goes into a lot more detail then my failing memory If I have time I will edit this post and add the link to the video.


  • $\begingroup$ It would be nice if you actually linked to the video $\endgroup$
    – Ángel
    Oct 24, 2020 at 22:03
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    $\begingroup$ Mountain can't significantly help you to get into—and stay in—space, because you need the velocity, not so much altitude. You'd need to climb above the geostationary orbit (that way you'd get the speed just from the rotation), but for a normally spinning planet that is many times higher than the radius of the planet (almost 6 times for Earth) and you can't have a mountain bigger than the planet, because the mountain would be the main mass and shift the stationary orbit correspondingly further out. See also what-if 58. $\endgroup$
    – Jan Hudec
    Oct 25, 2020 at 14:11
  • $\begingroup$ Never heard of a space elevator? $\endgroup$ Oct 25, 2020 at 19:24
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    $\begingroup$ The main advantage of using a high mountain as a launch site is that you skip the thickest parts of the atmosphere, allowing you to spend more of your fuel on velocity, rather than wasting it bullying through air. It's more efficient. $\endgroup$
    – Ruadhan
    Oct 26, 2020 at 8:50

If stars align with a tonne of factors - yes:

You haven't specified exactly:

  • how close they are to each other, just "Closer than the moon".
  • whether they're tidally locked
  • the mass of the planets
  • the speed of rotation

So I'm free to fine tune these. If I tweak these by taking this wonderful maths and stating that the planets are close enough to share an atmosphere, but not without crossing their respective Roche limits. (Eg - the air flows, but they don't rip each other to shreds).

The planets are also distorted by each other into ellipses, bringing the surfaces closer to each other by several hundred km.

enter image description here

The distortion of the ellipses also affects the atmosphere and results in increasing the air density in the region between the two planets. The entire atmosphere will thinner on the outside, and there'll a permanent low tide on the outside.

It looks like its quite feasible that the surface-surface distance is only ~100-200km. Since the air density is higher between the planets, you may not even need supplemental oxygen for the trip.

This should be possible in a hot air balloon. If you reduce your buoyancy right as you approach the midpoint, you'll transition into the other planets gravity well and start slowly falling onto the other planet. This may need some wiggling or ballast tossing, and could take a lot of skill to get right.

Spider silk strands have a breaking length of about 110km. Once again, if all the stars magically align and the world is ideal for this mathematically, you could run a spider-silk woven rope between the two and use it to guide your hot air balloons as a primitive space elevator.

  • $\begingroup$ Wouldn't a hot air balloon go away from the point of densest air, rather than towards it? $\endgroup$
    – das-g
    Oct 25, 2020 at 15:32
  • $\begingroup$ @das-g I don't think the densest air will be at the barycentre of the system, I think there will be 2 maximum density regions at the surface directly below the other planet, as that's where the pulls are strongest. $\endgroup$
    – Ash
    Oct 25, 2020 at 15:36
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    $\begingroup$ I think the point between the two worlds would be a hellish place. The net gravitational attraction at the surface would be very low due to the upward force from the other world. Imagine what a hurricane would be like in ultra low gravity. I suggest that the whole area would be one giant rotational atmospheric vortex filled with debris. Water foam, sand, vegetation and anything else that might be moved by a hurricane. The electrical storms would also probably be fairly spectacular as well $\endgroup$
    – Slarty
    Oct 25, 2020 at 17:08
  • $\begingroup$ One word: Rocheworld $\endgroup$
    – straycat
    Oct 25, 2020 at 18:05
  • $\begingroup$ Perhaps you look again at the question you cite with the "wonderful maths". It gives a separation of around 1,000 km which isn't exactly the same as several hundred km. Vide also Robert Forward's Rocheworld and its four sequels, Bob Shaw's The Ragged Astronauts trilogy, and a Charles Sheffield novel the name of which I can't remember right now. $\endgroup$
    – a4android
    Oct 26, 2020 at 1:20

Before the snows came--back when Luna was large in the sky every day, and not just during the Festival of Tango--there were great flying creatures that we could ride on. The air that covers Terra doesn't extend high enough for us to fly there, but those among us with a cruel streak would tax their beasts nearly to death trying to get as close as possible.

Then The Interloper came. Nobody saw it strike Mnemos No More--then the smallest moon of Theia--but we saw the havoc in its wake and our best arithmancers agreed that it was heading towards The Twins Luna and Terra.

Our ancestors hid underground and prayed--and it is well that they did. But the greedy, and the insane... They took to the skies to meet The Interloper head-on.

By some twist of fate, the Interloper split the gap between The Twins without striking either one. The three bodies danced a fleeting chaotic tango of near destruction. The Interloper was visible with the naked eye for just two days, but the earthquakes lasted for years, and our orbit changed forever. Then the snows came, and the great beasts of the sky died away.

Just a year ago, not even the wildest among us would have believed that any of the fools that took flight against The Interloper had survived. In fact, many would tell you that it never existed. They say it's just a story used by the Pharaohs to keep us in line. (They're fools too). Memory fades too easily since the sky lost Mnemos.

Last month the Imams completed the largest telescope built since the snows came. During the Tango festivities, the Highest used it to survey the surface of Luna. He saw it spangled with festival pyres--and everyone knows that he cannot lie.

The children of the lunatic fliers survive, and they too celebrate Tango.


Actual Answer: No. Outside primary school, such a civilisation could not realistically achieve such a task… and please remember “empire” is a purely political term but you Asked about culture and technology; by which way, Worldbuilding SE suggests doing some basic research first.

If you want to build a world where it’s simply stipulated that a rocket can reach the moon, why not follow Wells, Herge or Wibberley, among others? It shouldn’t take hand-waving to make it obvious that visitors to your world will no more challenge what you say about your rockets than they would the true rules of real rocket science, provided only that your building is worth its own while.

If medieval-Chinese solid-fuel rockets could reach the moon why would even the Nazis, let alone America or Russia, modern China or India or Europe spend squillions replacing them? Similarly, your empire will need to wait hundreds of years to develop cold-war era rockets, by which time it will not be the same empire. Politics might endure; technology and society will change. Think Teflon and ball-point pens, among many.

Without cars or trains, why might your society be tempted to go to the Moon? “Because it’s there?” Does “pre-modern” allow telescopes, or can the people not see forests, oceans, rivers, etc? Either way, how would it be known at all that the other was “just another planet”, or that the humans’ home was a sphere?

If your objects are closer than our Moon and Earth and both “planet sized” why will they not collide?

How would you use parachutes for planetary landing - not splash-down; moving from orbit? Why is this trip one-way only?

If any of medieval China and Imperial Rome, renaissance Italy or Hellenistic Egypt are comparable, what lets them jump the century or two into an early Industrial Age? What makes you think employing geniuses gives access to knowledge, rather than wisdom?

When “One planet has humans and the other animals”, where did you find ”Terran" animals? Are the humans vegetarian, or what?

Didn’t medieval China famously not use rockets - or gunpowder - in warfare? Wasn’t the first use of firearms in 15th Century Europe, and of military rockets in the Napoleonic wars a lot later? Come to that, what less advanced civilizations did medieval China conquer? Wasn’t it too protectionist to be interested in foreign expansion?

  • $\begingroup$ The empire is not a clone of China as this is another planet, but it is a classical "ancient civilization", specifically based on medieval China, because that empire started from a small bronze age state, and gradually expended over land to fill almost the entire continent, vs a bunch of scattered sea based colonies as in Europe. Then during the empire's "Pax Romana" period the government starts funding science and technology. Sooner or later they would start thinking of going to that other green world in the sky. And yes they do have telescopes. $\endgroup$
    – Galaxy
    Oct 25, 2020 at 23:37
  • $\begingroup$ And jumping a century or two is likewise based on Earth's history too, but this is another planet with it's own history and it's own pattern of development. The period of time between a highly developed "ancient civilization" and an Industrial age is arbitrary. The Terran empire is basically similar to the Fire Nation from Avatar the Last Airbender, but without the magic. A stable empire graudally developing it's technology. $\endgroup$
    – Galaxy
    Oct 25, 2020 at 23:41
  • $\begingroup$ “Jumping a century or two” is based on the only comparison there is, bar hand-waving. “… they haven't had the industrial revolution yet” demands specific comparison. If “Sooner or later they would start thinking of going to that other green world in the sky” is a given, how many people here d’you think saw a lunar landing as anything more than a green-cheese chase, before about 1900? If China isn’t a suitable comparison, why is it in your preamble? If there was a ‘classical "ancient civilization" ‘ why not use that, describing it first? Come to that, does “classical” here mean “classic”? $\endgroup$ Oct 27, 2020 at 19:24
  • $\begingroup$ If this new stuff matters, why not Edit your Backstory? If you’re happy dodging the kind of detail a clunky old Galilean telescope won’t see, why sweat the rest? Why not squint at the moon with your naked eye, then look up Gallileo’s results. No real river or forest level, and what would identify “ocean” as against “that flat bit”, without an orbital telescope to tell you what your ocean’s look like from above? I forgot to ask, isn’t petrol about 300 times more explosive than gunpowder; modern rocket fuel more so yet still NASA needed 10 lb of fuel to put 1lb of payload into orbit? $\endgroup$ Oct 27, 2020 at 19:25

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