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What changes would you have to make to humans, the Earth, or the entire universe for humans to be able to go to explore the Moon and Mars with the technology levels of the Renaissance?

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    $\begingroup$ We can barely even explore Mars with our current tech level, I don't see how it could be remotely feasible that renaissance level humans could do it without access to technology that makes them unrecognizable from the Renaissance.. $\endgroup$
    – Bitsplease
    Jun 3, 2020 at 23:14
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    $\begingroup$ Not a direct answer, but a great way to give people an incentive for quickly developing spaceflight would be to replace the moon with a habitable earthlike planet. If the people could see that there are forests, oceans and meadows the imperialist mindset will create an early space race. $\endgroup$ Jun 4, 2020 at 8:24
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    $\begingroup$ Not an answer but you might be interest in reading the short story called "The path not taken" or something like it. It revolves around all other species developing space flight in their Renaissance and as such not progressing much further than that. It handwaves it away though but you might find it enjoying and useful in other aspects perhaps. $\endgroup$
    – Robin
    Jun 4, 2020 at 12:13
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    $\begingroup$ For an alternative take, try reading "Children of Time" by Adrian Tchaikovsky. Really good book, and features space travel by a species that cannot smelt metal. They use lightweight construction and gasses to lift structures to near orbital heights, massively reducing the energy cost and complexity of launching space vehicles. Won't be a steampunk spaceship, but you can expand your options with a novel approach! $\endgroup$
    – Jez
    Jun 4, 2020 at 16:48
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    $\begingroup$ @Robin I believe you're referring to "The Road Not Taken," by Harry Turtledove. $\endgroup$
    – notovny
    Jun 4, 2020 at 19:10

12 Answers 12

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The problems

I'm having a hard time being optimistic about this. The technological gap between the Renaissance and the present day is enormous. Our understanding of science and our abilities in engineering are nowhere near where they were half a millennium ago. Renaissance humans would need to make some enormous technological leaps to even fly like modern humans do, let alone go to space.

There are some particular places where Renaissance people are scientifically or technologically deficient:

  • Propulsion. If you want to get to space, you need to know how to make and (more importantly) handle highly reactive, dangerous, explosive materials. That's strike one - I don't see a way for Renaissance people to, say, produce or store liquid oxygen. Back then, we didn't even understand how combustion worked, let alone hypergolic rocket fuels.

  • Computers. You don't need a supercomputer to calculate the right trajectory for a spacecraft, but you probably still need a computer. Figuring out paths by hand would be extraordinarily time-consuming. I'd also argue that computers would be needed once in space, to ensure that the engines burned exactly as needed.

  • Our understanding of science. Kepler and Newton wouldn't come around for another century or two, meaning that we really didn't know why objects moved in space, nor how a spacecraft would move once it got up there. We lacked calculus and numerical techniques to computer rocket trajectories. Plus - and this could arguable go under the section on propulsion - we didn't understand chemical reactions. Good luck with those explosions.

These are only a few of the main issues. Renaissance people were several centuries behind on all of them.

Potential solutions

Here are some things that could at the very least make it slightly less impossible to get to space:

  • A lower surface gravity. If you need less energy to escape the planet, maybe you need less volatile fuels. Perhaps you can get by with gunpowder for the initial launch, then use the dangerous stuff once you get to space.
  • A more interesting night sky. Maybe there's a reason for people to look up at the sky more often - perhaps there are more comets, or more extreme auroras, or a second moon, or something else that makes scientists think, "Huh, that's worth studying." This has the potential to accelerate the study of physics and astronomy.
  • A planet conducive to flight. Maybe the world has lots of forests, inviting people to live high in the trees, or high mountain ranges. Characteristics like this would make flight more convenient, thereby inducing more study of aeronautics and, eventually, travel in the upper atmosphere.

These really won't make it technologically easier to get to space. What they will do is make people more interested in studying and exploring the sky. And if people get curious about something, they have a tendency to, well, study and explore it. All of sudden, going to space might sound pretty interesting, and by the time of the Renaissance, people are at least interested in working towards spaceflight, even if they won't achieve it for another century or so.

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    $\begingroup$ To be fair, the night sky is already quite interesting. It's just that modern urban light pollution blots most of it out. Out in the desert, you can certainly see the Moon, you can also see the Galaxy itself as well loads of brightness differentiated stars. $\endgroup$
    – elemtilas
    Jun 3, 2020 at 15:25
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    $\begingroup$ I'm going to call bull on computers being required, at least as most people understand computers. Radios yes (but those don't even need solid-state electronics, and with the right knowledge could be put together in Renaissance times), but there's no reason that navigation would need a computer (appropriate charts, specially designed purely mechanical tools, a telescope, and a slide rule should be sufficient for basic near earth navigation), or that the engines couldn't be mechanically controlled (though that would again require specific knowledge). $\endgroup$ Jun 4, 2020 at 1:44
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    $\begingroup$ @AustinHemmelgarn Even if in-flight computations could be done manually - and I'm not saying that can - you'd absolutely need computers to determine trajectories prior to the mission. You won't need a supercomputer for that, but a regular computer is definitely required. $\endgroup$
    – HDE 226868
    Jun 4, 2020 at 1:52
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    $\begingroup$ @HDE226868 And there's no reason that a specialized mechanical calculator couldn't be made to do that, at least for near-Earth trajectories. I'll concede that pretty much anything beyond the Moon would require too much processing time for such a device to be practical, but just getting to orbit and back on an impulse trajectory should be doable (if admittedly time consuming and probably with space requirements equivalent to or greater than ENIAC). And the concept isn't too far fetched for the time period, given stuff like the Antikythera mechanism. $\endgroup$ Jun 4, 2020 at 2:01
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    $\begingroup$ I think gunpowder would actually work in space, because it carries its own oxydiser. Guns can be fired under water after all (if you can keep the powder dry). >Gunpowder consist mainly of nitro-cellulose. Applying heat causes nitro-cellulose to decompose into carbon dioxide/monoxide, nitrous oxide/dioxide and water vapour as gasses in a violent exothermic reaction. The generated heat increases the speed of the reaction. $\endgroup$ Jun 4, 2020 at 8:21
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If the medievals were correct about the structure of the universe, then all you need to get to the stars (and everything else in the heavens) is a big enough ladder or staircase, not unlike the ill-fated Tower of Babel project.

Or you may be able to get there just by finding the edge of the world (see image below, 'Flammarion' engraving) Getting through the crystal spheres might be a challenge, but medieval technology should produce adequate drills or other tools for the job.enter image description here

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    $\begingroup$ I like this answer because it doesn't change the definition of available technology, but the definition of 'space' - and if the OP is willing to seriously adjust the direction of technological advancement and perhaps even gravity in their world, as the answers above suggest, why not the nature of outer space instead! $\endgroup$ Jun 5, 2020 at 9:19
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People who ask how to do X at type Y questions regularly forget...

Technology is a pyramid. The ability to achieve something depends on the experience, innovation, invention, etc. of all the predecessors. You can sometimes skip small parts of that — but you can't skip whole chunks of that.

What you're asking is to take the technological development of the 1950s–1960s (which was needed just to get off the planet much less travel somewhere) and push it back to the 1500s.

In reality, what you just did was make the 1500s the 1950s–1960s.

Therfore, the real question is, can the sociodemographic development of society be restrained such that 500 years of technological development can occur while still having a feudal society? (I'm assuming a European perspective.)

The idea was explored a little bit in the Star Trek Original Series episode "Bread and Circuses" where the Enterprise crew discovers Ancient Romans with TVs and modern military weaponry.

However, in reality, it can't be done. Sociodemographics are part of the pyramid I spoke about earlier. As your technology becomes more advanced, your economics and political structures must also become more advanced to "handle the load."

Should this stop your story?

No. That Star Trek episode proves that there's space for this kind of alternate history story. Consequently, you might want to open another question. Something along the lines of, "What political and social pressures would allow the government and social behaviors of 1550..."

Note: Please be specific, even "The Renaissance" is too ambiguous when it comes to social and technological development.

"...to continue to 2020?"

Note my use of 2020. We don't have the tech quite yet to fly to Mars today, so you're not really looking for the 1950s–1960s.

Your goal is to do something like Start Trek's "Bread and Circuses." You want the social atmosphere of the past and the technology of the present. It will be much easier to retard social development (IMO) than it will be to speed up technological development. Realistically it can't be done... but from a story perspective...

But in the end, you might need to introduce a "gimmick"

Star Trek's "Bread and Circuses" didn't use a gimmick to justify the advanced tech of ancient Rome. They didn't need to — that wasn't the story they were trying to tell. (And, to be fair, it really wasn't ancient Rome with advanced tech. It was a modern Rome that had preserved its traditions — not unlike the Native Peoples of North America who drive cars and run modern businesses but still celebrate their ancient past and culture.)

But, if we look at another episode, "A Piece of the Action" we find a different perspective. This episode was investigating the issue of cultural contamination (and that might be the easiest way for you to achieve your goal). A book about gangsters is left on a planet and the planet modifies its sociodemographics to conform with "the book."

In your story, you might need to come up with some form of "cultural contamination" (aka, A Connecticut Yankee in King Arthur's Court) that rationalizes the mix of ancient culture and modern tech. If you don't, I can't see a way for you to make it work.

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  • $\begingroup$ can you elaborate regarding the sociodemographic? i can understand the economy such as industrialization, but politic? $\endgroup$
    – Li Jun
    Jun 4, 2020 at 6:17
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    $\begingroup$ @LiJun Human culture is complex - even back then. Politics, economics, religion (very religious period), language, even cultural shifts across national borders. The average info we come across in a day today exceeds what they were exposed to in a year or even a lifetime. It was a time of superstition and, when you realize that, you discover that the science needed to get to the moon would have to contend with the Spanish Inquisition. And if you change the universe to make it easier, you change all of those things and more. In a nutshell, that's what I mean by "sociodemographic." $\endgroup$
    – JBH
    Jun 4, 2020 at 14:36
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You need a universe where:

  • Everything is MUCH closer. The Moon needs to be only a few thousand miles away, not hundreds of thousands. The other planets can't be much farther. Ancient folks had no way to build rockets or heat shields, nor to astrogate effectively, so the Universe must shrink to a small, calm pond they can handle.
  • Escape velocity is much, much lower, so gravity works quite differently. Low enough for a kite or sailed vessel to not return under some circumstances. (We'll conveniently overlook that this prevents our form of life from existing)
  • Air is everywhere. Space must be breathable. Ancients have no way to compress or store air for weeks of travel, so the Universe must be tweaked a bit.
  • Water and Food must be available on most celestial bodies (not the Sun, of course. That would be silly)

This means that your ancients can take voyages quite similar to sea voyages, using their nifty-but-similar ether-ships or air-ships. Like sea voyages, they can use the winds and currents of space, astrogate using the sun and stars, and replenish their provisions at their destination while they deal with traders or monsters or whatever.

This also means, by the way, that monsters can jump off planets and become roving space monsters preying upon voyagers, and that space-pirates may swarm near wealthier planets.

This has been done, of course, in literature and cinema already: Baron Munchausen's trips to the Moon and dalliance with the queen there (The Adventures of Baron Muchausen, 1988), are merely one example.

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Space is really big. Although it looks close, the distance to the moon is colossal. Mars is monumentally further.

The definition of 'explore' is loose - at a superficial level we can 'explore' the moon through a telescope (at least the side that faces us) which is easily in the realms of the Renaissance without any Universe changes. If however, you mean physically be there to touch and stand on it, then here are some ideas:

  • The moon to be denser than Earth such that it retains a breathable atmosphere, to allow us to breath oxygen on it (so we don't die when we walk on it)

  • The moon to be much closer, perhaps only a 142 stories above say Mount Everest - as the 142m (the Strasbourg Cathedral) was the highest building in the Renaissance - although the gravitation attraction of the moon at this distance would be very disruptive.

Now for Mars, we need to rethink things entirely:

  • Make the whole solar system have a breathable atmosphere
  • Enable Hot Air balloons to reach much higher, perhaps using a material that weighs almost as light as helium but holds together a virtual vacuum, to allow balloons to escape the Earth's gravity.
  • As then your explorers can float in space and breath, allow them the ability to alter their direction through makeshift beating of wings, so they can slowly make it to mars and land on it (which now has also a breathable atmosphere).
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    $\begingroup$ And I just thought - OR the Sun could produce eddies and currents in the Solar System Atmosphere, which could be caught by sails on your balloon. By navigating eddies and currents, perhaps you could 'sail' to Mars - well within Renaissance abilities. $\endgroup$
    – flox
    Jun 3, 2020 at 15:07
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    $\begingroup$ If interplanetary space is filled with air then the key change to the universe is that gravity and/or friction work very differently - objects can't maintain an orbit within a significant atmosphere. $\endgroup$ Jun 4, 2020 at 3:09
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There was a story, by I think Pohl, but I'm routinely wrong. No doubt someone else will chime in with the name.

The premise of the story was that at some point during technological development of a civilization, they reach a crossroads. They reach a point where they either discover electricity, or, essentially, "warp drive". The "warp drive" lets them fly through air, and travel between stars. The discovery so fundamentally shapes the civilizations understanding of the universe that once they go down one path, the other path is very difficult to discover organically.

So, in the end, you have a race of beings that essentially have warp drive during what would be roughly the Age of Sail for us. The story did not go in to detail beyond that this race proceeded to run around and conquer other systems, using their flying craft and gun powder kegs to attack the lower tech species.

To this end, since the "hard part" is figured out, they needed to do just a few things.

  • Figure out how to seal their ships from vacuum. Whether this can be done with tar and wood, I honestly don't know.

  • Figure out how to create oxygen. This can be done chemically. Recall these folks only need weeks or months of supplies for their travels.

  • Figure out how to scrub CO2.

  • Figure out how to navigate. Honestly, I don't know how important an accurate clock is as a necessity for space flight, compared to how necessary it is for terrestrial and stellar navigation.

  • Figure out how to harden their ships to radiation. The trick here is them coming to the understanding that outside of the atmosphere, radiation is a bad thing. They could have easily used lead as a shielding, however.

  • Glass. They would need some kind of glass to make portals in their ships for observation.

  • A telescope would be handy.

  • And, as always is handwaved away in spaceflight fiction, some mechanism to manage heat.

They would perhaps needs some mechanism to pressurize gas (notably oxygen), if only to be able to operate some kind of airlock (and thus repressurize it). On the one hand, they can probably just turn up the oxygen generator and pressurize from the ambient pressure in the ship. Or they could simply be stuffing canaries in little ports that open to the outside to see if the atmosphere is compatible or not (and, obviously, if not -- don't open the door).

This would not let them explore the moon on foot. But they could do low level flying surveys of hostile environments, enough to know there's nothing worth conquering there.

The fundamental issue, is that much of the foundation science may not be available at this time to enable spaceflight.

But on the other hand, it certainly gives great incentive to develop it.

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    $\begingroup$ I believe this is the story you're talking about. scifi.stackexchange.com/q/33341/53771 It is a Harry Turtledove novelette: "The Road Not Taken". This was going to be the basis for my answer since the question seems to allow hand waving. $\endgroup$
    – Erik
    Jun 4, 2020 at 19:06
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    $\begingroup$ Yea, that's it. And I always get it wrong, and can never find it. $\endgroup$ Jun 4, 2020 at 19:06
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Seeing as you are allowing changes to the entire universe then I'd suggest wormholes connecting the surfaces of planets and moons. These could be naturally occurring on the surface of objects generating a large enough gravity, leftover from an ancient alien civilisation or perhaps generated by Leonardo's experiments with handwavium and plot particles. To me this seems an easier way for readers to maintain their suspension of disbelief than imagining rockets and spaceships so out of period. It's a major change from our universe, but it's easy to imagine. I'll leave you to work on portable oxygen supplies and heating for your diving suits.

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There is one simple change, maybe too simple for your needs: a visit by technologically advanced aliens, who leave some easy-to-fly machines behind.

An advantage is that if the humans do not understand the technology, you do not have to explain it to the reader either.

I can think of at least two examples in fiction where exploiting alien technology without understanding it at all permits humans to travel the universe: David Drake's Ranks of Bronze, and the historical backstory of Catherine Asaro's Skolia universe.

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    $\begingroup$ Doesn't need to be aliens. Could be a long collapsed advanced human civilization that left behind reliable or self maintaining idiot-proof machines. $\endgroup$
    – DKNguyen
    Jun 4, 2020 at 14:16
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Getting to Orbit with Renaissance Technology

The trick here is to create an environment where you can enter and exit space without entry heating or rocket power.

  • You can reduce orbital velocity by reducing the average density of the planet. Earth is actually quite dense, owing in part to the amount of iron in the core. Earth's average density is 5,493 ${kg} \over {m^3}$.
  • The lower limit of doing this (w.o. significantly changing Earth's chemistry is 2,329 ${kg} \over {m^3}$ (about half) for a rock world. This might drastically impact the presence of a magnetic field for the planet, and the protection from radiation that the magnetic belts provide. Or 1,000 ${kg} \over {m^3}$ for a water world

  • Orbital velocity is $v = \sqrt{{G M} \over R}$. With half the density, $M_{new} = 0.5 M_{earth}$, so orbital velocity drops from 7,900 $m \over s$ to ---> 5,587 $m \over s$ for a rock world (3,533 $m \over s$ for a water world).

  • The length of the day will contribute. A 24 hour day provides 463 $m \over s$. A 12 hour days gives you 926 $m \over s$ for free towards your 3,500 $m \over s$ orbital velocity goal.
  • Winds can also make up the difference. The fastest winds documented in the solar system are about 500 $m \over s$.

Tweak your world as you like until an airship aloft on the peak winds can reach orbital velocity. Likely this world is shedding atmosphere like Venus or ancient Mars. This world will probably, one day, be unable to support life. But that's in the future.

I'd imagine you'd have a pressure-sealed glider-like device, that catches the high-speed equatorial winds and rides them round-and-round the globe until reaching the fine mist leaving this fictional Earth behind for interplanetary space.

Getting to the Moon

Propulsion without rockets. Some sort of natural mylar would need to exist on your world to make further explanation possible. You're primitive explorers would use the material for solar sails. Just like Gallileo didn't exactly understand how sails work, your Renaissance explorers don't need a full theory of why solar sails work, they can simply have discovered that they work.

Time. An 800m x 800m solar sail picks up about 5 Newtons of force. For a manned 10-ton galleon, you'll pick up 1.8 m/s of velocity per hour. Reaching escape velocity of 5,000 $m \over s$ (for the water world) from a starting point of 3,500 $m \over s$ will require putting on an extra 1,500 m/s, after you've made orbit, which would require 833 hours (34 days).

The Apollo missions used a transfer orbit instead to get to the moon in 5 days time. It might be possible to do this with solar sails, or taking advantage of how the atmosphere is escaping. However, I feel like those will be dead-ends; but it might be worth trying.

Landing. Even though it only has ${1 \over {10}}^{th}$ the gravity of Earth, there's no friendly wind or fast spin helping you back aloft from the surface of the moon. Landing would be a one-way event; but maybe explorers could have attempted, or just flown by.

Navigation. The Antikythera mechanism and Bible passages about wise men following a star for ground navigation strongly support that celestial navigation has been available, in certain circles, for millennia. The spring watch and sextant were available in the 16th century for use. The gyroscope would be identified in 1783, although using it for navigation wouldn't be developed until 1885

Life support. Holding one atmosphere of pressure is not beyond Renaissance technology. Paintings show Alexander the Great being lowered underwater in a glass bathysphere. The pressure of going underwater (1 atmosphere every 20 feet) is quickly a bigger structural problem than just holding on to anywhere between 0.5 to 1.0 atmospheres. Lithium hydroxide was used by the Apollo mission for oxygen scrubbing, and served in that capacity for roughly 2 weeks.

Logistics. Could get much more precise on this, but a round trip to the moon, then, would very roughly be around 2 months long (acceleration, and deceleration). The average journey from France to America in the 1600s is reported at 7 weeks.

Warmth. Staying warm was a big problem for the Apollo astronauts when equipment failed. Lighting a fire is an option, although it puts additional pressure on the oxygen scrubbers. Blankets might be a possibility.

Communication. A heliograph could be used to communicate with other ships, or ground stations on the moon.

Getting off the Moon

Even though it only has ${1 \over {10}}^{th}$ the gravity of Earth, there's no friendly wind or fast spin helping you back aloft from the surface of the moon. However, maybe fast rotation could help you cut into 2,300 $m \over s$ escape velocity. And maybe some clever former-castaways that found a way to live inside some speculative kind of shelter developed a rail-assist that provides the kinetic energy shortfall.

Imagining the moon, like the fictional Earth, is spinning fast enough to give you a 1,000 $m \over s$ boost towards the 2,300 $m \over s$, a spring-style rail "launcher" would $v = \sqrt{ 2 s a }$, need to be ~28 kilometers long and providing 3 gee of acceleration to work.

To Mars

Distance. Mars is orbiting the Sun, just like the Earth is. Depending on how close the two orbits are, the distance from the Earth to Mars can be as much as 401 million kilometers but is, on average 225 million kilometers.

Time. With the solar sail providing constant thrust of 0.0005 $m \over s^2$ (1.8 m/s per hour) for the trip to Mars, and launching from the moon for 2,300 $m \over s$ starting velocity, an extremely simplistic equation for the trip time is $s = {1 \over 2} a t^2 + v_0 t$.

Using this, the trip would take 400 days each way. (200 accelerating + 200 slowing down).

Way stations and supplies. Earth's L2, L4, and L5 Lagrange points are each about 1.5 million kilometers from Earth, or only about $1 \over 2$ of 1% the distance to Mars. They wouldn't be very satisfying way stations for pre-positioning supplies.

It might be possible for a convoy to be making a circuit to Mars and back, loaded heavily with supplies and minimal crew, and purposefully taking longer than necessary to get there so that they are available as supply points to larger crewed expeditions. That won't always be possible either, as the Sun eventually gets in the way.

I think a Mars mission would have to provide it's own air, heat, and other supplied for the full 400 day one-way / 800 day round-trip.

How much? 1 gram of anhydrous lithium hydroxide scrubs 450 cubic centimers (0.45 liters / 0.02 mol) of CO2. The average human breathes 2.3 pounds (1,041.9 grams / 23.6 mol) of CO2 per day. Therefore, unless you recycle it somehow, you need 1.2 kg of scrubbing material per day for the trip.

Going back, then, to our 10-ton solar sail vessel: for each member of the crew 480 kg (about half a ton) of air scrubbing material will be required for a one-way trip. 111 grams of fat per day (44.4 kg for the trip) of food and 2 liters / 2,000 kg per day (800 tons for the trip).

Water recycling will be necessary. Distilling isn't beyond the capabilities of Renaissance technology.

Radiation. Interplanetary radiation is no joke. For the sake of this, though, maybe natural selection has already put only those who can tolerate the radiation on the trip.

Landing on the Red Planet

It seems like you could use a concept like Opportunity and Spirit, aerobraking, then just crash into the planet (with airbags).

Getting Back Off Mars

Getting back off Mars is going to be harder. The radius of the planet is about half of Earth's, so the boost you get from rotation is lower (241 $m \over s$). The escape velocity, nevertheless, is pretty high (5,020 $m \over s$). There are no winds to provide a boost, either. It looks like ground teams would be stuck.

Unless...

Orbiting only 9,000 kilometers above the surface of the planet Phobos is comically close to Mars. However, its very low density (close to water) leads to very little effect on the Martian surface.

If Phobos, however, was a rock of pure Osmium 22,500 ${kg} \over {m^3}$, and the mass tweaked up a little, the Mars-Phobos L1 point starts to shift closer to the surface of Mars. At an almost Earthlike mass (4.78 $\times 10^{24}$ kg), the L1 point scrapes just a few kilometers above the surface.

With a few other changes to move Olympus Mons to the Mars equator, once per day it would be possible to jump from Mars orbit to Phobos orbit, into the metastable Mars-Phobos L1.

From there, the solar sail can be raised to build up velocity for the long trip back to Earth.

Home

Returning to Earth, the Renaissance explorer would enter the upper atmosphere at relative rest to the prevailing equatorial winds. Shedding the solar sail for the glider, the craft would slowly tack down the wind column to a temperate zone, eventually deploying a lifting envelope (a balloon) for the calmest region.

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In terms of realism, I'm with the other posters here in suggesting that the baseline of the Renaissance is simply to "short" to support spaceflight.

Consider Leonardo da Vinci. During his employment with the Duke d'Sforza, Leonardo sketched and designed devices recognizable as tanks, submarines, flying machines, diving suits, parachutes and even a primitive form of gas turbine. Rather amazingly, modern reproductions of many of these devices following his designs generally work exactly as advertised.

So why didn't the Duke d'Sforza embark on an ambitious campaign to conquer Italy and eventually Europe using these amazing devices? Outside of the fact that his personal history suggests he preferred to set people against each other and do the work for him, Leonardo was missing the all important compact power source for most of these devices. His tank would work on hard level ground when a bunch of burly guys turned the cranks, but hills and cross country terrain would easily defeat it. The flying machines might have worked to an extent as gliders or kites, but no human being could move the wings fast enough to actually attempt powered flight. (looking at some of the reproduced drawings is a bit misleading. Leonardo calculated the wingspan of a man carrying machine which is in line with modern hang gliders. He also had many marginal notes describing how the wings were jointed with freedom of movement in all 3 dimensions to allow the pilot control fo the machine).

Other devices simply would be vastly expensive to produce using the artisanal methods of production common at the time - imagine trying to hand stitch hundreds of diving suits for an invading army? Quality control would be an issue as well - soldier drowning because the different seamstresses were inconsistent with their stitching would be an unacceptable outcome. This does not even address the raw materials such as cloth - some might fail because it wasn't woven correctly.

Now add to this the lack of proper understanding and tools needed to carry out spaceflight. Leonardo would have known about rockets and intuited that a much larger firework rocket could lift a larger load or climb higher, but without the "rocket equation" would never have been able to calculate how many fireworks rockets would be needed (in fact, it would be impossible with simple gunpowder rockets, but he would not be able to understand why). Knowledge of things like changing air pressure, the vacuum of space, radiation, thermal stresses in extreme environments, the behaviour of materials in a vacuum and so on would be non existent - they would not even recognize there were potential problems to solve in the unlikely event Leonardo launched a man in a barrel perched atop a cathedral sized black powder rocket. While the image is awesome, the crater the device would leave would be less appreciated (assuming Leonardo or any of the assembled crowd survived the explosion).

As a minimum, you need to move forward in time. Much better materials, mathematics and even scientific understanding of the conditions of space were beginning to appear a century after Leonardo. While it is still highly unlikely that anyone could go into space in the late 1600's, there was a much more solid grounding of understanding, and much better tools available. Assuming anyone actually thought that way, rocket flight may have been possible in the 1700's, although we would be talking about short ballistic trajectories and deploying a parachute at burn out, much like a model rocket today. That sort of actual real world experience would then be able to pave the way towards true rocket flight, and likely inspire people to really start thinking about the problem, moving the discovery of the rocket equation, liquid fuels, air tight capsules and life support systems and so on much farther ahead - perhaps the late 1700's or early 1800's.

From there it is almost anyone's guess. If there was some sort of international competition among the Great Powers of the day to harness rocketry and utilize it for military or commercial purposes, then there may be a path to orbital flight and from there a "moon race". Beating Napoleon to the Moon would be an interesting "Moon Race", with the Austro Hungarian, British and Russian Empires frantically working against each other and Imperial France.

But the short answer is the Renaissance is simply too early for space flight, much less interplanetary flight, due to the limitations of knowledge and material science of the time.

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Since I see no science based tag on this question and you allow the laws of the universe to be manipulated, I think the most reasonable explanation is magic. Now I know this sounds like a cop out, but consider the following:

Before the Renaissance, the technology did not even exist to even know what exactly the planets were or how to not instantly die if you could get to one.

Now, if your civilization of humans contained individuals who could reshape reality with thier minds, then you have a great starting point. In this context, your civilization could do all sorts of incredible things to solve this problem like open portals to other worlds or make talismans that protect you from the vacuum of space.

But why the Renaissance and not the Earlier?

For magic to manipulate the laws of nature, one must first understand what they are trying to manipulate. Earlier theories about geocentrism and the firmament made it impossible for wizards to even ask the right questions to get the right results. Wizards who tried always wound up either in the clouds or in the vacuum of space. In the ancient world there probably was a real Icarus, hundreds of them in fact. All telling the same stories that if you fly to high, you will die. For this reason even trying to reach for the stars would become a forbidden taboo, one that many a wizard would pay the ultimate price for breaking.

However, once people like Copernicus and Galileo started to develop a firmer understanding of what the solar system and planets actually were, wizards were able to start developing the spells needed to actually get there because they could then account for things like planetary spin and tilt and orbits and astronomical distances.

Likewise it was not until the Renaissance experiments with glass bell jars that people were necessarily able to understand how breathable air worked well enough to able to make long term extra terrestrial breathing possible. Sure older explorers could place themselves in a magic bubble, but they would not know what to do about the air to keep it breathable as the oxygen is consumed.

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You might find some inspiration in the Star Trek: Deep Space Nine episode "Explorers" (s3e22)

In that episode, Sisko attempts to prove that the ancient Bajorans traveled outside their own solar system using solar sails on a small craft. The sails are even manipulated by hand much as you would on a sea fairing vessel.

They never really explain how the ship got into space, it was just kind of already "there". So getting off the plane would still be problem. But since there are not any motors used in the craft I would say the level of technology presented is about renaissance level.

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