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So what I'm looking for is some plausible idea of two human planets, orbiting each other closely, that despite having regressed to a medieval homage, were capable in this old era of powering solar sails through vague recordings and derelict technology, leaving their atmospheres to raid/settle/explore the other planet, hopefully without the need or explanation of how they maintained thousands of colonists/warriors with swords and shields en route, round-trips.

I've read and vaguely remember about the concepts of gravitational dissonances that can cause areas somewhere to be slower or faster when it comes to passage of time. Not sure what that is called. Yet, I also remember something else about if we were able to travel at 1g, etc in space, we'd reach places far faster like Alpha Centauri. That's what I had in mind so far.

The idea so far, is that a 3rd intelligent planet seemingly disappeared after an apocalypse/conflict that rendered both planets to food and climate catastrophe. Both the current home-worlds recovered completely, but without keeping most of their old tech, lore, and populations in the billions intact.

Somehow the 3rd planet after its flight, left behind a gravitational field for several millenniums which until recently dissipated, almost allowed immediate transfer of primitive starships from one atmosphere to another. This lore isn't necessary for the present scene, but it is apart of the foundation of why there are so many issues and population transfers between both planets that ebb and flow in technological development.

I could imagine some sort of Dune/40KMechanicus priesthood on both worlds facilitating the transports and their maintenance with the political consequences of granting religion that stranglehold, not my cup of tea, for both at least. Its the explanation and **sounding scientific ** I want for a French men-at-arms for example, that is able to escape Earth's atmosphere and enter Moon/Mar's atmospheres (Distance) only needing some 17th century submarine to go back and forth without breathing apparatuses, or nuclear engines, maybe some sort of gravitational slingshot that overcomes NASA-tech to survive in space. I know it can't be realistic, but some theories that could support it?

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  • $\begingroup$ I don't think you're going to get a proper answer in the context of your requirements that's scientifically plausible. Might want to relax that constraint a bit to "sounds scientific." Also, note that Earth-Moon and Earth-Mars distances aren't even in the same ballpark. For two co-orbiting planets the distance will be much closer to Earth-Moon. $\endgroup$
    – ben
    Commented Apr 14, 2019 at 2:03
  • $\begingroup$ You're right, I shouldn't demand scientific realism, just "sounds scientific". Okay so two co-orbiting planets that sounds interesting I like that. Can two planets in that state have their own moon? $\endgroup$
    – Naga
    Commented Apr 14, 2019 at 2:30
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    $\begingroup$ According to this Astronomy.SE answer, you could in theory have moons of moons (although none are known to exist), so moons of binary planets should be possible in theory. However, a more likely result would be that (like Pluto, Charon, and Pluto's smaller moons) the moons will end up orbiting both of them. $\endgroup$
    – Cadence
    Commented Apr 14, 2019 at 2:55

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The hardest part is leaving Earth behind.

Despite the enormous distance between the Earth and Mars, for instance, the majority of the fuel required for a rocket to reach Mars is expended just getting it out of Earth's atmosphere. Escaping Earth's gravity requires a delta-V (a measure of thrust, and therefore fuel expenditure) of ~11 kilometers per second, whereas you can find Earth-Mars transfer orbits lasting around 180-200 days to the tune of ~3 km/s.

So whatever your ancient civilization did to facilitate travel, they would naturally want to focus on getting stuff from the planet's surface into orbit. Once it's in orbit, propelling it is exponentially easier. In essence you're looking for some kind of space elevator. Maybe it's a big obvious high-tech thing, with a visible cable and base stations, and engineer-priests performing all kinds of arcane rites of maintenance on it. Maybe it uses some kind of gravity manipulation to simply lift anything in the designated area up gently, and then bring it back down. The possibilities are limitless.

Once your ships are in orbit, they can use relatively primitive rockets to get where they're going, but it's going to be a long trip - as mentioned above, the most efficient Earth-Mars transfers take over six months, and they have to launch in a specific window that opens about once every two years. Of course, for binary planets the results will be much more favorable; with a delta-V in the 4km/s range you can reach the Moon in only four or five days, like the Apollo missions did.

Put together: some kind of future-technological device lifts the ships into orbit using means that are beyond their ability to reproduce. They use crude rockets to make a months-long journey between worlds; this trip needs to be carefully planned and plotted, and launch on the right day, but not that much moreso than actual sailing vessels. When they arrive, a second space elevator facilitates their trip down. (Each planet could have several elevators if you're worried about the political or strategic implications of a single chokepoint.) Is it perfectly realistic? Well, maybe not. But it's not completely implausible either.

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    $\begingroup$ 6 month self contained atmosphere and 17th century technology are not compatible, they had submerging times measured in minutes to hours even with chemical oxygen production. $\endgroup$
    – John
    Commented Apr 14, 2019 at 4:02
  • $\begingroup$ Good ideas. I am thinking about your ideas combined with a space phenomenon that further leverages space travel/velocity from one planet to another combined with the advanced tech. I'm just in a conundrum because I really want to solidify the epic tech the regressed recovered, but by present day ensure whatever unique engines/buildings they were using to facilitate journeys is absent. $\endgroup$
    – Naga
    Commented Apr 17, 2019 at 1:12
  • $\begingroup$ Apart from air containment, they did not have any fuels capable of getting them anywhere near Apollo mission speeds. The best they could probably do would be black-powder solid fuel rockets, and there is no way they could control those well enough for interplanetary flight. $\endgroup$
    – Nosajimiki
    Commented Jul 1, 2019 at 20:22
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How about just going with ancient technology? Once, so advanced that it basically doesn't need maintenance or deteriorate but their owners are long gone. If there are two nearby planets there might be an automatic ferry.

Another option would be Stargate technology like in the movie and show, except that the civilization who built it is no longer around.

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  • $\begingroup$ I want to, but I really want to flesh it out enough that it can explain various events in pre-industrial history. I still primarily prefer the idea of something in space hurtling primitive ships using bits of ancient tech that in present time are completely decayed. But I will now use space elevators to an extent and some religion corresponding with it one of the planets thanks to Cadence. $\endgroup$
    – Naga
    Commented Apr 17, 2019 at 1:22
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I believe you are asking for a simple way to travel between two planets orbiting each other.

What if they were orbiting in a figure 8 with the same spot always facing the other planet--and EXTREMELY close?

The atmosphere would stretch out towards the other planet--they might even join so that you could fly between the planets with an airplane. as long as they rotated together around a single point and the same spot always faced the other planet, there shouldn't be massive tidal forces (I think)

Think of it as a single planet pinched in the middle or barbells.

The water would flow towards the center of gravity as well (Between the planets), in fact the water might even create a bridge between the planets that you could sail across.

Not sure what would happen with the atmosphere... the atmosphere at the outer edge of the planets would be thin to nonexistent like the top of a very high mountain and there would be no water.

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  • $\begingroup$ One of the Retief stories by Keith Laumer takes place on a planet exactly like this. I don't think it's actually possible though.Two planets THAT close together would be exerting massive gravitational tidal forces on each other. In fairly short order (geologically speaking) instead of two planets you'd have a massive debris field and eventually one planet with most of the mass of the original two. $\endgroup$
    – Morris The Cat
    Commented Jul 1, 2019 at 18:43
  • $\begingroup$ If either rotated I think you are correct--but I was thinking that if they stayed facing each other in the exact same point then there wouldn't be any tidal force, it would just be a single fixed system . $\endgroup$
    – Bill K
    Commented Jul 2, 2019 at 21:43
  • $\begingroup$ That's not exactly how tidal forces work. It's a differential in gravitational pull and in the system you're describing, they'd be VERY severe, whether the planets are rotating relative to each other or not. $\endgroup$
    – Morris The Cat
    Commented Jul 3, 2019 at 12:53
  • $\begingroup$ I'm not sure because I don't know the math/physics of it well enough, but the system I describe could be reduced to a cylinder--imagine a roll of paper towels with the mass of the earth spinning lengthwise... are you saying that would have tidal forces? Mathematically it should be identical to the system I described. (What it should do is compress into a ball eventually, so that is probably what would happen to my system and perhaps that's exactly what you are saying?) $\endgroup$
    – Bill K
    Commented Jul 3, 2019 at 14:40

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