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In my other question "Is it possible to build a bridge between planets?" several concepts of interplanetary bridges were formed. What I would now like to know is, are any of the proposed bridges able to be build and lived on and also would these bridges have gravity?

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  • $\begingroup$ I believe the answer is yes, but it will take research to work out details. Just getting air circulation Is tricky, even with plants along the way. A complete answer to this question may take significant research. $\endgroup$ – SRM Dec 26 '16 at 20:52
  • $\begingroup$ @SRM What I am worried about is the bridges constant stretching and moving making building and living impossible. $\endgroup$ – Mendeleev Dec 26 '16 at 20:56
  • $\begingroup$ Each segment of the bridge could be several kilometers long, with a grow/shrink of only a single kilometer. That leaves you several kilometers of stable region for forests, jungles, deserts, houses, etc. Assuming you can have those without substantial gravity. I don't have an answer for the gravity problem. @Mendeleev $\endgroup$ – SRM Dec 27 '16 at 3:36
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Yes, no.

I have an idea where the bridge is based on nanotechnology/smart matter so the walls can be self maintained.

A long manual journey will require that it be lived in, for the generation-long journey. They will be living in it for decades, so it must be designed to support that.

They need to “live off the land” for a journey of that length. So besides being quasi-living for self repair, it needs to provide etable crops for the travellers, air, and water. Otherwise you have no story.

As for gravity, the inside of a uniform tube won’t have any, and a non-uniform tube will not have enough to notice without delicate instruments.

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  • $\begingroup$ Well the bridge will not be the main focus of the story but you are right the travel will not be instantaneous but will be fast. $\endgroup$ – Mendeleev Dec 27 '16 at 15:18
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What is the minimum amount of handwaveium to get atmosphere and gravity? Well we have already introduced ludicrous quantities of handwaveium so its not hard to just hand wave the rest.

Just to build a cable that connects two planets and is stable we have had to introduce an indestructible handwavium cable several times longer than the circumference of earth holding a significant fraction of the weight of 2 planets.

Since we are handwaving a bridge material of almost infinite straight lets hand wave it does not stretch significantly.

Atmosphere not much more We build a handwaveium tube around the outside of the cable to hold in the atmosphere. Okay but gravity will pull air down out of the tube onto the two planets. So we need air locks that segment the tube to reduce the pressure planet side and trap atmosphere in the tube. These would be paired with modern day air filtration and co2 scrubbing systems. You would also need pumps to gradually move air back toward the middle as it drifts downward whenever the airlocks transition

Gravity Lots more Lets assume you mean gravity pulling you toward the center of the cable. (you get gravity pulling toward the planets for free).

Why do you want this? In the normal case you climb a ladder out of planets 1 gravity well then drift effortlessly several hundred thousand miles in 0 gravity then climb down a ladder into planet 2. Why get gravity just to walk the several hundred thousand miles in-between?

If you want it anyway there are 3 methods

  • Handwave artificial gravity generators
  • make the cable way thicker (once its diameter approaches the diameter of the moon you will start having noticeable gravity) this also may consume dozens of planets' worth of material.
  • make the bridge spin. Around the central cable create a cylinder that spins around the cable rapidly. The centrifugal force will act as gravity unless the cylinder is large in diameter it will have to spin quickly. Technically this feels as gravity pulling outward not inward but it is more feasible.
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  • $\begingroup$ Because the planets are connected by a tube (presumably) directly between them, you will reach a point where the pull of gravity from one planet is equal to the pull from the other. This is a Lagrange Point (L1 specifically). This also makes airlocks extremely important. The atmospheres on the two planets are essentially fluids and the bridge is a hose. This gives you a siphon, and unless both planets are exactly the same size, one planet will eventually siphon off the atmosphere of the other. $\endgroup$ – Tim Jan 28 '17 at 1:15
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    $\begingroup$ The air in the bridge will follow the same dynamics as air outside the bridge: It will get thinner as you get further from the planet, so multiple airlocks are a must or the atmosphere at the planet-side airlock doors will be greater than the exterior (planetary) atmosphere. Adding artificial (centrifugal) gravity will not help this. Adding enough material to the bridge to make gravity vaguely consistent throughout its length would increase the siphon effect but eliminate the need for airlocks. Now your bridge would look more like two beads $\endgroup$ – Tim Jan 28 '17 at 1:16
  • $\begingroup$ (the planets) with an oval bubble in the middle (the bridge). This would create some funky gravity though since "down" would gradually change 180 degrees as you travelled the bridge. Building a path to cope with that would be an interesting feat of engineering. $\endgroup$ – Tim Jan 28 '17 at 1:16
  • $\begingroup$ @Tim its pretty easy you have a ladder to climb out of the gravity of planet A, as gravity falls off you need it less and less until you can almost drift at the Lagrange point then you flip around and climb "down" into the gravity of planet 2. BTW why is the bridge an oval $\endgroup$ – sdrawkcabdear Jan 30 '17 at 17:33
  • $\begingroup$ The ladder thing you discuss is if you don't add mass to get real gravity. If you add mass to the tube/bridge to get real gravity as you suggest in your 2nd point, you have to add the most mass in the L1 Lagrange point of the bridge since the ends of the bridge have the mass of the planets to provide real gravity. Adding about the mass of the Moon at L1, would give you about the Moon's gravity at L1, Add it at an end and you have the planet's gravity plus the Moon's gravity. $\endgroup$ – Tim Jan 30 '17 at 18:34
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Even if it has gravity it won't be able to hold onto an atmosphere. That takes planetary-scale gravity. While we think of the surface gravity mattering for whether an atmosphere can be retained in reality that has nothing to do with it. What counts is escape velocity which is a function of gravity and size. Make your bridge out of enough neutronium (you'll need some handwavium to contain it) to give 1g to people walking on it and they'll still be in vacuum. Build a Dyson sphere with a milligee on the surface and it will have no problem holding onto an atmosphere.

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A concept not explored in the linked question, but which could work, would be a binary planetary system, both planets rotationally locked to each other. The whole thing is rare and improbable, but it's not impossible.

At that point, you can build a dual-tethered Space Elevator. (Space Elevators are long proposed systems where you drop a cable from space all the way to the ground and run cable cars along it).

If the planets are broadly similar, I think that, in theory, it should be possible to balance the forces between the two that the double Space Elevator stays up there all on its lonesome with minimal station-keeping.

It should not be too hard to construct a hollow space elevator "cable" which is a larger rectangle (Arthur C Clarke's Space Elevator does this in his book about it). Then you can put in flooring if the space is large enough and have people living on the levels.

Gravity would fall off naturally as you went up to the mid-point, to vanish there, though you probably could spin up stations attached to the cable to simulate gravity (the spinning might even make the cable more stable against oscillations). You'd have to maintain the atmosphere artificially, but it would be fine.

On a more crazy note, the anime Last Exile has, I think, a double-planet system in which their atmospheres are close enough to touch. This might make life more interesting with such a scenario. (Whether it's possible would take calculating whether the planets would be stable enough that close together).

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In the space-spider built bridge that I suggested, gases for an atmosphere could be kept in place by secretions produced by the space-spider the seal the outside of the elastic space-spider silk tube connecting the planets.

Keeping the air breathable might require that there be some sort of plant life symbiotic with the space-spider that would recycle local air in the space-spider silk tubes (which realistically would probably come in a series of segments a bit like a bamboo plant, to prevent internal winds inside the tubes from getting out of hand). The symbiosis would work because the plants would make the tubes livable for travelers and the offerings from the travelers (or the travelers themselves if they failed to provide offerings) would be the main source of food for the space-spiders.

Gravity would be much more challenging and might just not be possible. Space-spider silk would need to be incredibly strong even with planetary dynamics and a design I have suggested which would be much less intense than the kind of two planet binary co-rotating systems that others have proposed, as opposed to the two very small planets in exaggerated versions of two asteroids in the same orbit in an asteroid belt one after the other, or very small planets in a system analogous to being at the same angular position on two concentric planetary rings that are fairly close to each other.

Throwing in centrifugal force to provide gravity by spinning part of the structure on top of everything else, while possible to conceive, feels like a bridge too far (pun intended) in an already credulity stretching situation.

Indeed, gravity might not even be desirable, because gravity would mean that people on the bridge would have to walk tens of thousands or hundreds of thousands of miles or more to cross it, which would take forever, while in a near zero gravity environment, people could fly through the tube of the bridge and that might be much, much faster.

At the ends of the bridge, planetary gravity would gradually begin to assert itself, but it would probably make the most sense for the middle of the bridge to have near zero gravity.

Similarly, while one could scale it up to be large enough to live upon, the bigger the bridge's capacity, the larger it must be in terms of materials and stressing forces, so I wouldn't think that you would want a bridge any bigger than absolutely necessary for travelers along the bridge to camp out for an evening during their journey across it (which would be many, many miles and take a very long time on foot even free of the burden of gravity so that travelers could essentially fly through it from one end to the other). There might be a handful of caretakers or oasis keepers who might live on the bridge, but I wouldn't think it would make sense to make the bridge big enough to support a full fledged village or city.

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