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?
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.
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.
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.
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).
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.