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What I am proposing is a bridge making it possible to travel between two planets, I have some ideas but I would like to know if and how it would work. Maybe somehow stopping their orbit and locking them in place with a ring like structure or a flexible moving bridge. I don't care if it requires a strong unearthly material I just want to know what and how it would work abiding by the known laws of Physics. I don't want too know what material I would need just if it could be built within the laws of physics.

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    $\begingroup$ If you stop them, you will make them fall into the sun. Just saying. $\endgroup$ – Mołot Dec 17 '16 at 13:36
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    $\begingroup$ Not a dupe at all. This one asks for engineering solutions to a much smaller problem. That one just asked for material supply. $\endgroup$ – SRM Dec 17 '16 at 16:29
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    $\begingroup$ Yes: it's called a stargate $\endgroup$ – Benubird Dec 17 '16 at 19:24
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    $\begingroup$ This is NOT a duplicate of the other question!!! The other question and its answers are all about resource supply, not the engineering of the bridge! $\endgroup$ – SRM Dec 19 '16 at 3:46
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    $\begingroup$ As the author of the previous question, this is definitely NOT the same question, because Mendeleev's is about respecting known physics, whereas I specifically and gleefully gave physics a hard shove off a high cliff. $\endgroup$ – Whelkaholism Dec 19 '16 at 10:45

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Introducing an infinitely strong material but otherwise using physics, I guess it should be possible. But note that I'm not considering at all how you'd build something like this, just whether it would work once you have it. You would need to significantly change the orbits of those planets because naturally (without the tether) what is a stable orbit for the duo would not be a stable orbit for the planets alone. This would thoroughly upset any existing ecosystem on the planets. Although if you are into tethering planets together, then I guess maybe you have your ecosystem well enough under control and understood so that you can counteract the consequences to get around the expect mass extinction.

So given a long piece of rope with tensile strength as large as needed:

  • Tether them together. The easiest way to attach your rope to your planet would probably be to just construct a net around the whole planet from your rope (just dense enough so the planet can't slip out, say maybe three equally spaced not-really-parallels to the longitudes, and three equally spaced not-really parallels to the equator) and attach the interplanetary rope to that net.
  • For astronomic purposes, your two planets would be one (unless only one of them is colliding with another object, that would be bad): The planets couldn't move relative to each other,
  • but would have rotation around their common centre of mass. There would be a minimum rotation needed to keep them from crashing into each other. This would make an otherwise completely unstable orbit stable by putting tension on the rope (so that if your system is disturbed, instead of breaking apart, only the tension on the rope changes) This rotation would need to cancel the gravitational force they have on each other. Because on the surface of the planets, the planets own gravity is much stronger than the gravity of the other (distanced) planet, this wouldn't completely mess up the planets individual gravity fields (so at each point of the planet there would be reasonable inward force).
  • There would still be huge forces on the ropes, but we specifically assumed the ropes won't break. For the net around the planets, you would want to have some minimum thickness of the ropes so it doesn't cut the planet to pieces (it would still have a tendency to lift off the ground on one side of the planet while being forced deeper and deeper into the planet on the other side, but you could surely get that process to be so slow as to just be limit to the lifetime of the planet which is in the far future with which you are not concerned. Or you could replace the netting every couple millions of years).
  • You could have the planets rotate individually only if you build some giant bearings from your super strong material and let the planets rotate in those bearings. Much much more complicated than the net I suggested. With the net, you could have them rotate in parallel to the connecting rope, but not otherwise. You could have them move fairly erratically if you do introduce too much rotation like this (think gyroscopes).

Now the real question is what happens to the exact gravitational force and what the minimum distance would need to be to have that force behave sensibly.

(I did some work on it, but when I'm already at it I want to solve a fairly general case without too many simplifying assumptions, and this turned out hairy. Not difficult, but lots of calculations. The equations are all nice and well in the right coordinate system, but I need to transform here... You'll get it tomorrow)

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  • $\begingroup$ I wonder if the speed that they need to spin around a cenral axis will make the solar wind strong enough to make the planets inhospitable, I'd hope it would cause just enough to create a giant aurora around the planets. That would be pretty...cool. yep,cool. $\endgroup$ – Necessity Dec 18 '16 at 15:29
  • $\begingroup$ Are you sure they would maintain orbits if tethered? $\endgroup$ – paparazzo Dec 18 '16 at 16:23
  • $\begingroup$ If you can have an infinitely strong material, why not an infinitely flexible one? Then the planets can move relative to each other and the bridge will just flex, changing in length and orientation. $\endgroup$ – Michael Dec 18 '16 at 16:54
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    $\begingroup$ @Michael: Unless you get very funky with orbital mechanics at some point your tether is going to have to pass through the sun. $\endgroup$ – Joe Bloggs Dec 20 '16 at 13:46
  • $\begingroup$ If you're moving planets around why not put two planets in opposite positions around the Sun and have two bridges going above and below the plane of the system? That way you can put the centre of mass for the planets where the sun is and avoid the unstable orbit headache. $\endgroup$ – Joe Bloggs Dec 20 '16 at 13:48
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Earth. Mars. Yes. Look to Japan.

You have infinite supply of your super material, yes? Good. You'll need a lot of it.

This is a research paper on jointed structures in use today to withstand stresses caused by wind, water, ground motion, etc: https://nathaz.nd.edu/journals/(1999)Mitigation_of_Motion_of_Tall_Buildings_with_Recent_Applications.pdf

Section 6.3 discusses active dampeners, that is, intelligent systems to move things back into place when forces pull on them. Japan has an airport they built on water. The legs supporting the structure are jointed, not fixed, and can respond to tide or earthquake.

You need to build a ring around both planets that will serve as your bridge attach point. The ring will need active stabilizers. The bridge can slide along the ring. The bridge itself is an arc that goes up out of the eliptic plane so that when Earth and Mars are on opposite sides of the sun, it goes over the top. The bridge is built in short segments, each of which actively corrects its position relative to its neighbor. Many segments have slides underneath to stretch or collapse (think like an airport conveyor belt when it goes around a corner) as the distance between the two changes.

Every segment will need power. This may be harvestable from solar. You're on your own for those calculations.

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    $\begingroup$ +1 for accounting for the sun being between earth and mars. $\endgroup$ – Eric Johnson Dec 17 '16 at 19:41
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    $\begingroup$ The segments would need so much power that you could as well add "infinite energy" to the assumptions. $\endgroup$ – Nobody Dec 18 '16 at 0:03
  • $\begingroup$ @Nobody No, that's untrue. I'm only unsure if sufficient can be provided by solar panels, but the power required is actually quite reasonable. I'm still working on numbers (may be a few weeks) but back-of-envelope is on order of Voyager II per segment. $\endgroup$ – SRM Dec 18 '16 at 1:35
  • $\begingroup$ The ratio between the shortest and longest possible Earth-Mars distance is ~7.4. That's a lot of stretching. $\endgroup$ – Random832 Dec 18 '16 at 3:30
  • $\begingroup$ Why stretch? I thought slack was the obvious answer. I detailed it in my own answer. $\endgroup$ – JDługosz Dec 18 '16 at 5:12
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I'm very tempted to say that no, that's not possible in practice. At least not without severely stretching the laws of physics. But like Nobody proposed, you could perhaps make it work if you are willing to introduce a super-strong material, plus some other paraphernalia.

Let me introduce you to a few things that belong to the field of orbital mechanics. For simplicity, I will consider only two-body systems, not n-body systems (which are far more complicated to model).

  • An orbit is an ellipse
  • The ellipse can be more or less circular, as described by its eccentricity
  • One of the foci of the ellipse is at the system's center of mass

One of the problems that was solved by the work of Johannes Kepler was that before him, we generally thought of orbits as circular. That isn't the case, and forcing orbits to be perfectly circular leads to all sorts of problems in the long term. By realizing that orbits are elliptical, not circular, Kepler was able to derive a model that described the behavior of orbiting bodies much more accurately.

There is a simple reason why orbits are ellipses. When one body is moving away from another, the gravitational attraction between them is reduced, but so is their relative velocity. Eventually they turn and start falling toward each other instead of away from each other. We call the point farthest from the system's center of mass apoapsis and the point nearest to the center of mass periapsis. When discussing orbits around the Earth specifically, the terms are apogee and perigee, respectively.

Notice how I said "the system's center of mass" above? That was not by accident. We generally think of for example the Earth as orbiting the Sun, or the Moon as orbiting the Earth, but that is a simplification. What really happens is that the Earth orbits the center of mass of the Earth-Sun system (again, simplifying by ignoring all the other bodies in the solar system), and that the Moon orbits the center of mass in the Earth-Moon system. In these cases, the center of mass happens to lie within the more massive body. In other cases, such as the Pluto-Charon system, the center of mass lies outside of either body. Even a small man-made satellite in orbit of the Earth perturbs (tugs at) the Earth ever so slightly. This is Newton's famous apple pulling the Earth toward it in action.

To have a bridge-like construction between two celestial bodies, you need the two bodies to be tidally locked with each other; in other words, they have to always present the same side to each other. Tidal locking happens when one of the bodies in the system is significantly larger (specifically, more massive) than the other, and the distance between them is comparatively small; for example, Earth-Moon, or Sun-Mercury. Tidal locking is a gradual process, but eventually, the smaller body stops rotating relative to the larger body.

The problem with that is that the larger body is still rotating relative to the smaller one. So there is no good anchor point on the larger body!

The only possible solution I can see to this is to use either rotational pole as the anchor point, on both bodies. Then you need to figure out a way to keep the anchor points of the "bridge" from being torn off, but that's an engineering problem, not a physics one. It only becomes a physics problem when you are trying to find a material to build the bridge with.

Now, you've got a bridge. But there's another problem: The elliptical shape of the orbit between the two bodies! Take the Moon's orbit around the Earth, for example; perigee is at 356.4 Mm and apogee is at 406.7 Mm, with a nominal semi-major axis (orbital radius) of 384.4 Mm. The distance to the Moon changes from -8.3% to +5.8% compared to its "normal" value! And the Moon is pretty massive; at about $7.34 \times 10^{22}$ kg, it masses about a percent of the Earth's $5.97 \times 10^{24}$ kg. Unless you can perfectly circularize the Moon's orbit around the Earth first, whatever material you build the bridge out of is going to be subjected to extreme stresses.

You can solve that by introducing that super-strong material Nobody mentioned, and that I alluded to in the beginning of my answer. Now the bridge itself will hold, and may even theoretically be able to hold the Moon at the distance of the bridge's length.

But how are you going to anchor the bridge? If you anchor it to the surface (of the Earth in our example), that surface isn't going to be equally super-strong. And even if it was, the Moon is already massive enough to tug the Earth back and forth a bit in the dance between the two.

Much of that could be solved if you put the system's center of mass exactly in the middle, right there in space between the two. The easiest way to do that is to give both celestial bodies the exact same mass. Now you are looking at a true double-planet system, rather than a planet and its satellite, but I suspect that would be okay, since you specifically said "planets" in your question. This would take an incredibly unlikely coincidence during planetary formation, somewhere along the lines of the infinite improbability drive, but I suppose if you handwave sufficiently, it could in principle happen. It won't be a stable situation, though. A handful of large asteroid impacts could potentially upset the balance, and those happen, in terms of time relevant for planetary formation, all the time.

Barring that, you need the material that you build the bridge from to be sufficiently strong to withstand such forces, and you need the anchor points at both ends to be sufficiently strong to withstand those same forces, and you need the two bodies to be in a perfectly circular orbit about each other, and you need them to be tidally locked with each other (not just one of them tidally locked with the other).

That's sufficiently unlikely to be possible that I would say that while what you propose may be possible in theory it won't be possible in practice.

And walking that bridge would be taxing, but perhaps possible.

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  • $\begingroup$ I added some caveats to my answer, I was thinking only about systems already existing like this, not about building them. Like something built by forgotten ancestors/gods/whatever. Completely different train of thought. $\endgroup$ – Nobody Dec 18 '16 at 0:06
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I’m surprised that only SRM considered that the bridge follows the planets’ varying positions, and everyone else wants to lock the two bodies together.

But my thought is to make the bridge flexible rather than varying length as SRM described.

The bridge goes up out of the ecliptic in an arc between the two bodies, accomidating the largest separation. When the planets are closer together, the arc is taller and/or there are waves. The overall motion is controlled via a series of waves that travel back and forth; several frequencies and phases at the same time. It’s engineered so the net effect is to crenulate the line by the right amount to give it the proper length between the endpoints at any given moment.

The problem is that the arc passing over the sun will want to fall downward. It will need constant energy to counter that; perhaps it can be held aloft with solar sails along the length. Tacking or trimming the sails will give dynamic control to keep the whole thing moving properly.

As far as the terminals go, you could have it just hang into the top of the atmosphere like a non-rotating skyhook, at the pole. However, if you want primitive people to simply walk, without needing a user-supplied vehicle at each end, it could be anchored to a rotating mount, or the skyhook can trail light cords down to the ground.

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  • $\begingroup$ The primitive people would want a good set of walking shoes and quite a lot of time. How long does it take to walk 200 million km? Assuming 50km a day it takes a little less than 11,000 years to walk the bridge... even by car, driving at 200km/h straight for 16h a day it still takes 170 years. $\endgroup$ – Durandal Dec 18 '16 at 10:47
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    $\begingroup$ @Durandal A species composed mostly of mathematicians would not care about such practicalities. They simply wanted a surface big enough to finally print out the proof of the classification of finite simple groups. They also created life extension treatments to allow a hiker enough time to walk it through. en.m.wikipedia.org/wiki/Classification_of_finite_simple_groups Famous quote from that species: "How many roads must a Mind walk down?" "Just this one, but it's a bugger!" $\endgroup$ – SRM Dec 18 '16 at 12:20
  • $\begingroup$ @JDlugosz it occurs to me that the shortening/growing of my bridge design has a side-effect of making the arc uncollapsible, thereby preventing the need for solar-sail stabilization. Thoughts? $\endgroup$ – SRM Dec 18 '16 at 12:23
  • $\begingroup$ @srm what are you thusting against when lifting the arc up against gravity of the central sun? You need power to work the sliding sections, but that alone does not stop it from falling. I don’t see how simply having length-changing segments would address that. $\endgroup$ – JDługosz Dec 18 '16 at 14:42
  • $\begingroup$ Each segment would act like blocks in a keystone arch. I really wish comments could include pictures. Imagine if the St. Louis arch included extendable segments so it could widen or narrow at different times. It doesn't need constant lift. It rests its weight against itself. As the legs are pulled wider by planet orbits, it grows taller to maintain arch shape and stability. $\endgroup$ – SRM Dec 18 '16 at 19:05
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An alternative to (hugely) size-changing bridges would be one Dyson-ring like bridge around the sun, and "off ramp" or "spoke" bridges to the planets.

animated illustration

Still talking crazy scales and structures here, of course, but bridges that slide around the central ring might at least seem more plausible depending on setting. The spoke bridges would need to curve upwards and down again to allow planets to pass under them. (but this could be a mostly fixed shape). They would also need to connect to smaller ring-bridges around the planets with a similar setup to account for a planets daily rotation. At this point you might not call it a bridge anymore though.

Notes;

  • As the spokes slide around the central ring the ring itself could be irregular (a ellipse for example). I have illustrated it as a ring for simplicity.
  • One issue with this design, however, is at some points two spokes will need to pass each other. The spokes themselves can just be at slightly different heights, but where they connect to the ring there is a "engineering challenge" of letting the connections pass each-other.
  • Despite this being a mostly rigid design there would still need to be some length variance I believe as orbits are not perfectly identical lengths from the sun.
  • As mentioned in the comments a alternative design is also possible, where each planet has its own ring for its full orbit, and spokes only go between a planets "orbit ring" and the next one out. Probably more effort to build, but shorter journey times with this design as your not going to the sun and back each time.
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Dec 20 '16 at 16:08
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No.

Even if you manage to find two planets that are so perfectly aligned in an orbit that they don't seem to move, the forces between them in a case of a tiny-seeming movement would be enormous. Then there is the amount of material involves - the distance from Earth to Mars is 300,000 times the length of the biggest bridge ever made. It's more than 1,000 times the length of the Equator.

The most important consideration though is why. If you wanted to get effectively between two planets, just build a space elevator on both planets. Once you're in space, rocket travel between the two would require much less energy and fuel to travel than it would from the planet's surface. And you'd need a rocket - it's the only sensible way to travel in space (the only other option really is magnetic propulsion on your bridge, but trying to transmit electricity over tens of millions of km won't work very well). You want to keep the vehicle off of the surface to avoid drag... at which point there is no reason to have the surface at all.

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  • $\begingroup$ But: We were supposed to ignore material constraints, weren't we? And with sufficient rotation, the distance could be kept much smaller, even less than Earth-Moon, say. Plus I'm not sure, but it seems like transmitting electricity is essentially the same as transmitting light which is in no way limited by distance. You would want to use extremely high voltages to keep transmission losses small, but other than that... $\endgroup$ – Nobody Dec 17 '16 at 17:00
  • $\begingroup$ Possible - however the second part definitely still applies; it doesn't really make any sense to. $\endgroup$ – Matt Bowyer Dec 17 '16 at 17:11
  • $\begingroup$ @MattBowyer The reason why is the feat of a bridge although inconvenient is much more grand and can possibly provide living space and other important things. From a storytelling point of view I find it better. $\endgroup$ – Mendeleev Dec 28 '16 at 14:28
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It depends,

mostly on what type of structure you let slide as a bridge, and what kind of planets you have in mind. All the potential bridges I can imagine will have in common is: they are more like zero-gravity guide wires (or tubes).

Lets start with a simple system: tidally locked double-planet. The closest we have in the solar system to this configuration is probably the Pluto/Charon pair. A kind of tether between the two could be built and it would be not much different from a space elevator, conceptually. The key point here is that the two "planets" show the same side to each other all the time. Perfectly circular orbit makes this easier, but the tether could compensate for changing distance if its not. This version strikes me as at least not totally impractical, as the distance between the two planets would be small enough that it could be covered with imaginable materials and traversed in useful time.

Next, co-orbiting planets. Thats two planets inhabiting the same orbit around their star, but separated by angular distance. Saturn has a pair of minor moons in this relationship, but their names currently escape me. A structure connecting two such planets would take the shape of an arc along their orbital path. The "bridge" would be mostly free of forces, as long as its mass is neglible compared to the planets. Again, perfect circular orbit makes this simpler, as the length of the arc never changes. The forces acting on the arc could be small enough to be manageable, but the "bridge" would be very long - for an earth like orbiting pair it would be approximately 1 AU long (150 million km), assuming 60 degree separation. Traversing the bridge might be impractical in useful time.

Other configurations have been described by others answers, but I feel these go too far, that after building the "bridge", the planets stop being planets and form a new type of body together with the "bridge". I'm also not sure the planets themselves could stand up to the forces acting on them in that case (or rather I'm pretty sure they won't, unless you reinforce the planets themselves with some super-material).

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It is possible, under very specific conditions which are supposedly possible, but we have no evidence to support that they do exist in nature, and they'd probably form a type of natural bridge as it is so not really needed.

A much better path that can be considered a bridge is something that is considered for the future of the Earth-Moon system which is you build 2 space elevators and the minor distance between the 2 that can be easily traversed in moments. This allows for the continued rotation of bodies seperately, doesn't (for the most part) require super strong materials and still acts as a bridge.

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  • $\begingroup$ Minor difference: so you time it so you jump when the Earth cable is passing the Moon’s? Hmm, minor difference in distance does not consider the rediculus difference in velocity. And how does that idea even apply to two planets orbiting a sun? $\endgroup$ – JDługosz Dec 19 '16 at 12:13
  • $\begingroup$ Well you can launch from one to the other, but you don't have to. You just get up to the station and then launch off of it which you would not have the same launch speed doing. The Earth moon system is possible to do this with because the closeness and you'd pretty much be doing it if you built space elevaters on either so it is just a thing that will happen. Between bodies not in such a system you could build what people may call bridges, but not what the OP is talking about. Non-connected relay points to catch and launch like Mass Effect's Mass Relays. $\endgroup$ – Durakken Dec 20 '16 at 3:32
  • $\begingroup$ «You just get up to the station and then launch off of it which you would not have the same launch speed doing.» I don’t understand that. And how does two separate towers moving at high speed at as a bridge? $\endgroup$ – JDługosz Dec 20 '16 at 13:54
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So I came across this idea myself and I think I have a solution.

In my world (I'm using mine as an example) I have a moon that orbits the planet at the same speed and in the same direction as the planet's rotation, making it seemingly tidally locked (though the moon still rotates on its own axis). Assuming you have a super material and a ridiculously huge budget, you build the start of the bridge/tether/elevator what-have-you on the main planet, when you reach the moon, the base is elliptically built on the surface, being that the bridge only touches the moons surface at two points and the rest is raised from the surface.

Why though?

If the track on which this bridge slides is elliptical based with the orbit of the moon and its rotation, the elliptical allows for the bridge to be the same distance all the time.

Picture of orbit drawing

This design also creates a giant "orbital engine" which could be used to generate a ton of power.

Anyway, just an idea. Comment if my geometry is wrong here...

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  • $\begingroup$ Correct me if I'm wrong, but are you saying that the base that touches the moon will slide around the surface of the moon when the moon moves, allowing flexibility? $\endgroup$ – Jerenda Jan 18 '17 at 22:29
  • $\begingroup$ Sort of, think of an elliptically shaped belt around the moon, if the distance between the moon and planet is say 5 at its closest, and 10 at its furthest, the bridge itself is 5 and the largest part of the moons 'belt' as it were, is also five. I'll post an image in an update when I can and hopefully that will help you visualize. $\endgroup$ – Garrett Gaddy Jan 18 '17 at 22:45
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Yes, if the orbits of the two planets are perfectly coplanar (something that won't happen in reality unless your engineers move one of the planets.)

There are a lot of no answers based on trying to build a bridge--which is impossible--rather than the correct answer of building multiple bridges.

Bridge #1:

Build a space elevator.
Build a space elevator 120 degrees away.
Build a space elevator 120 degrees away.

So long as your planet isn't too massive this is at the edge of what can be built, unobtainium makes it easier but isn't essential.

Bridge #2:

Build a ring around the planet, supported by the elevators. You can do this with a single elevator if the ring and elevator are unobtainium, otherwise you need at least three points.

Bridge #3:

Another ring around the planet, outside the first ring but touching it with magnetic levitation support (or anything else frictionless if you're going the unobtainium route.) This ring is not rotating in relation to the central star. Obviously there is a considerable velocity difference here, some sort of transport craft will be needed for velocity matching unless you go with Heinlien's rolling roads approach. (Which would take a lot of velocity-matching segments!) For engineering ease the net force of #2 and #3 combined is zero, the elevator connections are only for stability. Unobtainium will make the construction of #2 and #3 much easier but it could be done without.

If the orbits are perfectly circular:

Bridge #4:

A ring around the star. This is connected to #3. Unobtainium or stationkeeping engines are required.

You build an equivalent system around the second planet.

Bridge #5:

This hangs from the ring around the outer planet, a counterweight hangs in the opposite direction to give a net zero weight. It extends to the inner ring, again speed-matching shuttles are required.

If the orbits are not perfectly circular it gets harder:

Bridge #4a:

This is in space between the two planets. Stationkeeping engines are mandatory as it is not anchored to anything. Unobtainium is required.

Bridge #5a:

This rides a track on #4 (speed matching issues apply) so as to remain aligned with the planet. It dangles halfway to the planet plus half of the orbital variation of the planet. It is counterweighted.

Bridge #5b:

This is connected to #3 and extends outward and is counterweighted. It has the same length as #5a. #5a and #5b are aligned so as to be coaxial at all times but are only coupled by magnetic levitation or the like as #5b will move up and down with the planet. A pornographic nickname is inevitable for this pair. Unobtainium is required. Keeping the counterweights from #5a and #5b from striking their opposite bridge will be quite a project.

A second pair of bridges connects #4 to the other planet.

If you wish to extend this system to additional planets you can reuse #1, #2 and #3.

I can see no means of handling planets which are not coplanar.

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One step that would make this idea somewhat natural to even begin to think about would be to have a pair of planets where the tallest mountain on each one extends above the atmosphere, making a space elevator unnecessary.

Perhaps, rather than rotating around each other, they would either share a common orbit at some distance from each other, like asteroids in an asteroid belt, or rocks in a ring around a planet, or would have aligned concentric orbits that have lead to the same amount of time in a one year travel around the sun.

Ideally, these planets would also be on the small side of what would still qualify as a dwarf planet, so that the gravitational pull of the planets on each other could be smaller relative to the gravitational pull of the sun on each of the planets. The smaller the dwarf planets, the lower their average distance from each other could be.

Then, you would connect the tops of the super-atmospheric mountain tops with a long glorified rubber band (or with a solid bridge with a glorified rubber band at each end) of something like super-duper spider silk. The rubber band portion or portions would guide the space equivalent of a mountain gondola (greatly reducing the material requirements by requiring only a small proportion of it to have a habitable atmosphere).

Perhaps some spider-like creature that could survive in outer space would maintain the rubber band-like portions of the bridge, repairing it as it is damages or strained by stretching and contraction over the course of a year, perhaps receiving the nourishment to do so from sacrificial offerings from travelers traversing the bridge (or by eating the travelers themselves if they don't offer sufficient sacrificial offerings, perhaps by forcing them onto a dead end spur of the bridge controlled by the space-spider). Thus, the spider would be both a caretaker of the bridge and serve a troll-like role with regard to the bridge.

The entire system (mountains, spider-strands, space spider), except the gondolas used to move along the bridge, could be natural, rather than man made, reducing the need for the people using the bridge to be particularly technologically advanced.

Indeed, perhaps these space spiders evolved in a planetary ring where the distances that had to be traversed with space-spider silk were small, got carried to an asteroid belt when a stray collision of planetary ring material accidentally carried some space-spiders there, and then a colony of space-spiders made it to the planets that were not too far apart from each other when an asteroid crashed there after being disturbed from its orbit by a collision. At each step there would be selection for space-spiders that could produce longer and stronger strands. Related species of smaller, less capable space-spiders in the same solar system could be found in ring systems and asteroid belts throughout the solar system, each adapted to the scale it usually faces.

If the gondolas are too much, or too technologically advanced, or impair the concept too much, perhaps the space-spiders could make flexible space-spider silk tubes (perhaps enhanced with minerals found in asteroid belts, planetary rings and rocky planets) that people on the bridge would simply walk through, rather than something that they would hitch a gondola to. The tubes might be 2-4 meters in diameter or so on the inside, and could extend down the sides of the mountains to which they are anchored down into the area with a breathable atmosphere.

UPDATE: New materials science technology for space is based on spider silk.

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What you need is two planets orbiting each other(in circular orbit) while their rotation is fixed and equal and parallel... Consider this example : take a dumbbell and rotate it while the axis of rotation passes through the center of the handle. Now think the weights are planets and the handle is bridge and you can imagine the scenario.

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    $\begingroup$ or a stretchy bridge $\endgroup$ – djechlin Dec 17 '16 at 16:44
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I agree with the answer of Nobody, but want to add, that the inner planet would move outwards and the outer planet would move inwards if you lock them. So you should ensure that this results in both planets rotating the common barycenter, without adding so much pressure on the bridge, that they destroy the other planet on the position where they are connected to the bridge (your bridge should handle it with its super strong material).

So the moment when you start the locking is very critical here and I recommend to choose a position where the bridge does lay as tangential to the orbits as possible, which will result in the "rotation around their common center of mass" described in other answers.

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No...this would only work if the two planets orbited their sun at roughly the same speed while always presenting the same face to each other, and the chances of the favored conditions for a double planetary system exhibiting these traits is so small as to be zero in practical application.

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  • $\begingroup$ Other answers have raised these same points and gone on to address them. $\endgroup$ – JDługosz Dec 18 '16 at 5:12
  • $\begingroup$ Yes, but they seemed to always diverge from the main question and concentrate on one point...I was trying to give an answer that summed it all up. $\endgroup$ – Harlemme Dec 18 '16 at 5:17
  • $\begingroup$ And your logic assumes that the bridge has to have a constant length and be inflexible (why are so many assuming that? But SRM didn’t and his answer is earlier than yours), and that there was no way to deal with planet’s rotation (none possible?). $\endgroup$ – JDługosz Dec 18 '16 at 5:21
  • $\begingroup$ Why not talk about how to solve the problems? For example the bridge could rotate around the planet. $\endgroup$ – Bellerophon Dec 18 '16 at 10:51
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(Slightly) more realistic solution

Space elevators are theoretically possible, so you put one on each planet (these would need to be either at locations that could always "see" each other, so having the cord avoid the sun is another issue...) Then you have a cord connecting the two. This cord would have a bunch of slack left over beyond the anchor point on the elevator, so the area traveled would be taunt, but as the planet's orbits move further apart the slack gets added to the taunt cord. This way the orbits changing doesn't become a problem. Still not totally realistic, but the material doesn't need to be strong enough to change planetary orbits.

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  • $\begingroup$ This changing distance was covered by SRM’s and my answers, in detail. You are not clear on what you mean by slack and taught regions, or how this would be stable (my answer covers stability). $\endgroup$ – JDługosz Feb 17 '17 at 5:39

protected by Community Jan 28 '17 at 16:02

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