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Imagine a SupraWorld being built around a gas giant like Jupiter. To have a 1g gravity on the surface the resulting SupraWorld would have to be less than twice the radius of the giant (my calculation; could be wrong). I have unobtanium but it only provides impossible structural resistance and nothing else.

My civilization will start by placing a large hexagonal flat surface (where the colonists live) with very large walls to prevent the atmosphere from escaping. More will come; eventually all the hexagons will join in a ring (the walls remove periodically as two hexes join) and eventually they will "close" as a sphere(the finished SupraWorld).

How do I prevent the single "hexes" from falling into the planet? My idea was to have them at the top of a huge "tower", with the colony at one end and a large near vacuum spheroid or whatever at the other end, floating into the gas giant's metallic hydrogen ocean. Gyroscopes would stabilize the structure preventing it from flipping. How credible is that?

SilverCookies
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Your tower idea sounds fairly expensive. Given your setup, each tower would be nearly one planet's radius (r) in height. Also, you would soon need more towers as your structure grows.

If instead you build a ring around your planet, it's circumference would be $ 2 \pi r $.

So soon after the sixth tower, your towers combined would have the same length as the circumfenrence of such a ring. I would definitely go for the ring directly.

You could build the ring segments in a stable orbit, then once completed lower them far enough so they can be join.

This will also provide a spectacular show, allowing for enormous fees for the commercial breaks...

After that, you can attach your hexagons to that ring.

As a bonus, you can completely skip the inverted pendulum problem and the gyroscopic stabilizers.

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Burki
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  • The problem remains: How do you prevent the hexes from falling into Jupiter when you lower them out of orbit? As soon as Jupiter's gravity takes over, you need to counteract it. – Hackworth May 23 '16 at 13:38
  • @Hackworth the ring i am suggesting will stabilize the whole setup. Gravity of the planet will hold it in place. The weight of the first hexagon will make it wobble slightly, but the force of gravity will mainly be countered by the strenght of the ring. – Burki May 23 '16 at 14:08
  • And how do you construct the ring without it falling into jupiter? – Hackworth May 23 '16 at 14:16
  • @Hackworth as mentionned, i construct it in a stable orbit. – Burki May 23 '16 at 14:17
  • Then how do you lower the ring segments so they can be joined together into a ring? Same question. – Hackworth May 23 '16 at 14:20
  • I would suggest using a faster, not higher orbit for the ring construction, so that you can connect all the sections at leisure (in zero-g) , rather than in a frantic synchronized descent. After the ring is complete, you'd slow it down with thrusters or some kind giant flywheels to conserve the energy. – Cyrus May 23 '16 at 14:22
  • @Burki even in orbit, I am afraid such structure would not be gravitationaly stable. Isn't that the problem that existed with Niven's RingWorld? – SilverCookies May 23 '16 at 14:25
  • @Cyrus that is a valid point. Although my solution might provide more spectacular images. :-) – Burki May 23 '16 at 14:25
  • @SilverCookies I think it would suffice if it was stable enough. Adding some Thrusters to add minor corrections should help that until the sphere is complete. After that, the gas pressure inside the shpere (that will be higher closer to the center) should stabilize the entire system. Mind you, i am a complete layman in these matters, just relying on some basic understanding of physics and some consrtuction knowledge – Burki May 23 '16 at 14:28
  • @SilverCookies You have the same problem anyway. If your finished structure experiences gravity from Jupiter, it cannot be in orbit either, and watever configuration it has, it will be unstable. – Hackworth May 23 '16 at 14:35
  • @Hackworth - Construction is straightforward, if monumental. The plates are originally inserted into circular orbit, although they are uninhabitable due to being in freefall. When enough have been inserted into a single orbital plane they are linked to form a ring. The ring is then despun to produce whatever local gravity is required. Of course, once linked the ring is only of neutral stability, and will need constant correction to keep it from drifting out of orbit. Since you've already got the thrusters which were used to despin the ring in place, this is not a problem. – WhatRoughBeast May 23 '16 at 14:56
  • @Hackworth Why would it be unstable? in my solution they are basically anchored to the planet – SilverCookies May 23 '16 at 17:14
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If you start with an open lattice, you can build a sphere with very little material. Think of chicken wire: make a form from that, then start filling in the surface a bit at a time.

You will have to use some kind of active control to keep it centered around the planet. If it has a metallic hydrogen core and thus a strong magnetic field, you can use magnetism. In fact, you can use flux pinning for the whole thing. Your unobtanium could be superconductors programmed to hover and keep station in the planet's magnetic field, without any need for super strength!

But, making flux-pinning superconductive structural elements on the nano-scale is exactly how I'd make a hard-SF supermaterial, as well.

Oh, and don't worry about the radiation. Robert L. Forward published a (real science) paper on how conductive tethers could be adapted to short out the Van Allen belts and drain the charge.

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JDługosz
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  • That's interesting! However, wouldn't you need a very strong magnetic field to keep such a massive strusture in place? – SilverCookies May 23 '16 at 18:36
  • The magnetic field automatically compensates for motion, and is supplied by the planet and "grabbed" by the superconductor. Or are you asking if the planet's field is strong enough? – JDługosz May 23 '16 at 18:38
  • If you have a supra-shell and just need to keep it centered, you don’t need as much strength as you would to hold up a single platform against gravity. The gravity cancels out inside the hollow shell! – JDługosz May 23 '16 at 18:40
  • yes I was wondering if it was strong enough – SilverCookies May 23 '16 at 20:28