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Another question made me remember this.

Consider a sizable base (building, station) in the atmosphere of a gas giant planet. It should hold a staff of a couple hundred, with supplies and scientific instrument for the mission.

One mission it has is to lower things on a loooong cable. I mention this because the tether technology is available to apply to these questions.

how does it stay afloat?

It needs to hover indefinitely. You can't make a boyant balloon in an atmosphere that's already made of the lightest gas. So hot air, or engines, or ride updrafts, or what?

what does it use for power?

Fusion from locally sourced hydrogen would be an easy answer. But suppose that's not available (or not practical); or maybe some other power source is wonderfully elegant if you can think of it?

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  • $\begingroup$ This seems to be a very similar question to your previous one. It is barely an extension. Maybe you should revise your first question and rephrase it, further specify the situation with the aspects you think are relevant. I'm pretty new here and I'm not sure how far the rules for this go, but there is also another topic-duet like this in this meta and I find the arrangement of them quite confusing. It makes all questions seem unprepared, and the information is fragmented over multiple topics in a way that helpful context is missing unless you know of both topic by chance. $\endgroup$ – InstantMuffin Jun 15 '16 at 3:00
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    $\begingroup$ Why do you say this resembles my previous question? I'm baffled. $\endgroup$ – JDługosz Jun 15 '16 at 3:02
  • $\begingroup$ I don't think it's possible. You do know that gas giants are horrible places for living, even up in the atmosphere (all of it is atmosphere). The wind speeds on a calm day are a about a hundred miles per hour and can easily cross 500 on a sad day. The gravity is a killer and anything larger than a certain size/mass would be either ripped due to tidal forces or pulled down and be pulverized within moments. I don't think this is going to work. Better invest in a gas giant moon. $\endgroup$ – Youstay Igo Jun 15 '16 at 3:19
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    $\begingroup$ On Saturn the gravity is as low as 0.9, so you're wrong. Maybe it goes with the wind, so no prob. As for being a horrible place, it's a scientific expidition. Location is the point. $\endgroup$ – JDługosz Jun 15 '16 at 3:26
  • $\begingroup$ Don't forget the beautiful Saturn rings. They remind you about the roche limit and the effects of Saturn's low gravity. When we talk about something as mind boggling huge as a gas giant, there's nothing like with the wind or against the wind. You never know when you enter the giant red spot and all your wind direction calculations go horribly wrong. $\endgroup$ – Youstay Igo Jun 15 '16 at 6:01
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Rigid-frame partially-vacuum airship

So you are correct in saying that a helium or hydrogen blimp wouldn't work on a gas giant. However, if you can make a vacuum or near-vacuum, you'll have much better luck. Of course, the pressure pushing in on the balloon means you need a rigid shell. This is the concept of the vacuum airship. https://en.wikipedia.org/wiki/Vacuum_airship

According to http://nssdc.gsfc.nasa.gov/planetary/factsheet/saturnfact.html, the density of the atmosphere of Saturn at a pressure of 1 bar is 0.19 kg/m3. Since we need a 100-person vessel, let's say we need a 1,000,000 kg vessel, a medium-sized destroyer in Earth-terms. Furthermore let's assume we make this a perfect vacuum just for ease of use. If I'm doing my math right, that's a slightly over 200m diameter sphere for your vacuum balloon. Keep in mind that this balloon is exposed to 14.7 PSI at all time at every point on the surface.

I don't think this is possible with current technology but with future advances in materials, such as those required to make tethers, perhaps it would be possible to make a vacuum airship base on Saturn.

As for storms, I have no clue how to handle that, unfortunately. Maybe the airship is just big enough that it isn't to battered by the winds, or maybe it can just weather them, I don't honestly know.

But for power, wind turbines seem like a good choice. If you're going to be battered by storms, you may as well turn them to your side. In addition, a good old nuclear fission reactor could work.

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  • $\begingroup$ The temperature in the atmosphere around 1 bar is on the order of -250C, that means "hot gas" is definitely an option, especially since the base would need to be heated, and insulated anyway. $\endgroup$ – Daniel Hill Apr 2 '18 at 3:51
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Flip the tether around so that the whole "cloud city" is tethered to another station/base/ship/moon in orbit of the gas giant. Better yet, why have a station in the atmosphere at all? If they can lower their instruments down into the atmosphere from space, why bother with something so dangerous and impractical as a "cloud city"?

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  • $\begingroup$ Synchronous orbit around Jupiter noted on space.SE would require a teather (now a space elevator) 90000 km long. The example of Saturn would be only 53400 km long. That’s about the diameter of the planet. $\endgroup$ – JDługosz Jul 27 '16 at 23:03
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How it stays afloat

Using Saturn as an example, the radius of the planet is 60,000 km at the equator. The D ring has an inner radius of 66,000 km. Since the D ring is orbiting the planet and is more or less stable, a space station could probably orbit that close to Saturn too...assuming there was no D ring crashing into it!

If you are going to lower things thousands of kilometers on a long tether, why not just plop your space station into low orbit and make the tether 6,000km longer?

Fuel and power

There is a near-infinite source of hydrogen just 6km away, so that would be the goto for fuel. The rings are almost pure water ice and the moons will have valuable volitiles like water, ammonia, and methane. These will be less energy expensive to move to the station than sucking things up from the planet.

If you have fusion power, that would be great. If you don't fuel cells would work great, too. Transform water to oxygen using electrolysis and solar power at some station farther from the planet (that never gets shadowed by the planet into night), then bring the oxygen back to the station to power fuel cells with hydrogen from the planets surface, or methane from the moons. You could also burn ammonia with oxygen to nitrogen and water.

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  • $\begingroup$ Synchronous orbit noted on space.SE around Saturn would require a tether 53400 km long to reach the cloud tops. That’s about the radius of the planet, not a mere 6 km. $\endgroup$ – JDługosz Jul 27 '16 at 23:07
  • $\begingroup$ The D ring has a radius of 66,900 – 74,510 and has moved a bit. Saturn’s radius is 58,232, meaning the ring is 13000 km above the surface, not 6. At this orbit the ship would be overtaking the planet’s rotation by an enormous orbital velocity, not hovering. Parking in a synchronous orbit, the inner rings would actually be in the way. $\endgroup$ – JDługosz Jul 27 '16 at 23:18
  • $\begingroup$ More like 6000km, you are right. What makes you think that Saturn rotates like a solid mass? Earth's liquid metal core rotates in different directions! Even a cloudtop platform would be lowering a tether through zones that might be rotating at different speeds/directions, have incredible winds, etc. Doesn't sound like orbiting at a different speed than the surface of the planet is much different than orbiting at the same speed and dropping a tether through a liquid hydrogen ocean rotating in the opposite direction. $\endgroup$ – kingledion Jul 28 '16 at 2:01
  • $\begingroup$ Good point about depths having different speed. I think it’s a different order of magnitude; e.g. speed of sound, not 3× the nominal rotation speed of the cloud deck. $\endgroup$ – JDługosz Jul 28 '16 at 3:06
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Buoyancy may not be viable on such a low-density atmosphere, unless you manage to make a vacuum baloon as in Keon's answer.

Also, since it's going to be pretty windy anyway, you might prefer something with wings to make use of that. A proper flying fortress.

Turbulence is going to be a female dog, so I am thinking that maybe your residents should live in a cylinder or sphere full of a very viscous liquid to act as a "womb". This allows people to survive intense accelerations (this idea was explored in Arthur C. Clarke's The Garden of Rama), and may be a key to surviving the shaking they are going to experience in a regular basis. The chamber they live in may be separated from the hull of the vessel by an array of shock absorbers as well.

As for energy, you'd probably want to harvest hydrogen from the atmosphere, and use solar when available, but I think a safer bet would be taking an obscene amount of RTG's along.

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