Gas giant question

[Darth Wong]

The coolness would be people living on it tens of thousands of years later and not realizing what it is. That wouldn't happen with these floaters.

[Ender]

Also, big problem with Jupiter is the radiation shielding you would need. Though depending on how far down in you were, the atmosphere could do it for you.

[Ender]

The coolness would be people living on it tens of thousands of years later and not realizing what it is. That wouldn't happen with these floaters.

Well yeah, but in that scenario it is just a slight twist on the common place "on a generation ship and don't realize it" scenarios.

[Ford Prefect]

Once you get inside the superstructure, you can presumably find monitoring stations and all manner of other equipment which allows you to see down, or airlocks.

In my mind, I imagine that the actual structure of the shell would be filled with industry. Not just vast automated factories but continent sized farms run by legions of robots. While it would be conceivable that no actual person would need to be down there, because if you can build this thing you can probably make it run itself, if there were it would probably be quite boring. Even windows so you could see the occasional aurora spreading across one of the giant's poles would probably lack impact after a couple of weeks in Autofab Menial Managment.

[Ford Prefect]

Charles Stross did it in Accelerando, though it was Saturn there.

I was actually wondering what authors had actually used it before. I'm fairly sure Yukito Kishiro, a manga author/artist, used a similar premise as well.

[Paolo]

Also, big problem with Jupiter is the radiation shielding you would need. Though depending on how far down in you were, the atmosphere could do it for you.

Assuming you can manage the upwards multi-TPa stresses for the solid shells, build two and fill the intervening space with low density stuff until you have the desired shielding.

[Lord of the Abyss]

Charles Stross did it in Accelerando, though it was Saturn there.

I was actually wondering what authors had actually used it before. I'm fairly sure Yukito Kishiro, a manga author/artist, used a similar premise as well.

It's not quite the same, but Tony Rothman in The World Is Round has his protagonists discover a world fifty times the size of their homeworld, yet inhabitable ( barely; the day's a year long and the inhabitants spend much of the time underground ) with the gravity of a much smaller planet. In this case, though, it turns out to be a black hole and not a gas giant inside the hollow planet; the window they find just shows blackness and huge cylinders, nothing as dramatic as a close up view of a gas giant storm would be.

The name of the book comes from the fact that the inhabitants have no idea that their world is round. They have no astronomy; going out at night is lethal. And their world is so huge that other signs are hard to find; for example, it's curvature is so flat that a ship sailing into the distance will fade out before appearing to sink before the horizon.

[Darth Wong]

The coolness would be people living on it tens of thousands of years later and not realizing what it is. That wouldn't happen with these floaters.

Well yeah, but in that scenario it is just a slight twist on the common place "on a generation ship and don't realize it" scenarios.

Usually, people on generation ships are primitives, which is why they can't figure it out. In this case, people could be technologically advanced. They could have even developed space travel, and still not understand why the fuck their planet is so big.

[Adrian Laguna]

When I read the OP, I was actually thinking of what the size of a shell built inside Jupiter would be for it to have 1 g of gravity. I'd also like to know what kind of pressure the surface of such a construct would be experiencing. Undoubtedly, it would be enough to make it uninhabitable.

[Winston Blake]

Charles Stross did it in Accelerando, though it was Saturn there.

I was actually wondering what authors had actually used it before. I'm fairly sure Yukito Kishiro, a manga author/artist, used a similar premise as well.

Paul Birch proposed a Supra-Jupiter 16 years ago.

[CaptainChewbacca]

I still think the coolest part of this would be that its a massive, earthlike planet but can still have earth-sized MOONS. Think about it, you could have an artificial world built, and then the moons are seeded with sentient races because the architects want to see what will happen when the various races start colonizing.

Of course, super-earth has to have a kickass laser defense system to shoot down/fragment incoming asteroids so that they DON'T perforate the shell.

OR, how's this for an emergency defense; IF something DOES puncture the shell, the underlying superstructure immediately 'uplifts' in a massive earthquake, to raise the perforation above the atmosphere until it can be repaired.

What if you climbed the impossibly tall mountain on your world, and looked over the edge into a gaping void of emptiness?

[Ford Prefect]

I still think the coolest part of this would be that its a massive, earthlike planet but can still have earth-sized MOONS.

The construct will probably have already devoured all the other mass in the star system in the processes of construction. It would almost certainly be a vanity project, so it may well have moons.

[Ender]

I still think the coolest part of this would be that its a massive, earthlike planet but can still have earth-sized MOONS.

The construct will probably have already devoured all the other mass in the star system in the processes of construction. It would almost certainly be a vanity project, so it may well have moons.

Nah, surface area is 1.5e17m^2, call it a density of 3000 kg/m^3 and you are looking at 4.5e20 times whatever the thickness is. That is a fraction of the mass of the rest of the system. Even if you make it a km thick it isn't even the mass of earth. Make it mostly carbon or something and you can easily pull the mass you need out of the atmosphere of the giant itself.

[Dracofrost]

Well, the solution to the strength problem I usually hear proposed is dynamic compression members, but I have no idea how close to being actually plausible those are, engineering-wise.

Does anyone else know if such things would even really work?

[Sikon]

Dynamic compression members are an idea to have a stream of pellets holding something up by momentum transfer, rather than a solid structure holding itself up.

For example, a metal or carbon-fiber-composite tower built off earth's surface can't reach more than a limited number of kilometers height before the stresses at the base exceed material strengths (and "tapering" such with a more and more massive base compared to the top allows greater height but at the expense of exponential diminishing returns).

Yet technically a space fountain holding something up does not suffer from such exponential diminishing returns and allows vast structures while remaining within possible stresses able to handled by real materials.

A setup of dynamic compression members is superior to the solid shell around Jupiter idea in the sense that anything is superior to the impossible. It is, though, introducing a lot of complexity, expense, and reliability difficulty to say the least.

In a way, such is like holding up a house in mid-air with a giant jet of water hitting its underside (or, for a closer analogy, magnetically redirected circulating pellets), such that if a delicate active system fails at any time the whole thing falls to destruction.

It's possible within the laws of physics and without exceeding the forces that molecular bonds between elements in the periodic table can handle. But whether it is the preferred, most practical engineering method for building houses or living space is quite a different topic, to say the least.

While proposals for a space fountain system making extraordinary high towers on earth reaching into space at least have a purpose, here the use has little real point to it. Build independent structures, a multitude of rotating habitats in orbit around Jupiter or elsewhere, and every real, practical benefit of significance can be satisfied.

[CaptainChewbacca]

Yeah, but rotating habitats isn't NEARLY as cool.

[Surlethe]

Hm... in my looking I found a rule-of-thumb that if you're going for earthlike gravity on the surface, the surface area will be equal to the gravity disparity times earth's surface, i.e. a body with 100 earth masses surrounded by a shell with a surface gravity of 1 g will have a surface area equal to 100 earth areas.

If you solve for surface area in terms of enclosed mass and desired surface acceleration, it actually comes out that the area is independent of the radius: A = 4πGM/a, where A is area, M is enclosed mass, and a is desired acceleration. So, yes, A is directly proportional to M.

[Guardsman Bass]

I'm wondering; if you have the tech and capability to actually build a monstrosity like this, covering an entire jupiter-sized gas giant with a Dyson Sphere style structure, wouldn't it be better just to create an actual terrestrial-style planet the size of either Jupiter or a smaller gas giant? Obviously, you'd have to make the monster with very low density to get good surface gravity, but you could still probably put a lot of metals in the crust if you wanted them for later use without affecting the planet's mass much. Not to mention that the giant's volume to surface area would be so huge that it would have internal heat for quite a while.

[Guardsman Bass]

What about a smaller size, like a Uranus- or Neptune-sized low-density terrestrial planet?

[CaptainChewbacca]

One of the big 'reasons' for doing something like this isn't just the living space, but the energy potential of the gas giant. Building one of these things just floating in the black, you'd lose that energy.

[Kuroneko]

This setup is almost custom-built for Uranus or Neptune. For Uranus, the ratio of sphere radius to the volumetric mean radius of the planet (1 bar pressure level) is 0.956. For Neptune, 1.07. Plus, both Uranus and Neptune have much lower ellipticities than Jupiter. Of course, an even lower mass would be required if the structure is close enough to the star to have a reasonable mean temperature, which would cause a gas giant of comparable mass to be larger.

For a spherical shell of outer radius r, thickness h, and overall density ρ, the gravitational pressure on the inner side due to a central spherically symmetric mass M is P = [GM/r²][ρh][1 + 2h/R + ...] ≅ gρh. It's unfortunate that having a high internal pressure is even more dangerous than low one in the event of a break, or else we could simply have a very high internal pressure to counteract the gravity. As it is, if internal and external atmospheric pressures are approximately equal, the required specific strength of the material is about (1/2)sqrt(GMg) = 1.29e8 Nm/kg, and is to low orders insensitive to both thickness and radius (the latter from keeping surface gravity g constant). This is about 2800 stronger than even carbon nanotubes.

On the other hand, any civilization capable of building on such a scale would be also capable of clearing the stellar neighborhood, and if this is also occurs far from interstellar clouds (like the Oort cloud) to lower the probability of incoming comets, so perhaps a highly pressurized shell would not be too far out there. An upcoming civilization that evolved on the shell would actually be capable of building Strangelove-esque "doomsday devices" simply by building nuclear weapons of sufficient yield to locally damage the shell.

[Junghalli]

One of the big 'reasons' for doing something like this isn't just the living space, but the energy potential of the gas giant. Building one of these things just floating in the black, you'd lose that energy.

Come to think of it, something like this wouldn't necessarily have to be a vanity project. Say you have people who colonize one of the moons of a gas giant drifting in interstellar space for some reason (exiled there as a "fuck you" by a government that didn't like them?), and their civilization manages to survive and eventually thrive. There's no star around, so the largest energy source in their "solar system" is the gas giant. This construct is the closest thing they could have to a Dyson Sphere.

[CaptainChewbacca]

One of the big 'reasons' for doing something like this isn't just the living space, but the energy potential of the gas giant. Building one of these things just floating in the black, you'd lose that energy.

Come to think of it, something like this wouldn't necessarily have to be a vanity project. Say you have people who colonize one of the moons of a gas giant drifting in interstellar space for some reason (exiled there as a "fuck you" by a government that didn't like them?), and their civilization manages to survive and eventually thrive. There's no star around, so the largest energy source in their "solar system" is the gas giant. This construct is the closest thing they could have to a Dyson Sphere.

I like the idea of a brown dwarf sphere-world in deep space very, very much. I will put it in my file.

[Dracofrost]

One of the big 'reasons' for doing something like this isn't just the living space, but the energy potential of the gas giant. Building one of these things just floating in the black, you'd lose that energy.

Come to think of it, something like this wouldn't necessarily have to be a vanity project. Say you have people who colonize one of the moons of a gas giant drifting in interstellar space for some reason (exiled there as a "fuck you" by a government that didn't like them?), and their civilization manages to survive and eventually thrive. There's no star around, so the largest energy source in their "solar system" is the gas giant. This construct is the closest thing they could have to a Dyson Sphere.

I like the idea of a brown dwarf sphere-world in deep space very, very much. I will put it in my file.

Sounds like a new interpretation of Cybertron. Wasn't it supposed to be bigger than a gas giant but not have crushing gravity of doom?

[Themightytom]

didn't cybertron also transform into a giant robot to fight unicron? i feel like transforming would be hard if you're a dense brown dwarf but i don't really know about stars.
or transformers..