# Building a celestial body

• posted on August 1, 2007, 5:48 am
Here's an interesting thought problem for you folks.
The gravitational acceleration a person experiences is derived from the mass of the body we're relating to, and the distance of the person from that body's center of gravity. On Earth's surface, this is nominally about 9.8 m/s^2, or what we call "one gravity." On Jupiter's "surface", the experienced acceleration is much greater.
However, if one could hold a position at 110,000 km from Jupiter's center of gravity, one would experience an acceleration of about one gravity, making the planetary experience, on the whole, much more comfortable. (Radiation belts aside.) If one were to build a shell around Jupiter with an outer radius of 110,000 km, one would also get a surface area of 19.6 billion square miles, or over 99 times the surface area of Earth. *Lots* of living space, and, unlike some other celestial megastructures, your atmosphere is stuck to your shell by gravity.
The question becomes, what would one want to build such a thing out of? I don't have a background in mechanical or architectural engineering, which is why I come to you folks. I imagine one would want concrete, as I picture the stresses forcing the shell to compress laterally across its surface. But how much concrete would one need? How much water (A scarce resource in space, though Jupiter happens to have a large ice moon) would be needed?
Another engineering-related line of thought I can't follow up on is the effect of tidal forces...Jupiter has several large moons. How much of an effect would they have on the shell?
Incidentally, this is part of a sci-fi world-building project I'm working on called Grokked Universe. Here's the relevant thread: http://guforum.shortcircuit.us/index.php?topic8.0

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• posted on August 1, 2007, 9:29 am

What to build from 3D-H ofcaurse ; 3dh is not a material but a method the only one that in architecture today fir the computer ,and make a direct link from projecting to manufactoring. But the reson the method shuld be chosen are, that all it requier is sheet material, that mean that if you want to build a structure, all you must look for, is sheet material of some kind , look up 3D-H or 3D Honeycomb --- not the ready made sheets of honeycomb cubes but the buildng method.

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• posted on August 1, 2007, 12:10 pm

I'm confused, man...which way's this doob goin? <suppresses cough>

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• posted on August 1, 2007, 1:14 pm
Dear Michael Mol:
...

*Want* to build it out of wimmun. *Have* build it out of unobtanium.

Concrete won't work, not enough strength for a shell that will support a human.

Enough to drink.

Our Moon swings the Earth around a little circle, and there is a "three meter high" lump that follows the Moon around. Jupiter is massive enough that it swings the Sun around a point just outside the surface of the Sun. You'd have real difficulties keeping the shell in place.

Dyson sphere. Ringworld.
David A. Smith

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• posted on August 1, 2007, 4:15 pm

Second reference to Dyson on the group in a month!

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• posted on August 1, 2007, 10:15 pm
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Alright Michael! High-five!

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• posted on August 1, 2007, 10:56 pm

Hi-5!
In both cases I seem to have made similar references to Dyson going steady with Mary Jane...

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• posted on August 1, 2007, 6:47 pm

Wouldn't that depend on how thick the shell was?

I'm not worried about the shell staying in place. (See my other comment about gravitational centering.) I'm more concerned about stresses induced in the rigid structure of the shell.

I read Dyson's original article, I own all the Ringworld books, and I just finished reading Niven's "Playground of the Mind" collection. (I checked it out at the library specifically for the "Bigger than Worlds" article it contains.)
Dyson doesn't posit a rigid structure. His theory accepts any structure (or arrangement of structures) capable of capturing and harnessing all of the energy output of a star. Using it as a habitable surface poses difficulties in terms of gravity and retaining an atmosphere. (Though I once calculated that, given all the mass in our solar system, one could build a a shell around our sun with the density of the International Space Station, an inner radius of 1AU, and thickness of 2km. So it's not impossible to inhabit such a structure.)

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• posted on August 1, 2007, 7:32 pm

Think for a minute. The shell has to support itself against 1g with a very tiny curvature, and it will have to support a point mass of a few hundred pounds. And do all this with 5000 psi maximum stress (or less). Not in this Universe.

It didn't make sense there either. Displace the centers of mass off center, and they *will not* recenter without work being applied. Gravitation is not a force.

Something this size is not rigid. Something the size of a building is not rigid. And you are right to worry about stresses. I think they are insurmountable with any matter.

Hard to build anything out of hydrogen ice.

David A. Smith

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• posted on August 1, 2007, 2:42 pm

I fear that's a question that only RebarGuy can answer.
I, however, specialize in terraforming, and think it would be far easier to just terraform Mars, or maybe even Venus or Europa than build a concrete sphere around Jupiter. As Pat inspired; if you have the knowledge to build a concrete shell, your knowledge would likely find it impractical.

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• posted on August 2, 2007, 1:46 am
wrote:

...
The merest of quibbles, I assure you.
It's about gravity. On the surface of the Earth, you DON'T accelerate. Instead, you find that a certain force is required from your feet to remain standing.
This is what we usually call weight. So the formulation of g can be put not as an acceleration of 9.8 meters per second per second, but rather as a ratio of force(weight) to mass i.e. 9.8 Newtons per kilogram.
This helps you remember you can move to places some of them near the poles where the weight due to a given mass is higher, 9.81 Newtons per kilogram or more.....
Brian Whatcott Altus OK

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• posted on August 2, 2007, 9:54 pm

Well, firstly, the *surface* of your object would have to be around the altitude you specified. IOW, if you created the object, and then covered it with a layer of growing medium ("dirt") of x meters, the surface of the construct would be 110,000 km - x m from Jupiter's center of gravity.
If your Object is a ring rather than a sphere, you'd also have to have walls to hold in the atmosphere.
(((IIRC, in "Ringworld", LArry Niven (*IIRC*) had a rig that rotated and therefore held everything against the inside of the ring.)))
As for materials, isn't a Ni-Fe composite the most common material in the asteroids? If so, wouldn't that be the logical material of preference?
Certainly, one planet couldn't supply all of the material, at least not without basically destroying the planet. But it seems to me (jsut on gut feeling, tho', no calculations) that the asteroid belt might provide enough material.
I think that destroying Jupiter's moons for the thing is a horrible idea. Esp. if there is any life on Europa or whatever.
BTW, re: using cables slung from an orbiting body and penetrating a planet's atmosphere for energy prioduction, that idea has already been proposed in a science fiction story (might have been by Ben Bova, but I read it many years ago so can't recall).
A shell around a planet might be less practical than a ring. For one thing, a rign would be a smaller target for asteroids and comets. For another, it would allow thermal and other radiation to escape from Jupiter. ANd then, of coruse, there is the amount of material required for a shell, as opposed tothe amount required for a ring.
As for holes - well, if you have the tech to do a shell or even ring, you have to tech to repair the thing....