04-01-2021, 04:34 AM
Imagine a hoopworld spinning in free space, far from any star. Without tidal forces from rotating near a star, there is no dynamic limit on its size.
I worked up the gravitational physics for such an object some time ago as an ISO in a story about a traumatized and possibly insane S:4 entity, but I think the ISO is still a cool idea in physics, with or without the entity, so I'm dusting it off as a proposed megastructure, and if you like it, you can imagine that somewhere in the MPA somebody might have decided to build one.
So, the version of the structure that I've actually worked up the numbers on is this:
It's a hoop, 40.85 AU in diameter. It is 13.66 solar masses. It has a mass density of 1.0 tons/m3, which is substantially less than a rocky planet like Earth and a minor (cross section of the hoop) diameter of 42361 kilometers, which is 3.32 times the diameter of Earth. It rotates around its own center of mass to balance centripetal force against gravity.
Working out all the foof, I get a surface gravity of 8.82 meters per second squared. (note, there is some variation in experienced surface g and some deviation from roundness, due to only part of the surface field potential being due to self-gravitation. In round approximations, it's small enough to ignore).
If you want 9.82 m/s, you can simply increase the low mass density a bit. With its low mass density, it's bound to have a lot of hollow inside spaces. But, I think 1 ton/cubic meter is achievable.
This gives it a surface area a bit over 5 million times that of earth. There is enough gravity and a deep enough gravity well to hold an atmosphere.
Unless something inside is using a lot of power, it's cold enough that any breathable atmosphere would fall as snow. If something inside is using a lot of power, then there could be a breathable atmosphere and comfortable temperatures on its surface. But it would be very, very dark, and any local ecology would probably live on heat rather than light.
If it's intended as a habitat, then its surface would be sectioned off into 'cells' a few thousand kilometers wide separated by atmosphere walls. That way if you have a some sections that are inhabited and getting heat/power and some sections that aren't, the moisture and atmosphere doesn't just blow away from one to fall as snow in the other. But at least anything that escapes over an atmosphere wall doesn't wind up dissipating out in the void; it just goes to the neighboring cell.
OTOH, if lacking such atmosphere walls, de-powering a few sections and bringing the surface temperature there lower than the freezing point of the atmospheric gases would continually suck the air and water away from everywhere else around the whole ring, MAKING an atmosphere wall by building up ice and air to an altitude where the air's too thin to continue condensing on it.
If relatively unmodified people were living on it, it would be illuminated by artificial lighting - probably like enormous 'streetlights' in the sense of very bright lights, probably with reflectors directing light down, mounted on very tall towers. Or, in areas near an atmosphere wall, the lights would probably be mounted on the walls.
I worked up the gravitational physics for such an object some time ago as an ISO in a story about a traumatized and possibly insane S:4 entity, but I think the ISO is still a cool idea in physics, with or without the entity, so I'm dusting it off as a proposed megastructure, and if you like it, you can imagine that somewhere in the MPA somebody might have decided to build one.
So, the version of the structure that I've actually worked up the numbers on is this:
It's a hoop, 40.85 AU in diameter. It is 13.66 solar masses. It has a mass density of 1.0 tons/m3, which is substantially less than a rocky planet like Earth and a minor (cross section of the hoop) diameter of 42361 kilometers, which is 3.32 times the diameter of Earth. It rotates around its own center of mass to balance centripetal force against gravity.
Working out all the foof, I get a surface gravity of 8.82 meters per second squared. (note, there is some variation in experienced surface g and some deviation from roundness, due to only part of the surface field potential being due to self-gravitation. In round approximations, it's small enough to ignore).
If you want 9.82 m/s, you can simply increase the low mass density a bit. With its low mass density, it's bound to have a lot of hollow inside spaces. But, I think 1 ton/cubic meter is achievable.
This gives it a surface area a bit over 5 million times that of earth. There is enough gravity and a deep enough gravity well to hold an atmosphere.
Unless something inside is using a lot of power, it's cold enough that any breathable atmosphere would fall as snow. If something inside is using a lot of power, then there could be a breathable atmosphere and comfortable temperatures on its surface. But it would be very, very dark, and any local ecology would probably live on heat rather than light.
If it's intended as a habitat, then its surface would be sectioned off into 'cells' a few thousand kilometers wide separated by atmosphere walls. That way if you have a some sections that are inhabited and getting heat/power and some sections that aren't, the moisture and atmosphere doesn't just blow away from one to fall as snow in the other. But at least anything that escapes over an atmosphere wall doesn't wind up dissipating out in the void; it just goes to the neighboring cell.
OTOH, if lacking such atmosphere walls, de-powering a few sections and bringing the surface temperature there lower than the freezing point of the atmospheric gases would continually suck the air and water away from everywhere else around the whole ring, MAKING an atmosphere wall by building up ice and air to an altitude where the air's too thin to continue condensing on it.
If relatively unmodified people were living on it, it would be illuminated by artificial lighting - probably like enormous 'streetlights' in the sense of very bright lights, probably with reflectors directing light down, mounted on very tall towers. Or, in areas near an atmosphere wall, the lights would probably be mounted on the walls.