I watched the movie Elysium the other day and it got me to thinking about the physics of space habitats. (I realize that Elysium was based off cool visuals and not science.) It also brought to mind the novel Titan by John Varley, which involved a much larger torus (1200 km diameter). All of these seem based off of the Stanford Torus. These seem to have the idea that you can have the habitat open to space, not just living in a “donut”.
So here’s my question. How do you keep the atmosphere in and at high enough pressure to be breathable? Will just rotating the torus fast enough to simulate 1G be enough to keep the atmosphere from bleeding out?
Well, Earth’s atmosphere is open to space, and that works. Though of course, on a long enough time scale, Earth’s atmosphere would be lost, too. On an orbital habitat, that timescale would be shorter, and so it might make more sense in the long run to enclose it anyway.
It is instructive to consider that the sea level air pressure on Earth isn’t due to gravity acting just on the air around you, but also pulling down on all the air above you. That is to say, air pressure is really the result of the weight of all the atmosphere above you, just as pressure in the ocean. On a rotating space taurus the centrifugal force (that realized from the change in direction due to rotation) will act on fluid just as gravity acts here save that additonal forces–the Coriolis and Euler components–as seen from a reference frame rotating with the station. (We see Coriolis forces here on Earth, too, albeit not caused by gravity but by the effect of the Earth rotating and dragging the atmosphere with it, which can spawn cyclones and hurricanes.)
Air pressure on Earth drops to about 10% of sea level pressure by 50 kft, so realistically, you’d need walls somewhat higher than that to keep a significant amount of air, and higher if you elect to rotate your station only fast enough to get, say, 5 m/s[SUP]2[/SUP] of simulated gravity. Since some air will escape by ionization regardless, it would probably make sense to put a roof on the station. But then, narrow torrodial stations don’t actually make a lot of sense to begin with from a structural standpoint for as surface area requirement required to house thousands or tens of thousands of people; a long oblate spheroid or long tubular make a lot more sense for a large scale habitat.
I was thinking the same - it takes about 100 miles deep of atmosphere 1G to get 14.7psi of pressure. Building side walls 160km or more tall seems like a waste of resources, when complete enclosure or roofed-over torus is a more economical or simpler construction.
It might work out OK if your torus is a ribbon 1 AU in diameter and a million miles wide, and your walls are mountain ranges at the edges of the ribbons extending up about a thousand miles.
The advantage is that you can center the ring on the sun and get natural sunlight. The disadvantage is that it’s an impossible megastructure on multiple levels, only practical in sci-fi.
Moreover, even if you did have the technology to pull it off, why would you? In the universe of Ringworld, plenty of hostile forces have the kind of weaponry that could destroy a structure like that. You have to assume if you really could build a 1 AU ring, other enemies would have weapons of destruction theoretically capable of destroying it.
So why not make a whole bunch of much smaller and safer habitats with the same technolgy and wonder materials? Spread them out over multiple stars as well.
The Ringworld Engineers probably thought it was a good idea at the time. Considering they were
Pak Protectorsescaping the perpetual world wars of their homeworld, they probably figured the greatest threat they faced was more of their own kind, and they thought fleeing and creating a megastructure was good enough for that.
Still, examples of “smaller safer habitats” are the Culture Orbitals. (Of course, Culture Orbitals having near-omniscienct near-omnipotent Minds watching over them, so that also makes sure they don’t wander out of place or get punctured by meteors.) An Orbital is still a Big Dumb Object, but at least it’s at a scale closer to the idea of an “open-topped” space habitat holding atmosphere in by centrifugal gravity and side walls. (Of course, Culture technology would eliminate the need for centrifugal gravity or side walls, but that’s the size scale we’re looking at.)
Since the GQ question is answered, I’d like to briefly digress to ask what in Pak psychology would cause them to forget that they were bringing their own perpetually warlike selves along for the trip?
This is just a little nitpik- I love Niven’s books and especially the Pak. I think it’s a travesty that the book “Protector” is so hard to find.
Eh, nothing about Ringworld makes any sense at all if it’s in the same continuity as Protector. If we can only keep one of those two, better that it be Ringworld.
Well, except for the various impossibilities with the Ringworld itself, plus the fact that the Earth rotates backwards, and the improbabilty that the Pak would put a hyperaggressive warlike species like the Kzin upon it.
Back to Elysium. Would 10 mile high side walls kind of cut it? I’d think you’d need walls closer to 50 or 100 miles to avoid excessive loss of atmosphere. From my recollection of the move, the walls look to be a small fraction of the radius, maybe 5 or 10%. That would put the radius at around 1000 miles. I did not get such a sense of very large scale from the movie. Still, I think the ring was visible from Earth.
No; ten mile high walls would not cut it. Air pressure at ten miles is by a strange coincidence about 0.1 bar; with atmosphere walls that high you’d be losing atmosphere in a constant flow that would rapidly deplete the habitat.
However all you’d need is a relatively thin membrane across the top that could withstand 1/10 atmospheric pressure (unless it were hit by a meteor). I think that a lower and more robust atmosphere roof would probably be a better bet.
One idea I’ve had for an atmosphere roof on a rotating space habitat is a double membrane with sealed water-filled cells sandwiched between them. The extra weight of the water would resist the air-pressure of the habitat below, and if a meteoroid punctured the roof the weight of the unpunctured cells would squeeze the hole shut, forming a temporary seal. A layer of water would act as radiation shielding as well. Perhaps you could refill the cells using little flexible pipes as well, although that might not be necessary.
There are holes in the sky where the rain gets in. But they’re ever so small, that’s why the rain is thin.
Spike Milligan
Suppose you did ten mile high walls. Atmosphere leaks, but you replenish frequently. (You have a whole world of desperate saps to tap for resources, after all.). Will you eventually build up a ring of orbiting gas that could slow down future losses?
No. The gas that escapes over the top of the wall has the momentum imparted to it by the ring, so in addition to the pressure pushing it up it will also fall outward relative to the center of the ring, lost to interplanetary space. There are a number of other reasons you probably don’t want the top of your habitat exposed, and in any case, a large space station with a narrow width versus the radius of rotation has numerous undesirable characteristics in addition to being a very inefficient use of mass. It looks cool in “Our Future In Space” pictures and a certain first person shooter game, but functionally makes little sense.
One advantage a narrow ring has over a cylinder is a reduced tendency for tumbling. A single long cylinder will need more stabilisation than a ring if you want to avoid a second axis of rotation developing.
Actually, an object will spin stably about either its highest-moment axis or its lowest-moment axis. It’s only when you spin about the medium axis that you get tumbling. So long cylinders are fine.