For years I’ve had flashes in my mind’s eye of an endless sky, and of towers and buildings floating in that sky. A particularly persistent image was of a woman standing at a tower window looking out over an ocean of cloud, with no ground beneath the tower. It was this recurring daydream that brought me to the question of whether such a place could exist. So began my design of the world of Virga.
SF has been using the idea of pressurized rotating space colonies for many decades. The first person to popularize them outside of SF itself—and the first to extrapolate their potentially gargantuan size—was Gerard K. O’Neill, in his book The High Frontier. This book is a must-read for any writer or designer trying to come up with a near-term space civilization. O’Neill’s largest pressurized cylinders (the “island three” habitats) would be made of steel or titanium and rotated to provide one gravity on their inner surfaces. Island Threes could be up to 3 kilometers in diameter for steel structures, and up to 14 Km in diameter for titanium. Vast windows would let in sunlight; inside would be a habitable, earth-like setting suitable for parkland, cities, or what-have-you. In the 1970s, 14 kilometers seemed like a reasonable technical limit to the size of a rotating pressurized vessel—at least, if you stuck to real material and not “unobtainium.” The outward pressures on the vessel from spin-gravity and air pressure increase as the vessel gets larger, eventually overwhelming any structural material. If you could build a cylinder out of diamond you could get diameters of 1000 Km or more—but who can work diamond?
Of course, in the early 1990s, unobtainium was obtained—we discovered carbon nanotubes. With a theoretical strength a hundred times that of steel, nanotubes have the potential to be the ultimate construction material. Once we figure out how to mass-produce and weave single-walled nanotubes, we’ll be able to build space elevators. (Progress is being made much faster than expected—see this article for a recent development.) But we could also build other things.
A space colony cylinder woven out of fullerene could be at least 5000 kilometers in diameter—if designed to rotate. (This is the design I use in Lady of Mazes for the ‘coronals.’) The assumption of colony designers has always been that you want land under gravity inside such a structure. But in any rotating cylinder of such vast size, most of the interior volume is wasted space—and as the cylinder’s size increases the interior volume increases faster (this is the famous “surface-to-volume ratio”). If the atmosphere is rotated along with the shell, it will probably collect in a blanket a few hundred kilometers thick lining the shell (depending on how rapidly the rotational gravity falls off). Above that will be vacuum or near-vacuum, and the volume of this empty space will be huge. Anything in it will be unaffected by the rotational gravity of the cylinder—in fact, the greatest force affecting an object in this central space will be the gravitational mass of the cylinder structure. At the center of the cylinder, this will have a neutral effect, so anything put there will stay there. And in a cylinder 5000 kilometers in diameter, you could put whole moons in that space.
But what if you don’t rotate the air along with the cylinder? What if you fill the cylinder with a uniform oxygen-nitrogen mix at sea-level pressure, and just spin the cylinder around it? You’ll get cyclonic winds near the shell, but there might be a livable calm layer at the surface that’s deep enough for comfortable habitation. Above the turbulent layers is a vast weightless but habitable airspace. (I’ll leave questions of entrainment of the atmosphere to the rotation of the cylinder and braking effects on the cylinder from the air to the physics-inclined.) This volume of habitable space is so gigantic compared with the surface area of the cylinder that, from the point of view of a civilization moving into the place, it’s much more attractive. After all, you can move stuff wherever you want it. You can make gravity where you need it by spinning things—houses, cities, countries . . . In fact, why rotate the cylinder as a whole, at all? That just takes energy, puts strain on the structure, and causes all kinds of hazards from moving air and land masses. After all, any one of the smaller rotating structures could still be hundreds of kilometers in diameter.
The lesson of truly gigantic space colonies is that the volume is more important than the surface.
Virga is such a construct—conceived as a sphere, not a cylinder, and containing numerous asteroids and cometary bodies that, melted, provide mobile oceans. As a non-rotating sphere, however, Virga takes on new dynamic qualities and these have to be accounted for. Virga is not like an O’Neill cylinder. The details of why it’s different are what make the Candesce books’ setting unique.
For instance, air scatters and absorbs light. If we’re going to build a balloon 5000 Km in diameter, we can’t just put windows in it to let sunlight in. The light will only penetrate a few hundred miles before it is diffused, absorbed and re-radiated at lower frequencies (redded-out). It is exactly this effect that makes sunsets red—and at sunset, the light is only traveling through a few hundred miles of extra air. So if you put windows in a balloon that’s thousands of miles in diameter, you end up with a lit outer layer and a deep ocean of darkness taking up the bulk of the balloon (or if you shine enough light in to reach the center, you incinerate anything in the outer layers). You can’t have one light source; you need dozens, even hundreds in order to light the whole interior. Hence the national suns that play such a prominent role in the plot of Sun of Suns.
(I don’t talk much about the technology of these suns. For those interested in exploring them further, I assume that they’re stadium-sized Farnsworth Fusors, built with vacuum-tube era technology.)
If there’s no external sun, there will be issues of heat transport to consider. Heat will eventually leak out the skin of the balloon, so there’s a net flow in that direction. In cross-section, Virga’s heat transport in fact looks a lot like a star’s—but without a central heat source there’s no real circulation and no global pattern of air movement. Since it’s not evenly heated, the outer skin may get cold enough to freeze the air in places, forming sinks that drain the atmosphere away. We need controlled, even heat circulation, so we’ll borrow a page from star design and put a bright, hot energy source at the very center of the balloon. This is Candesce, the Sun of Suns—the largest fusion heat source in Virga.
Candesce creates a global circulation pattern by driving heat outward evenly. The cooled air circulates back inward forming what are called Hadley cells: vast, semi-stable vortexes like the ones that hover invisibly over the northern and southern hemispheres of the Earth. These cells create the equivalent of geographical stability for a world with no gravity. You can’t organize collections of things (houses, cities etc.) on stable parcels of land; nor can you put them in the same orbits because there are no orbits here. But they can all ride the same jet streams and circulate together. The local suns and the circulation cells created by Candesce are what make stable nations possible in this weightless world. (Attentive readers will realize that the place referred to as Meridian in Sun of Suns is, in fact, a Hadley cell.)
So there you have it. Surface-to-volume ratios suggest that the bigger the space colony, the less useful its land area is compared to its air volume. With carbon nanotubes, we could conceivably build pressurized vessels that are thousands of kilometers in diameter. Lighting them and solving heat transport problems leads us to a world of dozens of micro-suns and one central heat source—and a livable volume much greater than any planet could enjoy.
There’s so much more that could be talked about—the ecology of the place, Dyson trees and water circulation . . . and I’ll be exploring more of the implications of this world in future books. But it would be foolish of me to give it all away now. For the moment, I hope you enjoy the conclusion to
Sun of Suns.
