Engineering Virga

In a Science Behind the Story article I wrote for Analog magazine, I described the engineering logic behind the world of Virga:

A space colony cylinder woven out of carbon nanotubes 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 than the surface area (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? . . .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.

As I said above, my design for Virga is conservative.  My main constraint for the diameter of the balloon was that I thought people wouldn't believe that one bigger than 10,000 km in diameter is possible.  I knew when I was designing Virga that it could be a lot bigger, but I had no idea just how big.  Vernor Vinge ran some numbers for me on the overall dynamics of the place, and concluded it would work; but he didn't bother looking into how big it could be.  Crowlspace has handily calculated the maximum size of a Virga-type world, and I was astounded:

. . .A gas sphere of Earth mix, pressure and temperature starts collapsing under its own gravity when it’s 34,761 km across. If we change the gas mix - say 50:50 helium/oxygen - and lower the molecular mass, the radius goes up. If we decrease the pressure the radius also goes up. For that heliox mix at 0.4 bar pressure the sphere is about 229,000 km in radius.

Imagine a sphere containing heliox at 0.4 bar pressure and 400,000 kilometres across - enough volume to fit almost two dozen Jupiters. Interesting thing is that the sphere doesn’t have to hold the gas in by brute strength if it’s thick enough - self-gravity of the sphere and the gas mass provides the counter-pressure. If we make it from diamond (the strongest carbon allotrope, density 3.5) then it only has to be 1,345 metres thick for its gravity and the gas’s gravity to provide sufficient counterpressure.

And the mass? Just 0.4 Earth masses for the shell, and 0.655 for the gas - thus just a bit more than an Earth mass. Such masses are ludicrously large for us mere mortals to contemplate, but for the postulated Post-humanity of current SF such a project may well appeal. And if it can be done, out in the Cosmos there may be Someone who has already done so.

So my Virga is pretty petty-ante compared to what's possible! 

There's pretty rigorous logic to the rest of the design as well--although there's one area where I think I may have fallen down (more on that in a minute).  The sheer size of this construct turns out to necessitate the local suns that play so big a role in the story.  Here's why:

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 Rayleigh scattering and absorption by dust and water droplets reddens and smears it.  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 are a physically necessary part of the design--if it's to be a comfortable environment for humans.

As I said above, the one place where where I might have fallen down is with regard to Virga's air circulation.  Circulation is necessary because otherwise you run the risk of having the atmosphere condense and then freeze onto the outer skin of the sphere.  (This will only occur if it's orbiting in the outer part of a star system, of course--but in my books Virga does just that.)  My original thought was to model Virga's internal circulation pattern on the internal structure of our sun (and other stars).  In these bodies, a central heat source drives Hadley-cell circulation that rises and falls; you can see that sketched in the image above.  This constraint necessitates a power central heat source:  and thus was born the sun of suns itself, Candesce.

Candesce is a great object, but my overall scheme may not work.  After all, stars have massive gravity fields, and Hadley cells turn out to be artifacts of heat transport under gravity.  Virga has (almost) no gravity--so why would Hadley cells form?  This is a problem.

I have two potential solutions.  One's almost a cheat:  I just don't know whether the microgravity environment of Virga might not be strong enough to create Hadley cells.  It may.  But, as Vernor pointed out, we can't know that sort of detail until we run some detailed simulations.  Ouch!

The other possibility is that we drive the air using electrostatic means.  This isn't as silly as it sounds:  in the novels, the artificial suns are multi-terawatt Bussard-style polywell fusion reactors (say that five times fast!), which fuse boron to produce almost pure electricity.  Much of that energy is dissipated as light, and more as heat; but gigawatts of electricity remain, and there's no ground to, well, ground it.  So of necessity, we end up with huge electrostatic gradients throughout the Virgan atmosphere. 

A novel I haven't written yet might concern the politics of sunlighting, and how adding a new sun to a local cluster might change the electrodynamics of the whole system, perhaps unfavourably to one or more countries.  In any case, there's a couple of potential uses for all of that sparking power (which even in Sun of Suns, I recognized had to be dissipated--in that novel, Hayden Griffin trails a ground wire behind his jetbike to do just that):

• Maybe Candesce drives the global circulation patterns by shooting out thousand-kilometre-long ion streams.  These wouldn't be gigantic lightning bolts, they'd be experienced locally as streams of lightning-prone weather; but they might be enough to regulate the air circulation the way I want.
• Maybe the nations of Virga use electrostatic fields to keep together.  As one wag points out in Sun of Suns, it's formation flying that keeps a Virgan nation unified.  The local sun creates a huge electrical potential, which local towns and forests etc. could tap into to steer themselves in concert with it.  I like this idea, because it simultaneously explains how countries maintain formation, and provides the wonderful image of flying nations steering their way through the Virgan airs.

The lesson here (and indeed, all through Virga) is that constraints are a creative force in worldbuilding.  That's one of the reasons I don't invent impossible technologies unless I absolutely have to. It turns out that if you stay within the parameters of known science, you're forced to solutions that are far more interesting than ones you could come up with by breaking physical laws.  Virga is nothing if not the ultimate object lesson in this principle.

--Oh, and by the way, in the fourth Virga novel I describe how to build such a monstrous construct.  It turns out to be easier than you might think--but you'll have to wait a while before I reveal those secrets.