The trouble with Dyson Sphere and Ringworld and Orbital is that they attempt to capture a significant fraction of a star’s energy output; the Sphere all of it, the Ringworld a few percentage, the Orbital a few percent of a percent. But a star is so enormous, on the human scale, that even an Orbital is monstrously big. I submit that it is bad engineering to build a habitat up to the size of your power source; far sounder to scale power source down to the size of your habitat.
This blog series describes “Wet Stars”. These are habitats as populous as a Ringworld, but robust enough to shrug off a volley of asteroid strikes. A Wet Star is a kind of “Steppenwolf”, or habitable rogue water-world. A sunless world can harbor an under-ice ocean if it is a bit larger than Earth, or has a thicker atmosphere; fission in the rocky core keeps the ocean liquid and out of thermodynamic equilibrium for gigayears; plenty of time to form a biosphere.
I like this; it means that you don’t need stars for life. Stars are wasteful; we barely use ours; once we master fusion then stars will be obsolete. Who needs a star when you have a polywell fusor?
The universe has already made 95% of all the stars it will ever make; so stars are not an expanding niche. But there’s plenty of material left to make rogue planets from.
A Wet Star is an enhanced Steppenwolf. It is a sunless inhabited planet, powered by its own nuclear energy; but where the Steppenwolf is powered by its core’s dumb fission, fading with time, the Wet Star is powered by its biosphere’s smart fusion, constant over time.
The Wet Star’s structure is as follows: its core is a Moon-sized sphere of magnetized iron, for gravity and for a magnetosphere; this is surrounded by a thick shell of high-pressure ices; surrounded by a deep ocean, surrounded by an atmosphere. It is mostly dirty ice and dirty water, four to five times the radius of Earth, with 1 gee surface gravity. The deep ocean is heated by the waste heat of millions of fusion-powered deep-sea vessels. Each vessel is a fusion-powered submarine arcology; each with up to millions of inhabitants; making a possible population of trillions. This is comparable to a single Ringworld, or a flock of Orbitals, but much smaller in size, because it populates volume not area. I said the Wet Star has a ‘biosphere’, but really it has a ‘technoshell’.
Because distances in a Wet Star are less than a light-second (unlike Orbital or Ringworld or Dyson Sphere) it’s possible for the millions of vessels to communicate quickly, and thus form a planetary internet, or nooshell, for exchanging information about positions, currents, traffic, temperature, chemical composition, and much else.
We, on Earth, now have a planetary information system, but that system is not a Multivac, not a benevolent Hub, nor a tyrannical Colossus. It’s not a Personality set above us; it’s a Network, it’s made by us, it is us. So, too, is the Wet Star’s Net; the Wet Net.
The basic design intuitions of the Wet Star are: 1) webs, not hubs; 2) simple robust liquid flows, not complex fiddly solid structures; 3) deep ocean for food, fuel, gravitation, and shielding; and above all 4) who needs a star when you have a Bussard polywell?
The Wet Star’s biggest problems are; motility, magnetosphere, and technoshell coordination, especially in heat regulation. How to get a million independent city-states to burn just enough hydrogen to keep the ocean liquid, but not boil it off? That’s where the Wet Net comes in.
The Wet Net also make the Wet Star able to react as a unit in the case of collision, radiation or other threat. The vessels usually react simply by retreating to the bottom of the deep ocean; sufficient shielding against asteroid or supernova blast; but they have other resources too.
The vessels reproduce, age, die, and are broken up for parts, inhabitants and material. They share language, math, information and inhabitants but are otherwise independent. The vessels can meet to exchange materials or inhabitants.
(These encounters resemble sexual encounters, for docking and sex must solve the same technical problem of getting vulnerable small organisms from one hospitable vessel to another, through a hostile environment. It’ll be easy for a writer to play this for laughs; for instance if you want to go from one vessel to another then you have to wear one of those white Woody Allen sperm suits.)
The Wet Star’s ocean is heated from within, and cooled at the surface; therefore it convects. The water wells up at both poles, then circulates towards the frozen equatorial belt, then descends and circulates poleward. The equatorial ice-belt is the wet star’s only solid surface. Taken as a whole, the wet star is fixed at the triple point of water. Its atmosphere is water vapor, CO2 and other greenhouse gases; but also some oxygen and helium; these being left over from fusing the hydrogen from ocean water. The helium is light and warm enough to escape the wet star; it outgasses helium constantly, and in consequence the wet star slowly loses mass. Trillions of years from now that might cause trouble; but by that time a wet star could ‘refuel’ by plowing into a Saturn-like world’s ice ring.
The wet star’s atmosphere also convects, and it has weather. There are constant thunderstorms at the equatorial icebelt.
The wet star’s magnetic core gives it a magnetosphere; but perhaps that would not remain magnetized for the terayears I see possible. Perhaps the Wet Star’s technoshell could cooperatively manipulate ionic currents in its constantly-convecting deep ocean to generate a truly organic magnetosphere. It’s a trillion-year continuous maintenance chore, but necessary in a radiologically hazardous Galaxy.
Of course a Wet Star doesn’t need a Galaxy any more than it needs a
star. So it could just up and leave... given motility. But how to mount an effective engine on an inhabited slushball as big as Neptune? Perhaps... give each of those billions of vessels a neutralino beam, which they all fire continuously in unison? Or maybe hook a cable to geosynch at the ice belt, and fire an ion drive from geosynch? It may take a while to change course, but what’s time to a wet star?
The above are my speculations to date. I have mentioned this to Pournelle, friend of Niven, and he put it on his website. I also sent it to Iain Banks. Of course this is just a rough sketch; numbers and feasibility need checking.
G. David Nordley wrote me to say:
“The ‘fast’ way of giving delta-v to such an object would be to perform a parabolic trajectory around a much more massive object that happens to be moving in the right direction. Looked at from a great distance, it is as if the objects “bounce” off each other, like billiard balls. It could pick up some mass at the same time, of course. The object will be too massive and too cold to lose mass by evaporation, indeed it will need to get rid of accreted mass from time to time if one is thinking in terms of multibillion year time scales.”
“For low acceleration course adjustments, a nuclear powered rocket could hover out in ‘front’ of an Earth-mass planet (one of the asteroid-moving schemes, scaled up by about 20 orders of magnitude). Let us imagine an ion rocket array with a thrust of 20 million kN, (about 2 million kg force) and an exhaust velocity of 100 km/s with a continuous supply of mass and energy from the planet. It would change the planet’s velocity by about 1m/s in a million years and consume about 6.2 E16 metric tons of mass (about that of an asteroid or small moon). Also, the planet’s magnetic field could interact with the galactic field to steer the planet. While ‘new physics’ may come into play, especially at that time scale, I like to confine my imaginings to the physics we know; it’s almost always adequate.”
“The magnetosphere is less of a problem than you might think. Build a pipeline full of superconductors around the world on the sea floor and pump current into it. A loop that big gives you a nice magnetic field with large but manageable currents.”
“Even with no internal power source, one will get liquid water temperatures powered only by starlight, if one’s atmosphere is deep enough. Starlight in this part of the galaxy gives one an effective temperature of about 25K. Start with an effective temperature of about 20-30 K at the tropopause, then with a lapse rate of 6.5K/km one gets to 280 to 290K at about 40 km deep.”