© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

19.2.4  Large Scale Biospheric Engineering

Assuming a Type II civilization can find all the materials and energy it may require, normal expansive development and material growth may proceed. As it matures, the stellar culture may place more and more artifacts in solar orbit in an attempt to soak up every last joule of the sun's output. They may not want to go to the trouble of turning their star off, or they may lack the technological acumen to do it, or they may find the idea unethical, unnatural, and morally repugnant.

So while it is not inevitable, the chances are good that at least some extraterrestrial Type II cultures wish to preserve their sun in its natural state. On the basis of this assumption, Freeman Dyson predicted more than twenty years ago that the end result of interplanetary industrialization may be a shell of artifacts completely encasing the star. Seen from a great distance this "Dyson Sphere" would radiate only waste heat at a wavelength of about 10 jim in the deep infrared.¹⁰²²

Theoretically, a normal solar system should have plenty of mass with which to construct a solid Dyson Sphere around a star. (At least one writer has likened this to "a ping pong ball with a star in the middle."⁶⁷³) If the entire planetary mass of Sol's system was spread into a solid shell at the radius of Earth's orbit, the resulting artifact would run a bit less than 10 meters thick. The interior surface area would be the equivalent of a billion Earths, about three hundred million billion square kilometers of habitable land.

There are a number of mechanical difficulties associated with the solid Dyson Sphere. If rotating, the equatorial section would bow outward due to centrifugal force. Similarly, the "poles" would lack support and flatten, causing dynamic instability and collapse of the structure. If stationary, solar tidal forces would give rise to large compressive stresses.* The shell seems too thin to maintain proper rigidity. Furthermore, the gravity on the outer surface would be only 0.6 milligees so no reasonable atmosphere could be held. Objects or gases or people living on the inside would fall gently into the sun. There have been many attempts to save the Sphere by positing antigravity devices at critical points along the surface, but these heroic efforts remain unconvincing. For a Type II society, at least, a solid Dyson sphere is out of the question.

Anyway, the Square-Cube Law predicts that we'll get more square meters of living area out of each kilogram of building materials used if we construct the smallest habitable structures possible. For this reason, Dyson himself proposed that the Sphere should be comprised of swarms of relatively small space habitats. If we use Cole Planetoids equipped with large, very thin solar collector mirrors, we could build about 10¹⁴ of them interior to Earth's orbit. Each would weigh about 10¹³ kg, and could be pressurized with a breathable oxygen atmosphere. If they were constructed with a doubling time of 25 years, only 1200 years would be required to complete the transition to a mature Type II stellar industrial civilization.

Larry Niven has come up with a fascinating alternative to the Dyson Sphere. His proposal: A giant Ringworld (Figure 19.3), a great ribbon of matter shaped like a hoop with the same diameter as Earth's orbit.⁷⁵³ The great structure whirls around the sun at 1240 km/sec to provide a constant 1-gee gravity across the Ring. If the entire planetary mass of our solar system was used, the Ringworld would measure about 1000 meters thick. At last -- a project truly worthy of a stellar culture.

Figure 19.3 The Niven Ringworld⁷⁵³

Ring mass = 2.5 x 10²⁷ kg
Radius = 1.5 x 10¹¹ meters (1 AU)
Wall height =  2 x 10⁶ meters
Floor thickness = 1000 meters

Walls 2000 kilometers high at either edge of the Ringworld ribbon, aiming toward the central star, would be enough to hold in most of the atmosphere. An inner ring of shadow squares -- orbiting panels to block out parts of the sunlight -- provide a day/night cycle for Ringworld inhabitants. By bobbing the structure up and down, the apparent angle of the sun changes and we get seasons. You could even see the stars, as well as a beautiful checkered arc, traversing the nighttime sky.

The interior surface area would be equivalent to three million Earths, and the artifact would require on the order of 10³⁶ joules to assemble -- a project well within reach of a Type II civilization. The engineering effort would necessarily be a massive one. Looking at the outer surface of the Ringworld, Niven says:

Seas would show as bulges, mountains as dents. Riverbeds and river deltas would be sculptured in; there would be no room for erosion on something as thin as a Ringworld. Seas would be flat-bottomed -- as we use only the top of a sea anyway -- and small, with convoluted shore lines. Lots of beachfront. Mountains would exist only for scenery and recreation. A large meteor would be a disaster on such a structure. A hole in the floor of the Ringworld, if not plugged, would eventually let all the air out, and the pressure differential would cause storms the size of a world, making repairs difficult.⁶⁷³

More than one Ringworld could circle a sun, although this would require additional mass borrowed either from the local star or from a neighboring system. Many different intelligent races could wrap noncoplanar Ringworlds around the same star, with differing radii to avoid collision and provide a variety of temperature regimes.

There is also the possibility of harnessing the basic Ringworld structure for interstellar travel on a massive scale. Niven elaborates:

The Ringworld rotates at 1240 km/sec. Given appropriate conducting surfaces, this rotation could set up enormous magnetic effects. These could be used to control the burning of the sun, to cause it to fire off a jet of gas along the Ringworld axis of rotation. The sun be-comes its own rocket. The Ringworld follows, tethered by gravity. By the time we run out of sun, the Ring is moving through space at Bussard ramjet velocities. We continue to use the magnetic effect to pinch the interstellar gas into a fusion flame, which now becomes our sun and our motive power.⁶⁷³

Pat Gunkel has designed a structure analogous to the Ring but of considerably lower mass. Imagine a strand of hollow metal macaroni, about a trillion meters long but only a kilometer or two in cross-sectional diameter and a few hundred meters thick. Gunkel joins the two ends together in a great hoop around the sun, sets it rotating in smoke ring fashion to get artificial gravity on the inner surface, and calls it Topopolis.⁶⁷³ With a complete artificial biosphere set up inside, and solar collectors set up outside for energy, this huge tunnel world would have a habitable surface area of only about 20 Earths. But each one we make costs us only about 3 x 10²⁷ joules and weighs only about one-thousandth as much as the Earth.

So far we've discussed projects for Type II civilizations. Let's have a look at a few designs that will require the muscle of a galactic Type III civilization.

Dr. Daniel Alderson proposes a massive structure shaped like a giant phonograph record with the star in the center hole (Figure 19.4). Gravity will be uniform and perpendicular to the Disk everywhere except at the edges. A slow spin should partially counteract the sideways inward pull of the central star and provide a stable celestial pole. A 1000-km-high retaining wall should be constructed on the lip of the inner hole, to prevent the leakage of atmosphere into the sun.⁶⁷³

Figure 19.4 The Alderson Disk⁶⁷³

The Alderson Disk (see illustration previous page) weighs in at about 6 x 10³³ kg, or about 3000 solar masses. The innermost radius is about 50 million kilometers, just inside the orbit of Mercury; the outermost radius is set at 600 million kilometers from the sun, about midway between the asteroid belt and Jupiter. The Disk is 5000 km thick, so the surface gravity is 0.14 gees (about like Luna). If the air is pressurized to 1 atm at the surface, then the total weight of the atmosphere is only 2 x 10²⁹ kg, less than one-tenth of a solar mass extra. Since gravity is so low, the air thins out very slowly with altitude: 40 kilometers up, the pressure is only down to 0.5 atm which is still breathable. Also, note that both sides of the artifact can be inhabited.

Since the Disk is far more massive than the central star, the sun should be bobbled up and down to create seasons. Computer-controlled Shklovskii Grasers mounted on the inside edge could induce vertical motion, and would also serve to nudge the star back to center if it begins to stray towards the inside annular edge of the Disk. The energy required to assemble the Alderson Disk should be about 5 x 10⁴⁴ joules, which should be fairly trivial for a galactic culture.

One science fiction writer waxes enthusiastic about the idea:

The Disc would be a wonderful place to stage a Gothic or a sword-and-sorcery novel. The atmosphere is right, and there are real monsters. Consider: We can occupy only a part of the Disc the right distance from the sun. We might as well share the Disc and the cost of its construction with aliens from hotter or colder climes. Mercurians and Venusians nearer the sun, Martians out toward the rim, aliens from other stars living wherever it suits them best. Over the tens of thousands of years, mutations and adaptations would migrate across the sparsely settled borders. If civilization should fall, things could get eerie and interesting.⁶⁷³

Due to its size, the Alderson Disk would probably have to be a cooperative venture of a group of Type II cultures (perhaps 10-100 of them) or of a single emergent Type III galactic civilization. The additional land area made avail-able is enormous. The useful living surface would be the equivalent of more than 4 billion Earths.

The value of large-scale economic cooperation is fairly obvious. Perhaps, in view of this, we should take a second look at the solid Dyson Sphere concept. Is it possible for a galactic community to build one?

Probably. It will be recalled that the main problem, other than simple lack of mass, was the matter of dynamical instability. Even if the hollow sphere was rotated fast enough to support the equatorial zone of the structure against collapse, the polar regions would fall in and destroy the Sphere. Veteran science fictioneers Jack Williamson and Frederick Pohl pondered the problem and came up with a most ingenious, workable solution. Here, in their own words, is how it's done:

Surround the star with several layers of ring-shaped tubes. The tubes themselves are stationary, but they contain a heavy, low-viscosity fluid flowing fast enough to create the centrifugal force required to support the tubes and loads above them. One set of parallel tubes holds up the “equator“ -- which isn‘t really moving -- and other sets, tilted at suitable angles, support the regions near the “poles.“ The tubes are also heat engines, with the fluid driven by energy absorbed from the sun and flowing through generator stations which supply power to all the inhabited levels. Master computers adjust the velocity of flow to fit the loads.²⁸³⁴

The Sphere is massive -- about 15 solar masses. The shell is equivalent to a wall of solid steel more than 3 km thick, but since much of the room is taken up by living quarters, passageways and so forth, the hull is actually more than 60 km deep. The total energy needed to assemble the Sphere would probably run on the order of 10⁴³ joules, but to propel it through space at 17%c (the stated velocity) will require an additional 4 x 10⁴⁶ joules of energy.

Though the surface gravity is only 1% that of Earth, this is still enough to hold a tenuous atmosphere just above the external armor plating. Part of this is interstellar gas, but most of it is a helium-oxygen mixture left over from nuclear power plants which use water for fuel. The oxy-helium air is just barely breathable at the lower altitudes, with predictable consequences:

The outer surface was at first an endless plain of bare metal, but much of it is now covered with soil from accumulated cosmic dust and the industrial wastes dumped from the occupied levels. Plant life has evolved there, supported by the energy-flow from below through a process of thermosynthesis. These plants are often luminescent, so that vast landscapes glow with varied color. There's animal life, adapted to the low gravity and to varied local conditions of light or darkness, heat or cold, wild storms or unending calm -- with no rotation and no external sun. Most of these beings evolved on the surface, but some are migrants from below. A few are human.²⁸³⁴

* This may not be fatal to the project. According to equations for gravitational tidal stress on large structures provided by Dyson,¹⁴⁵⁰ the minimum materials strength needed to hold a solid Dyson Sphere together should be no more than 10¹² N/m². Flawless diamond has a theoretical maximum compression strength of ~10¹² N/m² and shearing strength ~10¹¹ N/m² so it may be possible to work something out.²⁸³⁸ At compressions above 10¹¹ N/m², the normally colorless diamond takes on a delicate light brown color.²⁹³⁹

Last updated on 6 December 2008