The Dyson "Sphere"
The great physicist and freethinker Freeman Dyson once proposed (circa 1960) that it would be possible for an advanced civilization to build a collection of independent satellites around a star, each of which would be capable of serving as a solar power generator. Such satellites, he imagined, might be built in a variety of configurations-- some habitable, some not. They could transmit energy to other satellites-- or planets-- via masers, thereby enabling the civilization to draw immense amounts of power from its star.
Variations of this idea have been developed in science fiction. In one Star Trek Next Generation episode, the Enterprise encountered a solid sphere that completely enclosed a star. The sphere, as it turned out, was abandoned by whoever had built it-- but it is clear from the storyline that the sphere was intended to serve as a habitation for its builders. The advantages of such a sphere are clear. It would be capable in principle of capturing every photon emitted by the enveloped star and would therefore generate immense amounts of power. In the episode the sphere was called a Dyson Sphere even though Dyson's proposal only called for a large number of separate satellites, not a completely enclosing sphere.
But in fact the construction of such a sphere is impractical, for several reasons. First and foremost is the amount of material required. Imagine such a sphere built around our own Sun at a radius equal to that of the Earth's orbit, and imagine further that it were 1 kilometer thick. The volume of such a sphere would be about 100 times greater than the total volume of the Earth. There is simply not enough solid matter in the entire Solar System to produce a solid object that large. Not even the Asteroids have enough total matter, as the entire volume of the Asteroids in our Solar System is only about 4% of the total volume of the Moon.
One would also have to consider the effort involved in transporting such a huge quantity of matter from distant reaches of space. Where would the raw materials be obtained? No doubt they would have to be harvested from other nearby solar systems. From our initial calculations that would require trips to perhaps as many as 100 such star systems. But then wouldn't it be easier and more practical to travel to those other systems to build permanent colonies? True, doing so necessarily results in the distribution of the species over an immense region of space, making trade and communication difficult at best. And over time genetic drift will separate the colonists from their ancestors genetically as well as spatially. It is even possible that distant descendants of the colonists would not even recognize their progenitors as being of the same species.
An alternative would be to build a large number of satellites each supporting a sunlight collector of immense area. The collectors could be made of relatively thin material. The purpose of such a satellite system would be simply to capture and retransmit the star's power. Let's imagine that all of the solid material of Mercury, Venus, Mars, Pluto, and Eris (a planetesimal almost twice Pluto's distance from the Sun) could be converted to such a purpose. Let's suppose further that this raw material could be fashioned into a material of the same thickness as aluminum foil (.2 mm) that would be capable of converting 10% of incident sunlight to raw power. That would result in enough material to capture sunlight over an area of approximately 5.84 x 1018 sq.km. (Note: In making this calculation I've assumed that each planet is of uniform density, and that all solid material in each planet is equally applicable to the task. Both of these assumptions are certainly false.) The surface of the Earth, for comparison, is roughly 5.11 x 108 sq. km., so this would represent a total surface area roughly 10 billion times larger than the total surface area of the Earth. By contrast, the surface area of a sphere with a radius of the Earth's orbit about the sun is roughly 2.812 x 1017 sq.km., so in principle there would be enough material (under our assumptions) to capture 10% of the total radiant energy of the Sun. That works out to about 3.827 x 1025 Watts (ref: 21st Century Astronomy, by Hester, Smith, Blumenthal, Kay, and Voss, pg. A-8), or approximately 250 billion times the current world total energy consumption (taken as 15 terawatts.) For all practical purposes that represents an unlimited amount of power for any conceivable human society that the Earth might be able to support.
There are many engineering problems with this plan. While these rough calculations show that it is possible in principle to capture a significant percentage of the Sun's total output, there still remains the problem of delivering that energy in some useful means to a habitation (such as the Earth.) That much total energy in an uncontrolled state would simply fry the Earth. The power generated by such a satellite system could be used to build immensely tall skyscrapers or to otherwise fashion the world to our tastes only if a system for collecting and controlling it has been implemented. It would undoubtedly be a challenge to maintain such a system. Stray meteors may damage some of the satellites in such an array. The position of each satellite would need be monitored by the minute to ensure that it doesn't collide with others in the system. The Solar wind might damage critical electrical components. And if all of the energy from such a system were delivered to the Earth for consumption, the heat generated by that much power would have to be dissipated somehow, lest the Earth liquefy or even vaporize.
The satellite system I've described would not be habitable. Each satellite would be designed purely for the purpose of collecting the Sun's energy and delivering it to a civilization. But one could imagine satellites serving two purposes-- habitation and power generation. Such a system would necessarily cover a smaller total area for the same total volume of raw material and would therefore be capable of capturing a smaller total percentage of the Sun's raw energy. But it could, in principle, greatly increase the total habitable region of the Solar system.
To be as satisfying a dwelling as the Earth any such habitation will need an atmosphere, gravitation (or a reasonable facsimile thereof), protection from damaging radiation, and, ideally, a terrestrial ecosystem. Reproducing such an environment in the barrenness of space would be difficult and costly. But overall this is a far more realistic plan than that of building a habitable solid sphere large enough to enclose the Sun. While the notion of such a sphere might be appealing from a narrative point of view, it isn't realistic. The beauty of a system of independent satellites is that each can be funded separately, and that each satellite can return financial rewards within a relatively short time. Those rewards would come in the form of living space and generated energy, both of which can be profitably marketed. And that makes it possible to imagine entrepreneurs finding the funding to develop such systems. A project to build a solid sphere large enough to completely enclose the Sun is impractical from a purely entrepreneurial point of view, regardless of its engineering complexities, and is therefore impossible to implement.
In principle, the possibilities of such a system are precisely as Dyson first imagined them. Dyson observed that as human societies have evolved, demand for energy has grown at an ever increasing rate. Building habitable power generating satellites would provide a way for humanity to capture an ever increasing amount of energy in small enough increments that each satellite could be funded as an entrepreneurial venture. Once such a venture becomes profitable there will be nothing to prevent humanity from adapting the raw materials of the Solar System to the expansion of human habitation. In fact, the simplest method of implementing such a system depends on each incremental unit being funded, powered, designed, controlled, and managed-- or governed-- separately.
There is one more issue to consider. Imagine that the Earth's orbit is populated with a system of satellites, each of which consists of a habitable realm surrounded by an umbrella of solar power cells. Such a system will block out the light of the Sun in the distant reaches of the Solar System and would thereby make it less likely that people would want to travel-- or look-- beyond their cozy ring of habitation. And that could make humanity more insular and less likely to travel to other planets or stars. Personally I doubt that this will ever be a serious problem, for two reasons. First, I doubt that humanity will ever be able to build an enclosure of satellites of such density that it could block out all visibility of the universe beyond. Besides, a satellite orbiting the Sun would be as capable of looking outward toward the ever expanding immensity of the universe as it would be capable of fixing its gaze on the Earth. And second, I doubt that humanity will ever lose its interest in exploring new frontiers. I think of Dyson's vision of a vast array of satellites as empowering humanity to take the next giant leap toward the far reaches of the galaxy.
Copyright (c) 2011 by David S. Moore. All rights reserved.