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Journal 4/20/17 Imagine you wanted to construct a ring planet, like in Halo. What kinds of challenges do you think this would present? Make a list. Objective Tonight’s Homework To learn about some of the theoretical megastructures astronomers have thought up Start working on final projects by laying out timelines Ringworlds and Dyson Spheres Every year, humanity needs more and more energy to function. Imagine that we keep getting more and more advanced. The farther we go, the more energy we’ll need. One way to get energy is to trap it from the Sun. We could then convert it using efficient solar cells. How far can we take this idea? Ringworlds and Dyson Spheres We can get 3 results: What’s called a “Dyson Sphere”. This is a loose sphere of material that completely encloses the star. A “Ringworld” as created by Larry Niven in 1970. This would circle around the star at Earth’s orbit. A “Halo” ring, as a planet-sized ring orbiting around the star like a planet does. Ringworlds and Dyson Spheres Before we get into engineering, let’s look at simple materials. How much stuff would it take to build something like these? Let’s start with Mercury. Mercury is 70% solid iron. What could we make if we completely mined out Mercury to get our metal? A Halo ring 10,000 miles across, 1000 miles wide, and 325 miles thick. A Ringworld 186 million miles across, 1000 miles wide, and 92 feet thick. A Dyson sphere 186 million miles across and 0.15 millimeters thick. (too thin!) Ringworlds and Dyson Spheres Halo rings would be big, but relatively easy to construct, so we’ll skip discussing them here. The more interesting choice is the Ringworld. In this setup, our ring is placed around the sun at the same distance Earth is. While Mercury could provide us with enough material to make something like this, we’re still presented with some interesting challenges. Ringworlds and Dyson Spheres A lot of people have done calculations for Ringworlds. The first is how carefully you’d have to place the ring. Math has shown that if the ring is even an inch off-center from the star, it would only take 6 years for the wobble to get so bad that an edge of the ring hits the star. Tidal forces would also cause problems. The ring would be stretched and bent enough as it goes around the star that oceans would likely slosh out of the edges of the ring unless walls many miles high were built. Ringworlds and Dyson Spheres The ring would also have to be shielded from meteors as well. Since even an inch off center would cause problems, even a small impact could ruin things. Dyson spheres are trickier. A lot of science fiction shows them as solid spheres. But it doesn’t take much math to show that a solid sphere around a star would never hold. But just for kicks, let’s look at a few statistics. Ringworlds and Dyson Spheres If our Dyson sphere had the same radius as Earth, it would have: - A surface area of 1.09x1017 square miles. For comparison, Earth is 1.97x108 square miles. This is a billion times more surface than Earth. - Assuming the whole thing was covered in perfect solar cells, this would generate 3.95x1026 W of energy. For reference, this is a billion million times more energy than all of humanity uses in a year. Even if we covered the sphere only 10% with cells and they were only 10% efficient, it would still be a trillion times more energy than all of what humanity uses. Ringworlds and Dyson Spheres Let’s back things up to our Ringworld then. With perfect solar cells, this would absorb 6.28x1021 W if we covered only 10%. This would still give an insane amount of room to live, but also generate a billion times more energy than all of what humanity uses. At 92 feet thick, this Ringworld is also getting to be scientifically within the realm of at least being imaginable. Exit Question Why is a full Dyson sphere not realistic? a) It would require too much material b) It would feel too many stresses from gravity c) It would be hard to keep it centered d) It would be unnecessarily big e) All of the above f) None of the above