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Send up a rocket with water in it, then freeze it in the shape of a giant lens, keeping it in earth's shadow to minimize sublimation. A series of lenses could be made for focusing.
Why hasn't anyone tried this yet?
Znamya
http://en.wikipedia..._%28space_mirror%29
Orbital mirrors [8th of 7, Aug 26 2009]
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// Why hasn't anyone tried this yet? // |
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Launch mass. A really big reflecting telescope can be made using a thin film forming a parabolic surface, coated with a suitable metal. For the mass of water required to make a big lens, you could loft a great deal more in the way of a lightweight support structure and reflective material and you wouldn't need to keep it out of the sun, just compensate for temperature changes. Photovoltaic panels would make good sunscreens ..... only the detector system needs to be kept really cold. |
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hmm... The ice could be spray coated with the reflective material, then when the ice sublimates away, you're left with the material in a good lens shape! |
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Not only how would you shape it, how would you shape to a degree of precision great enough to act as a good lens?
Also, what are you focusing the image onto? |
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all the ice I've seen has bubbles in it. |
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Make it from recycled astronaut pee - I'll leave it's imaginative names to future generations. |
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[po]: I don't think bubbles would form in space, simply
because there's no air. |
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Surely you don't need to focus the sun's light anywhere -- simply *de*focus it. Some of the light striking the lens will still go towards the earth, some will scatter. Either way, it would reduce the amount of light reaching the earth. |
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Of course, if merely scattering the sun's light is sufficient, then there are surely more cost effective ways to do it. |
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Basically, you want to have minimum mass for maximum shade. |
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//Not only how would you shape it, how would you shape to a degree of precision great enough to act as a good lens?// |
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Using Nasa's get-it-right-the-second-time method, they will freeze the lens, find massive flaws in it, then fly up a corrective lens to fix it. |
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Sorry for the wait [Quest] - I'd form it by progressively freezing larger amounts to a small ice core to make a roughly parabolic shape, then planing it to make it smooth, then possibly spraying it with something more permanent than ice so the billion-dollar-mission could leave a mirror(s) that would last for more than a few weeks. |
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The goal is to create a giant reflector telescope. As to keeping it from being shattered, it would have to be far enough away to avoid the debris field around earth, but other than that, I don't know what else could be done for it. |
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[kaz] - The ice would be a temporary support structure for the more permanent layer sprayed or wrapped onto it, which could then be planed and polished... as to where the image would be focused, that's a tricky one -- possibly another spaceborne platform that could either be manned, like the ISA, or that would just transmit the signals electronically back to earth. |
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The point is to make a giant reflector telescope... although it could definitely double as a spaceborne parasol as [goldbb] suggests. |
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This Idea might work in the middle of Antarctica better than in Space. When ice is very cold, it can be literally hard as a rock (like glass). The modern "spin casting" technique can be used to make quite a large mirror-base with an accurate parabolic curvature (Note: gravity is required part of the process, so that won't work in the zero-Gee of Space). And of course the "casting" temperature is merely the temperature of melted water, not the maybe-thousand degrees like you need to cast glass. |
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Note that operating telescopes need their mirrors to be at the same temperature as the local atmosphere (prevents certain light-aberrations). This is why we need the middle of Antarctica, where it is always colder than the freezing point of water. |
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[po],. we simply ensure the casting-water freezes slowly, and there will be no bubbles. |
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There oughta be a way... thinking over the "slowly freeze" process, a mass block of ice will change shape because water expands on freezing. You might get away from shape-changing by freezing an extremely thin film, say, just humidify the air and let it freeze out - but that doesn't go slow and doesn't produce clear ice. Need the good parts from each... |
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With technology like this, we could build an icicle connecting the earth to the moon. |
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