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Imagine an old-timey parachute. It has a hemisphere on one end and a weight on the other end. Make a really big one. Cap off the hole in the parachute end, no need for that in this application. Put a packet of mercury in the middle of the chute. Put the thing in space. Start it spinning on all three
axes.
The centripetal force will spread the mercury out to the sides of the fabric, and also pull the fabric away from the counterweight, and also pull the sides of the parachute away from the center. After it has spun long enough to stabilize the mercury stiffen around the sides of the chute somehow. A lightweight metal frame should do the trick.
Now despin the thing and cut off the counterweight. After the ripples die down you are left with a very large, very smooth parabolic mirror to use as part of a telescope.
Actually you only need two axes of rotation.
Bend-Forming of Large Electrostatically Actuated Space Structures
https://www.nasa.go...d_Space_Structures/
[a1, Jan 31 2023]
Super_20Deep_20Space_20Hubble
...where I noticed the other item [a1, Jan 31 2023]
Ionic liquids, which mercury isn't
https://physics.sta...aporate-in-a-vacuum
[a1, Jan 31 2023]
Fun with gallium
https://www.cell.co...590-2385(22)00693-2
[a1, Feb 01 2023]
Gallium mirror
https://www.youtube...watch?v=z7PdTWqu7mg
[a1, Feb 01 2023]
The Vapor Pressure of Mercury
https://nvlpubs.nis...y/IR/nistir6643.pdf
[a1, Feb 01 2023]
Yet even at room temperature and 1 ATM
https://www.bing.co...ion%26form%3DSWAUA2
[a1, Feb 01 2023]
[link]
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This is interesting, spinning is how they make big space telescope mirrors on the ground, using a technique of making it in space would allow you to just ship up enough material to make a 100 foot across mirror, a lot easier to do that than to send up a already made 100 foot mirror. [+] |
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I assume you'd put some kind of solidifying agent in the mercury to make it a solid mirror once it's shaped. Plastic or something? |
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How do you //despin the thing// without distorting its shape? |
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//How do you /despin the thing/ without distorting its shape?// |
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You despin it very slowly and let the attraction to the fabric be stronger than the attraction of mercury to itself. |
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I think you must have seen this (linked) elsewhere today. |
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Now that I had time to get some details on mercury - um, no, sorry... "Mercury is not an ionic liquid and it has a high vapor pressure - it will evaporate in a vacuum with even a small amount of added heat." (link) |
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Oh. Well there goes that idea. Gallium? |
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Gallium might be better choice, at least it has a near zero vapor pressure. Highly reactive with a lot of other compounds though. It’ll literally dissolve aluminium. Even in air at room temperature a molten drop will forms oxide “skin.” Fascinating stuff but may offer some handling and engineering challenges. |
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//Oh. Well there goes that idea. Gallium?// |
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Yesss massster it wassnt usss we never touched the preciouss mercury massster but its gone gone away away <sobs> |
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//I assume you'd put some kind of solidifying agent in the mercury to make it a solid mirror once it's shaped.// |
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//it will evaporate in a vacuum with even a small amount of added heat.// |
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Mercury will be very much a sold at space temperatures. Even if it's a problem, a very small amount of oil will float to the inner surface, that would prevent any possible sublimation. You might have issues using it as a solar observatory. |
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// Mercury will be very much a sold at space temperatures // |
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Hmmm.... That's what I get for posting the first reasonable looking hit. I'm digging through a long paper (link) to try to make a more informed guess. Good chart on page 5. |
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(edit to add) Okay - you win. If [Voice]'s experiment can be shielded like the JWST is, the mirror would (probably) not evaporate. But it's incident radiation you have to pay attention to, not the "space temperature." That's why the JWST is always aimed very carefully. |
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//it's incident radiation you have to pay attention to// |
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It's mainly because heat is so difficult to get rid of in space. |
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Y'know, this still may not work, Keeping it cold enough not to vaporize in a vacuum, it'd be frozen solid. But keep it warm enough to be liquid, it WILL outgas/vaporize in vacuum. |
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Explain that triple-point thing again? I was asleep that day in Chem 101. |
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//Explain that triple-point thing again// I think that's where the gravity of earth, Mars and the milky way all balance out, isn't it? |
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No, that’s a galactic Lagrange point. The triple point is the pressure and temperature at which a substance can be in all three phases of matter - solid, liquid and vapor - in equilibrium. Not to be confused with equivalium. |
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I think that’s an important distraction, don’t you? |
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So what's a triple Lagrange point? Is that where the Mars melts in your pocket? |
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No, that's a hike. Triple Lagrange is a kind of ice cream treat invented in France. It has walnuts, cherries, pineapple, and chocolate. |
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Sounds good except for the pineapple. But wouldn't that many ingredients make it a Quadruple Lagrange? |
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I know, just leave out the pineapple. |
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Is this in competition for the Balloon?? |
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