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Parachutes! IN SPAAAAAAACE!

Inflate parachutes and do science with them
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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.
Voice, Jan 31 2023

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]






       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. [+]   

       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?
doctorremulac3, Jan 31 2023
  

       How do you //despin the thing// without distorting its shape?
pocmloc, Jan 31 2023
  

       //How do you /despin the thing/ without distorting its shape?//   

       You despin it very slowly and let the attraction to the fabric be stronger than the attraction of mercury to itself.
Voice, Jan 31 2023
  

       I think you must have seen this (linked) elsewhere today.
a1, Jan 31 2023
  

       No, but that's awesome.
Voice, Jan 31 2023
  

       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)
a1, Feb 01 2023
  

       Oh. Well there goes that idea. Gallium?
Voice, Feb 01 2023
  

       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.
a1, Feb 01 2023
  

       //Oh. Well there goes that idea. Gallium?//   

       Yesss massster it wassnt usss we never touched the preciouss mercury massster but its gone gone away away <sobs>
pocmloc, Feb 01 2023
  

       //I assume you'd put some kind of solidifying agent in the mercury to make it a solid mirror once it's shaped.//   

       //it will evaporate in a vacuum with even a small amount of added heat.//   

       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.
bs0u0155, Feb 01 2023
  

       // Mercury will be very much a sold at space temperatures //   

       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.   

       (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.
a1, Feb 01 2023
  

       //it's incident radiation you have to pay attention to//   

       It's mainly because heat is so difficult to get rid of in space.
bs0u0155, Feb 01 2023
  

       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.   

       Explain that triple-point thing again? I was asleep that day in Chem 101.
a1, Feb 01 2023
  

       //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?
pocmloc, Feb 02 2023
  

       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.   

       I think that’s an important distraction, don’t you?
a1, Feb 02 2023
  

       So what's a triple Lagrange point? Is that where the Mars melts in your pocket?
pocmloc, Feb 02 2023
  

       No, that's a hike. Triple Lagrange is a kind of ice cream treat invented in France. It has walnuts, cherries, pineapple, and chocolate.
Voice, Feb 03 2023
  

       Sounds good except for the pineapple. But wouldn't that many ingredients make it a Quadruple Lagrange?   

       I know, just leave out the pineapple.
a1, Feb 03 2023
  

       Is this in competition for the Balloon??
blissmiss, Feb 04 2023
  
      
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