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

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


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


Oh. Well there goes that idea. Gallium?
-- Voice, 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


//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


//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


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


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



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