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Comment by Jyaif | Hacker News Reader

How would they keep the liquid from evaporating? Would they need to enclose it?

If the liquid is kept in the shadow it will be at a temperature of zero kelvin (-273 °C). I guess that at this temperature mercury won't evaporate.

Then on the contrary how do they keep it from freezing?

By heating it

If the phase diagram permits it...

They need to find a solid that does not just sublimates when heated in the void.

Won't it still boil due to the near-zero pressure?

Depends how cold you can keep it. Even if it doesn't boil it could sublimate away if not kept extremely cold.

Permanently shaded regions don't get much below 25K, maybe up to 70K. http://lroc.sese.asu.edu/posts/979 Still good enough for mercury of course.

If its highly reflective maybe it won't heat up much in sunlight.

It can't be in sunlight, or the telescope would be useless.

The problem would be more in choosing a material that has low enough melting point so that making up for the radiative loss to keep it melted wouldn't be a big issue and at the same time vapor losses can be managed. Which is why they propose separate materials for the mirror body and its surface I guess.

Why would it be useless? I'm sure dark is preferable but there isn't an atmosphere on the moon and most of light should reflect away from the sensor.

Of course it should probably never point at the sun unless they want to melt their sensors.

No mirror is perfect, it scatters some light anyway. Just a tiny bit of scattered sunlight will drown anything they're trying to see.

Could it be a liquid like mercury? Would that evaporate?

Mercury does still have a vapor pressure and evaporates at room temperature so it could evaporate some but it would be in the shade so it would need warming to remain liquid more than cooling so the evaporation rate could be controlled and would probably be very very slow around it's freezing point.

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