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Should there be a visible steam plume from the Apollo astronauts sublimation cooler in the lunar EVA videos?

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    $\begingroup$ Look around you and you won't see water in the air even though there's plenty of it. You see clouds because water forms drops big enough to scatter light. $\endgroup$
    – GdD
    Commented Jul 11, 2017 at 8:18
  • $\begingroup$ Pure water steam is invisible. If there are tiny droplets of liquid water within the steam, you see the mist caused by the droplets, but you don't see the water vapour itself. $\endgroup$
    – Uwe
    Commented Jul 11, 2017 at 9:50

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Wikipedia says:

As an example, during the Apollo 12 commander's first EVA (of 3 hrs, 44 minutes), 4.75 lb of feedwater were sublimated, and this dissipated 894.4 Btu / hr.

That's 2.154 kg of water over 224 minutes, or 10g/min, or 0.16g/s.

Water vapor is invisible to the naked eye (2, 3). What you see e.g. when boiling water is not the water vapor itself, it's small droplets of liquid water (fog, mist, steam) which are suspended in the air and carried by the air flow.

With only so little mass escaping the suits, those droplets do not form in any substantial amount, and there is no air that could carry them around. Thus, there is nothing to see.

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    $\begingroup$ 0.16g/s is about three small drops (a drop, as used in medicine, for eye droppers or stomach drops is 0.05ml). Drip three drops per second on a hot stove and see if you can spot the vapor... $\endgroup$
    – SF.
    Commented Jul 11, 2017 at 11:45
  • $\begingroup$ Makes sense to me. I was under the impression that steam would immediately condense in the vacuum of space and freeze. I was thinking crystals should form and fall to the ground. $\endgroup$
    – Tim Finch
    Commented Jul 11, 2017 at 23:51
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    $\begingroup$ When the astronauts take a leak while on a mission and expel the result into space, it boils violently. The vapor then passes immediately into the solid state (a process known as desublimation), and you end up with a cloud of very fine crystals of frozen urine. $\endgroup$
    – Tim Finch
    Commented Jul 12, 2017 at 0:21
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    $\begingroup$ @TimFinch: Vacuum of space is 0.[very little]. Atmosphere at sea level is ~1 bar. There's no such thing as negative pressure. There's a scale that's offset by 1 bar (e.g. Psi-guage) which is used in technology, but rarely useful in space applications. $\endgroup$
    – SF.
    Commented Jul 16, 2017 at 18:37
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    $\begingroup$ @TimFinch The tiny holes of the heat exchanger are filled with ice, but by sublimation this ice turns to vapour. If all the ice of one hole is sublimated, a little drop of water fills the hole and turns toice again. The heat exchanger plate is not useless when all holes are filled with ice, that is the normal operation to avoid loss of too much water. $\endgroup$
    – Uwe
    Commented Jul 17, 2017 at 14:36

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