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Water in the vacuum of space


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We've all seen water out in to a vacuum chamber and seen it begin boil. People therefor have got the idea that the same happens when you throw it out in space. But I don't believe that's the case.

It's true that any liquid's evaporation point lowers with the surrounding pressure. But in order for a liquid to vaporize it needs energy from somewhere. When there's no air the water can't get any energy from the surroundings (We say that there's no radiation directed at it). But it has to vaporize and the only source of energy it has is it self. Therefor it pulls energy from itself which will cause it to freeze, while it vaporize.

So we have a situation where the water will vaporize and freeze at the same time. But I can't figure out what the water would look like afterwards. My current theory is that it would be something like snow but I'm not sure.

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i believe you are correct. After an initial 'burst' of vapour, you'd be left with a solidish mass, probably passingly similar to snow or slush.

Over time, this would sublimate into the vacuum around it as it absorbed energy from radiative heat etc.

The question i'd like to see answered is "would thermite, or similar metal/oxide fires burn in vacuum"?

Edited by kahlzun
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Depends on where you are. Outside the frostline it'll end up frozen solid. Inside it will start to sublimate.

Metal fires in space: Yes, they do. That's how solid rocket motors work, the fuel in an SRB is not all too different from thermite.

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Depends whether it's in sunlight or not. In direct sunlight there's plenty of incident energy to vaporise it, at least in earth orbit. In shade Kahlzun is right, there would be a burst of vapour as the water loses most of the energy it had when it was released into vacuum and then it would freeze. This is why there is ice in the permanently shaded parts of craters on the moon, but on the exposed surface it can only exist beneath the surface layer or else the sun would vaporise it.

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The question i'd like to see answered is "would thermite, or similar metal/oxide fires burn in vacuum"?

I don't see why not, at least those that don't require an external source of oxydiser outside what's provided for by the reaction.

That's after all the way rocket engines work, and we know those work rather well in a vacuum :)

Now the thermite reaction fits the bill: Fe2O3 + 2 Al → 2 Fe + Al2O3

Remains the problem of keeping the materials in contact so the reaction can take place, but that's an engineering problem.

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In vacuum, there's nothing to keep the faster particles in water to jump out and deplete the amount of energy in it. After initial boiling, you'd be left with a porous lump of ice. If that ice is always in darkness (permanent shadow in crater etc.), it will stay very cold.

Ice close to zero degrees C sublimes at a considerable level. That's how we freeze dry foods.

Thermite mixtures would work, but you wouldn't see any flame as you need a sufficiently dense gas pressing onto it to develop dancing, glowing gas, whether the gas reacts with it or not. You'd just see glowing and the container holding the vacuum would be peppered by molten iron. Probably a similar sight to ion engines, but not bluish.

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It's true that any liquid's evaporation point lowers with the surrounding pressure.

No. It's not true. The boiling point lowers. Boiling point is the temperature at which the vapor pressure is greater than ambient. Vaporization happens at any temperature. Water at room temperature evaporates. It just does so more slowly. And as it evaporates, it cools down. Unless humidity in the room is 100%, water left on its own will be at lower temperature than the air in the room.

You are correct in predicting that water will cool to the freezing point. But it will not stop evaporating then, either. It will continue to evaporate even when frozen. So the correct sequence of events for a blob of water that got accidentally released into the vacuum of space is that it will first begin to violently boil, breaking down into many tiny droplets. These droplets will cool to freezing point and freeze. The result will not be quite like snow, because freezing will be very fast. It will be more of a frozen mist. That frozen mist will then, over time, evaporate completely.

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