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The Background:

I am going to be asking a series of questions that will be relevant to forming some sort of a picture of human space commerce.

Let's say that Earth-based human civilizations have discovered a series of ancient jump-gates that allow them to travel within a large and varied interstellar network.

There are not many clues, apart from the jumpgates, as to who left this system behind. For the moment, I am assuming that there is no bias to the kind of systems included in the network: i.e. its not like systems with earth like planets make up the majority of the planets in the network. So, "system types" have roughly the same probability of occurrence as if one were just taking a cross-section of space and scanning it.

Put another way, the gates simply connect a large number of close-by star systems, rather than a large number of only useful star systems.

While genetically-engineered humans exist in this "universe", no sentient non-human aliens have yet been encountered.

The Question:

How easy would it be to find potable water in space? Can passing-by ships simply "mine" ice comets for water (take off chunks of ice and melt it, et voila!)? Or, is it that while water is common enough to find, infrastructure would be needed to separate it from other stuff it might be found with, making it usable for humans?

I ask in order to determine if travelling through space is akin to travelling through deserts: few sources of potable water, and "caravan rest stop" like structures are an important, if not absolutely necessary, piece of infrastructure.

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There are sources such as comets, but of course they are reasonably dangerous to approach. Though having some kind of scoop might work to follow a comet and 'catch its tail'.

Though water is very recycle able and while some will be lost into space through different means a 'refuel' of water shouldn't be an extremely common need. More common need might be to scoop up hydrogen and other elements to use as a propellant for the ship. catch in the front and send out the back at high speeds.

But as far as water, it's water and a good filtration system can purify it. Even if you can't find H2O if you can find Oxygen you are golden, since Hydrogen is everywhere especially around gas giants. Our solar system has water on the Moon, Mars, Europa and comets in large enough quantities that we can detect it from here. So barring any accidents for emergencies between care use and recycling a ship shouldn't have an imminent need, though being careful, topping off and knowing where the water sources are on any trip/route is still a very good idea.

EDT: After reading one of the other posts, it turns out that Oxygen is the 3rd most common element in the universe, and large stars can have it in the solar winds. So it could be scooped up with hydrogen while traveling interplanetary in a solar system. Though I think finding a more concentrated source would be more useful.

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    $\begingroup$ Yeah, there's a lot of Water Ice in the solar system. Rings, asteroids, comets, moons, etc. Finding Water Ice isn't hard, purifying it might take some work though as it could be mixed with anything. $\endgroup$
    – Tim B
    Feb 20, 2015 at 9:23
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You don't spend water

Well, technically you can (e.g. if using it for propellant somehow); but normal biological processes - human life support, farming and everything else only cycles a fixed amount of water. In a closed system, you don't generally need new supplies of water - you recycle the existing one.

Cleaning up the water already in your system is cheaper and simpler than cleaning up water "found" on another space object and you don't have to go and fetch it. So the major source of potable water is the water that was potable yesterday.

You'd need new large sources of water only if you expect your colony (and thus your biomass) to grow, but sustaining an existing ship or colony can be done without such a source for a very, very long time. Essentially, you only need to replace water that you lost in some accidents or damage to your ship. Cleaning up your existing water is technologically simple and the major requirement is only a reliable source of energy.

A prime example is our existing spaceships. Even in the current ISS, there's no chance we're bringing in new water all the time - the water that astronauts drink is recycled.

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  • $\begingroup$ Water cannot simply be "recycled" at 100% efficiency. Some is lost to the atmosphere, and some goes into organisms - and is not released until death. If you just "recycle" your existing water you will slowly lose it as your plants and people consume it, or it is lost to the air, or it is lost in pipes and drains, or spilled on the floor, or contaminated - until yes, you have the same amount of water - but it is not usable. $\endgroup$
    – Zxyrra
    Nov 4, 2016 at 20:44
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    $\begingroup$ @Zxyrra water "lost to the atmosphere" and water spilled in on the floor (and eventually evaporated to the air) is a prime source of recycled water - as in the real example of ISS, where water is scrubbed from the air. That's the whole point - assuming that your colony doesn't grow and you have the same weight of people as you did last year, your plants and people don't consume any water. The water absorbed in plants is eaten by people; the exact same water is then sweat out and urinated by people, and the exact same water is recycled and used to drink and water plants, again, forever. $\endgroup$
    – Peteris
    Nov 4, 2016 at 21:31
  • $\begingroup$ That helps clarify, thanks. & I never intended to say it is "consumed" as matter is neither created nor destroyed - but if the population wants to increase, which while not necessary may be wanted, an increasing amount of water will be trapped in human bodies pre-decomosition; water needs will rise over time that one stockpile may not match $\endgroup$
    – Zxyrra
    Nov 5, 2016 at 0:39
  • $\begingroup$ @Zxyrra growing needs with growing colony was mention. Any rock is almost 40% oxygen by mass, jupiter is ~90% made of hydrogen - so there no problems to get water as much as you need - it will cost energy but why not. $\endgroup$
    – MolbOrg
    Nov 5, 2016 at 1:34
  • $\begingroup$ @Peteris - To comment on your ISS example, they are definitely less than 95% efficient. As mentioned here IF the ISS was able to get to 95% efficiency they think the water found in food would be sufficient to allow them to stop shipping in water itself. Yet they do continue to send up water. I don't know how much, but it's on every cargo list. It is very unrealistic to assume 100% efficiency in any system, or even very nearly 100% efficiency. $\endgroup$
    – GrinningX
    Nov 5, 2016 at 20:24
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Good primary concern, water. I'll assume that the size of the opening in your jump-gates limits the size of the ships which pass through them. That rules out our hauling massive quantities of water along with us and leaves us at the mercy of water we find along the way.

My big concern would be radiation. We can probably filter out most of the material impurities from melted space-ice but if it glows in the dark, all bets are off. At least we would be able to determine that the water is not pure and pitch it back into space, but it would be a little like sea-water. Once your crew is thristy enough, they will drink it anyway.

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  • $\begingroup$ That's a good point, that I should clarify: "I'll assume that the size of the opening in your jump-gates limits the size of the ships which pass through them." Assume the opposite: there are other concerns that determine ship size, but not jumpgates. $\endgroup$
    – bzm3r
    Feb 20, 2015 at 4:18
  • $\begingroup$ I didn't think of water being contaminated with radioactive particles, so +1 for that. $\endgroup$
    – bzm3r
    Feb 20, 2015 at 4:22
  • $\begingroup$ @user89: It would not be that difficult to chemically purify water (distillation comes to mind). Only if the contaminant is e.g. super-heavy water(i.e. di-tritium-oxide) you won't have a chance, since it is chemically equivalent to water. But since tritium has a half-time of about 12 years, there won't be much of it in space. Another concern is that distillated water is detrimental to your health if consumed in large quantities. $\endgroup$
    – M.Herzkamp
    Feb 20, 2015 at 12:17
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    $\begingroup$ Couldn't you "undistill" it by adding trace elements back in? $\endgroup$
    – Henry Taylor
    Feb 20, 2015 at 12:39
  • $\begingroup$ Radiation mostly isn't a problem: from a chemical perspective, there's no real difference between filtering out something like dissolved uranium and filtering out dissolved arsenic or lead. $\endgroup$
    – Mark
    Nov 5, 2016 at 0:52
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Remember that space travel happens at extremely high velocities, and that reaction-based drives consume a huge amount of fuel. Most spacecraft will carry just enough fuel to (1) accelerate at the start of the journey to a very high speed, and (2) decelerate at the end of the journey so they don't crash.

Unless your ships are using some sort of reactionless drive, they're moving way too fast past those asteroids, and it's just not worth it to expend the fuel to match velocities with the asteroid and mine water. It's much cheaper to just bring the water with you.

Fortunately, as another answer pointed out, you don't really expend water during space travel, so it's not likely that you'll run out.

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Limiting factors to shipbuilding generally are its size and weight. Now, with clever piloting, a small ship could dock to a larger asteroid and turn that chunk of ice into a large part of its hull, basically everything but engines and lie support being mined out of the thing. underway, this gives our ship a surplus amount of water to be used as reaction mass for the engines, but it does also increase size and weight a lot.

Removing impurities from the ice is pretty simple with physical and chemical processes, and water itself has a very low chance of being radioactive (Deuterium, Tritium and the radioactive oxygen isotopes are not very stable), so cleaned water from space ice would be pretty safe for consumption, once some salts are put back in - which in turn can be processed out of the human excrements. If you want to go the extra mile: split the water to hydrogen and oxygen and just burn it to get water vapor and heat (maybe even use this method in the heating of the ship? Less interaction with possibly radioactive materials if you just take the electric power from the generator and generate a fuel for the heating with it). And most of the water mined from the asteroid will go straight to the engines anyway, so cleaning and splitting it up is just a step to keep the engines running smoothly.

But if you want an equivalent: As long as you manage to grab an asteroid close to your starting point, you can travel for possibly a hundred years without ever running out of water, even if ou use it up at a rate of 1% per year as fuel while loss due to other reasons is neglectable (which is kind of an estimate for generation ships).

If you don't need water to turn it into fuel/reaction mass, you don't really need to take that much water with you, and it would be more akin to a modern cruise ship: make sure you pack enough so your water cleaning plants can stay online all the time and you still have an overhead in fresh water.

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  • $\begingroup$ Feel Perpetual motion type 1 engine in this one; the dark side of the Force are they. $\endgroup$
    – MolbOrg
    Nov 5, 2016 at 12:21

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