# Where would a technologically advanced group of people forced into space by a calamity obtain water from?

The fleet in my setting are forced to leave the planet due to a volatile virus, and leave in pre-prepared spacefaring vessels capable of transporting ten million people per ship. The ships have algaculture-based farms for sustenance, but food isn't nearly as important as water. Water can be recycled from urine, but urine can only be recycled so many times before it loses all of its benefits. Are there chunks of ice in space that the ships could collect and use for water? The ships could come with a massive tank of water to keep everyone hydrated for a lengthy amount of time, but it obviously wouldn't fully be recyclable.

EDIT: Can urine be infinitely recycled for water? I was under the assumption that less water content and more waste content would be made each time urine is recycled.

The ships are equipped for travel through both interstellar regions, and within solar systems. The main goal is to find another planet with permanent living conditions that suit their needs. Another Earth-like planet if you will. There's obviously not another Earth-like planet anywhere near our solar system, so these ships are going to be long term living conditions until another Earth is found in a far away galaxy.

• "before it loses all of its benefits" - can you elaborate, please? The water must be lost to space to become truly unavailable. – Alexander Apr 30 at 22:20
• Everybody's favorite source of water in space: Reclaimed From Sewage! – user535733 May 1 at 1:34
• @user535733 Reclaimed From Sewage! works for the water on Earth too. The reclamation is a bit more sophisticated, using the heat from a fusion reactor, huge evaporation ponds, perhaps cryogenic storage, but sewage reclamation it is. Ever since they invented life. – Adrian Colomitchi May 1 at 4:32
• To answer the edit: Yes, you can infinitely recycle water. When a desalination plant in a city makes pure water, it's doing it with water that is billions of years old and was pissed out by dinosaurs. So long as you have power, you can make pure water. – Sir Adelaide May 1 at 7:59
• @SirAdelaide in theory you can, however in practice some is lost to stray chemical combination. – John May 1 at 21:31

As long as the system is closed, it's impossible for matter within the system to be lost. That's the law of conservation of mass - mass cannot be destroyed. So as long as the people in the ship don't do anything stupid like jettison water out of the ship, they'll always have as much water as they started out with. (Well, as much hydrogen and oxygen as they started out with.)

You are right that there will be problems, and I'll address the two problems and solutions here. The first is that not all the water a human drinks is lost through urine. There's a fair amount of it lost through breathing, sweating, or solid waste. But that can all be recovered in various ways as long as the system is closed, i.e. extracting pure water from the waste and using a dehumidifier on the air.

The second problem is the second law of thermodynamics, which states that entropy is increased through every exchange of energy. Meaning that while all the mass on the ship stays the same, the entropy inevitably increases, and that's bad - that's the 'loss of benefits'. Water with increased entropy is hydrogen and oxygen, after all. The crew will need someway to get more energy to combat the entropy increase - i.e., when stuff breaks down, they need power to fix it. That can be done simply by using energy sources, i.e. solar panels, or just have a really capable nuclear reactor on board the ship. Nuclear reactors have insanely high levels of energy and they should be good enough for the generation ship to reach the next planet, assuming there's enough fuel.

• Also worth pointing out that 100% closed systems are really really hard to keep going. Even if you assume your hull and doors are perfect (and they likely won’t be) you still have to worry about somehow reclaiming water from chemical reactions (corrosion is not your friend here) and keeping all the various parts of the system in perfect equilibrium even when presented with unexpected problems or unforeseen flaws. Just have a look at biosphere 2 for an example of best laid plans needing extra injections of raw material to keep going. – Joe Bloggs May 1 at 8:59
• Uh... seeing as hydrogen burns, isn't water the higher entropy state? – Matthew May 1 at 15:16
• @Matthew No. Gases will almost always have more entropy than solids, not to mention that the energy potential within the bonds of water are higher than the energy from hydrogen and oxygen gas, even though it's not as accessible. Checking to see whether or not something burns is a crude measure of determining entropy, and isn't always right. – Halfthawed May 1 at 16:01
• I've been playing a lot of Oxygen Not Included lately. The only time you (should) lose water is when you crack it into oxygen and hydrogen, to breathe and burn for fuel. - I'm not sure how it relates to entropy, but the deathknell in the late game (as you travel through space living/burrowing inside an asteroid) is the inability to disperse heat in that 'closed system'. – Mazura May 2 at 1:02
• You can also lose water to various chemical and biological processes. For example, there will be some water in the mildew that will form in high-humidity areas. – NomadMaker May 2 at 2:24

Comets. Comets have been described as floating balls of dirty ice, and mining comets for water has actually been proposed for space travel and has been used as a source of water and ice in science fiction for decades (the Planet Express crew trying to mine Halley's Comet in Futurama comes to mind).

• not just comets plenty of asteroids are ice balls too. – John May 1 at 21:33

"Are there chunks of ice in space that the ships could collect and use for water?"

Saturn and gas giants like it:

With an estimated local thickness of as little as 10 m and as much as 1 km, they are composed of 99.9% pure water ice with a smattering of impurities that may include tholins or silicates. The main rings are primarily composed of particles ranging in size from 1 cm to 10 m. - Wikipedia

That's about half as much ice as the Antarctic ice shelf.

There are numerous other sources of ice, but contamination by volatiles and such varies.

• That's only useful when you're traveling inside of a solar system. There's no ice to collect if you're traveling in the void between stars. – Halfthawed Apr 30 at 23:16
• @Halfthawed To be fair, we don't know that. – Asteroids With Wings May 1 at 15:50

Bring much more water than you need for drinking. It is handy stuff.

Water is good if you are thirsty. It is good for lots of other things too. Your algae will use it to turn CO2 into carbohydrate. It is good radiation shielding. It is good micrometeorite shielding. You can store energy with it, splitting the hydrogen and oxygen and recombining them. You can use it as reaction mass, hurling it behind you at speed to propel your ship. You can use it offensively, hurling ice chunks ahead of you to clear a path.

Of all the stuff to bring a lot of, water is the most useful one.

In addition to all the other answers, do also note that you don't need to necessarily obtain water, all you need is hydrogen and oxygen and you can create water.

Hydrogen is the most common element in the universe, but most of it is inconveniently in stars, gas giants or spread tenuously in open space. However hydrogen containing gasses could be obtained from places like the atmosphere of Titan. Some planets contain minerals that include hydrogen as well (generally wetter ones like Mars), as do some asteroids. If truly desperate you can also collect it from the solar wind and the interplanetary or even interstellar dust.

Oxygen is less common, but probably more accessible. Pretty much all rock contains significant amount of oxygen as part of its chemical makeup. Lunar regolith for instance is 42% oxygen.

In a closed recycling system you are more likely to be losing oxygen, since it is the more reactive chemical and is liable to become bound in various oxides that no longer participate in the cycle. So it is convenient that it is the more easily replaceable of the two. (And also the reason why almost everything contains oxygen in the first place.)

• water is actually one of the most common substances in the universe. It is the most common compound. Water is easily the most common source of oxygen in the universe. – John May 2 at 4:10

The "best" plan (imho) would be for the ship(s) to head to a moon (like Europa); mine enough ice/water to fill the ship's tanks; then head out towards the target solar system.

Europa (a moon of Jupiter) is thought to have layer of water/ice ~100KM deep.

Once the ship(s) leave the inner solar system they could perhaps collect ice (water) from the Oort Cloud which extends far beyond the orbit of the planets. The problem would be "collecting" the material - given the velocity the ships would be travelling at. Maybe a structure looking like a "solar sail" could be adapted to funnel material to the ship. But - to be honest - I can't see how it could be made strong enough.

Most likely other planets. It might not be water, but if it's a sci-fi explanation you're looking for there could be another liquid which we know for certain doesn't harm humans due to extensive science.