Water in Space: What Happens?

The Earth is one of those extremely rare, special places in the Universe where water can exist, stably, as a liquid. So much of it exists here on Earth, that if you were to add up all the oceans on Earth together, it would weigh more than 10^18 tonnes, more massive than the biggest asteroid ever, and about as massive as Pluto’s giant moon, Charon.

But water only has a very small window in which it can be a liquid. For instance, if you took some warm water up to a very high elevation, it would start to boil, and become a gas! The higher up you took it, the lower and lower your boiling point would be.

Why? Because higher altitudes on Earth mean lower pressure. If there’s not enough force pressing the water into a liquid phase, then there’s no force binding the water molecules together. If you simply allow them to diffuse away, they will. And that’s the definition of a gas, which is what you’ll wind up with.

On the other hand, water has no business being a liquid at low temperatures, either. You can see — from this diagram below — that if you start with liquid water, you can turn it into a gas by lowering the pressure, but you can also turn it into a solid by lowering the temperature.

So my question is this:

If you took a glass of water into outer space, would the water freeze or would the water boil?

This is a question that seems awfully tough, because in addition to knowing about water:

We also need to know about outer space. Space is a lot of things: cold, dark, and empty come to mind right away. And they come to mind, pretty much, as soon as you leave the Earth.

Well, the temperature of space is, at its coldest, just the temperature of the leftover glow from the Big Bang. This radiation, known as the Cosmic Microwave Background, bathes the entire Universe in a temperature of only 2.7 Kelvin. That’s less than 3 degrees above absolute zero, or -455 degrees Fahrenheit! But there’s also — literally — no pressure in space. So, what happens? Who wins? Does the water freeze or boil?

Oddly enough, the answer is first one, and then the other! It turns out that having a pressure vacuum will cause the water to boil almost instantly. In other words, the effect of boiling is much, much faster than the effect of freezing.

But the story doesn’t end there. Once the water has boiled, we now have some isolated water molecules in a gaseous state, but a very, very cold environment! These tiny water vapor droplets now immediately freeze (or, technically, desublimate), and become ice crystals.

We’ve observed this before. According to astronaut observations, where they’ve observed their urine get expelled from the ship:

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.

Sounds like it would be a fantastic thing to watch, doesn’t it? Well, we’ve done almost the same thing on Earth. What happens if you take boiling water and, on a very, very cold day, throw it up into the air?

The water finishes boiling and becomes a gas, the gas freezes (or desublimates), and ice crystals — a.k.a. snow — results! And that’s what happens to water in space. Oh, to be able to try this at home…


  1. #1 rfguy
    June 29, 2009

    I have tried this at home! But then, I do live in Canada…

  2. #2 Jamie
    June 29, 2009

    That was brilliant! :)

  3. #3 Mu
    June 29, 2009

    For a practical application of Fig 2, move to Los Alamos, NM. At 92 C, you boil spaghetti al dente in 20 min, and your potatoes take an hour plus.

  4. #4 Clinto
    June 29, 2009

    We used to do this as kids growing up in MN. Science, sometimes, is so much like magic :)

  5. #5 Sili
    June 29, 2009

    Why not get a pressure cooker, then?

  6. #6 MadScientist
    June 29, 2009

    That snowflake looks disturbingly familiar. Did you get permission from the photographer? If it’s who I think it is, I’m sure he’ll be happy to let you use the photo (and you can put a link to his web page where many high quality photos can be found).

    @Mu: A good quality pressure cooker with a pressure adjustment works very well – if you can find it. You can just crank down the pressure setting so that water boils around the usual sea level boiling point. I’d take a normal pressure cooker and modify it, but I’m not saying anything because I wouldn’t want to be sued if something goes wrong. :P

  7. #7 Ethan Siegel
    June 29, 2009


    Nope, don’t know the photographer; just took it from google images. If you know the guy, send me his site info (and/or a link to his photo) and I’ll link to it in the article.

    As for the snow/boiling water trick, it seems that about -40 degrees is where you want to be for this stunt to work. I should’ve tried it the one winter I spent in Madison, WI…

  8. #8 Jeff Mitchell
    June 29, 2009


    Check out this old straight dope, which happened to recently be reposted to the front page of straightdope.com

  9. #9 Mu
    June 30, 2009

    As for the pressure cooker suggestion, it’s the perfect way to produce mashed spaghetti and al dente potatoes, due to problems of getting the timing just right. Also, I found starches in certain kind of pressure cookers quite dangerous, since the foam gets into the valve mechanism and makes them sticky.
    For a funny anecdote, when I first moved their I grabbed a pack of cake mix and looked at the “high altitude” instructions, which were for 3000 – 6000 ft. I was baffled what to do at 7500.

  10. #10 Jodie
    June 30, 2009


    You write the coolest blog. I learn something new almost every time I read it. I’ve always wondered what happened first in space – freezing or boiling.

    I should have guessed, because every material has a thermal capacity, i.e. an ability to retain heat. So it stands to reason that it would respond first to the instantaneous drop in pressure, and then as it gives up its heat more slowly, drop in temperature to the freezing point. Makes sense, but I never stopped to think about it. I thought the “Urean Nebula” just went straight to ice, and I wondered why it didn’t boil.

    Answer: It did!

  11. #11 Cory
    July 6, 2009

    Forgive my ignorance – where does the heat in the liquid go that quickly?

  12. #12 Mu
    July 6, 2009

    The heat gets lost during evaporation and consequent expansion of the resulting gas into the vacuum.

  13. #13 Chunky soup
    September 3, 2009

    thats not a snow flake …its a pee flake….

  14. #14 finesa
    September 8, 2009

    hei..would you like to teach me how to make 2nd diagram (liquid,gas and solid)??
    reply me pls.thanks

  15. #15 jeux import uk
    September 8, 2010

    can u say me where does the heat in the liquid go quicly ?

    Thanks i haven’ understand

  16. #16 Jenny
    September 20, 2010

    Enjoyed the post. I was curious as to the final droplet size distribution. I assume that once they are fully cooled, sublimation drops to a very very low rate due to the extremely cold temperatures. My interest: contaminant movement in space and possible incursion into space shuttle bay.

  17. #17 daymond bush
    September 28, 2010

    that help me alot but Seriously it helps cuse my teacher ask me so i finely have the answer…tnx¡

  18. #18 William Waterway
    September 10, 2011

    • This article makes me wonder as to how water first came into existence, while helping to further educate about water in the our galaxy and universe.
    On the same subject, we have the following quotes from the award-winning philosophical, spiritual, science exploration book published in 2001 called, “The Holy Order of Water, Healing Earth’s Waters and Ourselves”:
    “Cosmic clouds containing water were recently discovered rotating around black holes that are suspected to be near the center of our universe. These cosmic clouds of water indicate the creation and distribution of water throughout our universe at an early age.
    • One water distribution method we find throughout universe occurs through comets. Water is one of the major constituents of comets – since they are made up of about 90% water in the form of ice mixed with dust. “When Halley’s Comet last visited our solar system in 1985, scientists reported that ‘the comet’s dominant constituent is water ice, and that much of the tenuous 360,000-mile-wide cloud surrounding it consisted of water vapor.’ At one point, it was estimated that the comet was losing up to 70 tons of water a minute!”

  19. #19 Nishan Paudel
    September 11, 2012

    I really mean what would happen if water is heated in space. If it turns into gas how?, If stays in same position how,? water must change into gas on heating but to it goes with moving up and down. But in space this process does not take place as there is no gravity.

  20. #20 Mark Nunes
    Visalia CA
    November 12, 2012

    Water turns into crystals in space. crystals are snow. Snow sticks to snow. I’m wondering if this could be a clue to how planets form in the solar system. There seems to be plenty of water in space. Especially on planets and moons. Could the mystery of how planets form be solved by the fact that snow is sticky? Crystals smashing into one another forming snowballs. The snow balls grow and grow till they have enough gravity to attract dust. Anyway I’ve heard it said that people don’t seem to know how planets are able to form as gravity is not enough to cause dust to clump together.

  21. #21 prathap
    sounth india,india
    March 29, 2013

    i am telling that in under the earth there is no water, because h2-hydrogen is only available and wheno-oxygen enter then its changed into water=h2o.

  22. #22 bob
    Pacific Ocean
    July 9, 2013

    yolo you only live once

  23. #23 biellakah
    July 21, 2013

    ohh .very tragic to know bout water

  24. #24 russell Bamberger
    August 20, 2013

    I can understand water boiling in space but I dont understand the freezing part. How does the liquid pass its energy in a vacuum when there are no adjoining particles ?

  25. #25 russell Bamberger
    August 20, 2013

    Ok I will also speculate that what the astronauts observed was not frozen Urine, it was a liquid crystalline formation that appeared to be solid.

  26. #26 paul
    August 24, 2013

    What effect if any would viscosity have on freezing/atmospheric pressure of water-based liquids? In orbit if an astronaut opened a squeeze bottle of honey, would the honey boil out then crystallize in a fine cloud?

    If you had a squeeze bottle of salt water and exposed it to space, would the minerals from the salt water be left at the bottom of the bottle? Like seeing sea salt and stuff?

  27. #27 Jim Hinkey
    aransas pass Texas
    September 28, 2013

    If this is what happens to water in space, how are comets explained then? Aren’t they mostly ice? How is this possible?

  28. #28 Sean T
    September 30, 2013

    Jim Hinkey,

    Did you read the article? Water in space first boils, then freezes. Thus, it ends up as ice, just like the ice that makes up the comets.

  29. #29 Sean T
    September 30, 2013

    Russell Bamberger,

    Freezing (technically desublimation, but I’ll stick to freezing here) requires that there be a heat transfer from the water vapor that is doing the freezing to the surroundings. No heat transfer, no freezing (otherwise the second law of thermodynamics is violated). This can be accomplished by radiation, namely by emission of infrared photons. This certainly can occur without any surrounding medium.

  30. #30 Sbeatty
    December 3, 2013

    I have two say amazing:):):):):):):):):):):):):):);););)@ya

  31. #31 Sbeatty
    December 3, 2013

    Do you have any other science project for my son,he’s a teenager so it’s hared getting a good science project.

  32. #32 Sweaty
    Apple valley
    December 3, 2013

    No I don’t have any more science projects

  33. #33 jade
    January 3, 2014

    If people are worried about water levels rising because of global warming why don’t we bring water to outer space so our land won’t be compleatly covered.. Maybe I sound stupid saying it. Yes it will turn to gas then freeze but if scientest find a container that can hold the molecules in place that might work. Just a thought

  34. #34 mahesh chandan
    July 8, 2014

    My question- hypothetically if there wasn’t any external conditions like pressure temperature and gravity affecting waterm then will a droplets of water remain static in vacuum? I personally think it wouldn’t stay static because of its internal energy and would be floating around this leads to a point that no matter in the universe stays static and is trying to spend its energy in a way ..if im wrong pls do correct me..”prmahesh007@gmail.com

  35. #35 Pratik
    October 13, 2014

    Thanks for an elaborate explanation.

  36. #36 Shawn Waida
    November 19, 2014

    Hello, does anyone know, in relation to the graph on here, what the sp, mp, and nbp mean? I was thinking melting point and such, but then nbp doesn’t make sense for me, although the sp is an arguable point. Hence my confusion.

  37. #37 ankit
    February 8, 2015

    It was very useful and interesting.

  38. #38 Icon21
    March 25, 2015

    So my question is as a refrigeration engineer, if we have standing water in a closed circuit and place this circuit under a vacuum of say 500 microns, will the water just boil away rapidly or will it eventually turn to ice? If we based my question on the pipe work and surrounding air of around 5c?

  39. #39 Wow
    March 26, 2015

    As a refrigeration engineer, you should either be able to answer that question yourself (in the case the qualification is relevant enough to merit display) or it wasn’t really germane to your query.

    First, what do you mean by “under a vacuum of 500 microns”? mmHg? Secondly, absent knowing that, you’ve made it stable at 5C, so looking at the phase diagram of H2O answers that.