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Everyone always says that there has always been, and always will be, the same amount of water on planet Earth.

What about space travel?

They obviously need water in space, so they must take it up with them. Take the International Space Station: astronauts must survive up there for around 6 months. And yes, I know they have ways to recycle it, but it must eventually be "refreshed".

I know that it makes the only teeniest, smallest of differences (a few hundred gallons taken from billions of gallons), but could that phrase be proved wrong?

Thanks in advance!

Note: I put this on the Earth Science SE because I couldn't find anywhere else it would fit.

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    $\begingroup$ "Everyone always says that there has always been, and always will be, the same amount of water on planet Earth." -- it would be helpful if you could cite some sources for this incorrect claim. $\endgroup$
    – Pont
    Commented Jul 30, 2016 at 10:01
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    $\begingroup$ It's not actually correct to say "they must take water with them", there's lots of water in space, so if space exploration every grows, there's water in lots of places. There's glaciers on Mars. There's small pockets of ice in shaded craters on the moon. There's ice in comets, there's even ice in shaded parts of Mercury that never get direct sunlight. If space travel ever gets really extensive, it would make more sense to collect ice from space rather than to keep launching it from Earth. $\endgroup$
    – userLTK
    Commented Aug 3, 2016 at 9:40
  • $\begingroup$ @userLTK What I meant was the International Space Station. I don't think that they would "mine" for water on Mars. (Although that would be really awesome) $\endgroup$
    – tycrek
    Commented Aug 5, 2016 at 3:38

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Everyone always says that there has always been, and always will be, the same amount of water on planet Earth.

Your premise is wrong. The surface and atmosphere of the Earth keeps gaining and losing water all the time. I have no idea where is that coming from, but no scientists say the amount of water is constant.

  • The Earth's atmosphere is leaking gas to space. This includes mostly hydrogen gas, which is a major component of water, and also some minor amounts of molecular water.
  • Earth is gaining water (and a bunch of other stuff) from meteorites and dust hitting the Earth. Dust accounts for the majority of the mass flux to the Earth.
  • The Earth's surface and atmosphere are getting huge amount of water from volcanic eruptions. H2O is the major component of volcanic gas in most volcanic eruptions.
  • Earth is losing water to the mantle in subduction zones and in lithified sediments.

All of these water mass fluxes are orders of magnitude larger than the amount of water carried to the international space station. It's like asking whether you can deplete a beach from sand by taking single sand grains out of it, when you're gaining and lose huge amounts of sand by ocean wave action.

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    $\begingroup$ Helium is also the next-most lost gas, and unlike Hydrogen we can't replenish it. Eventually earth will lose its Helium, whereas with Hydrogen we can crack open molecules like water to get more. $\endgroup$
    – Logan
    Commented Aug 1, 2016 at 14:15
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    $\begingroup$ Actually, the Earth produce Helium, slowly, by radioactive decay. Most of Earth's Helium is trapped underground so we'll never lose "all of it", but there is currently a helium shortage that likely won't end anytime soon. It's not worth the trouble to drill for helium directly. It's mostly collected out of natural gas deposits. $\endgroup$
    – userLTK
    Commented Aug 3, 2016 at 9:36
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    $\begingroup$ @userLTK ox.ac.uk/news/2016-06-28-huge-helium-discovery-life-saving-find related $\endgroup$
    – Gimelist
    Commented Aug 3, 2016 at 9:40
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    $\begingroup$ @Michael That's awesome. I wasn't aware of that. $\endgroup$
    – userLTK
    Commented Aug 3, 2016 at 9:45
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Consider: If they evacuate the left-over waste product into space, as the station is NOT in geosynchronous orbit, the waste will eventually come back to Earth. Obviously, before then, the actual water content will be dissipated because of the vacuum of space (like freeze drying), but it will still have the opportunity to be re-absorbed as the molecules are drawn into Earth's atmosphere.

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  • $\begingroup$ Depending on which way it's vented and at what velocity! But gotta think that's quite well within validity. A very good point Charles $\endgroup$
    – JeopardyTempest
    Commented Feb 18, 2017 at 1:01

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