I have heard rumors that they use compressed water to save space on transportation to the space station.

Is there any possibility of this occurring at any point in space travel history?

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    $\begingroup$ First of all water is very hard to compress, or, practically uncompressable. Second, water is a relative dense payload so space isn't as much of a concern as mass $\endgroup$ Aug 29, 2018 at 17:01
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    $\begingroup$ No. But dehydrated water is commonly used to save mass :) $\endgroup$ Aug 29, 2018 at 17:35
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    $\begingroup$ @OrganicMarble - That info's a little dated. They used to do that, but they found that re-hydrating it resulted in large amounts of dihydrogen monoxide, known to be the result of many fatalities. $\endgroup$ Aug 29, 2018 at 18:55
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    $\begingroup$ Please avoid posting questions on this site based on things like "I have heard...". Without a specific, referenced claim, it's very difficult to make sense of what's being asked. $\endgroup$ Aug 29, 2018 at 20:25
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    $\begingroup$ Compressing water needs an extreamly high pressure (about 4000 bar) requiring a very heavy container. It is much more important to save mass instead of volume. Uncompressed water in a light container would save a lot of mass. $\endgroup$
    – Uwe
    Aug 29, 2018 at 20:25

2 Answers 2


The rumors you have heard are not right, even a huge pressure would only result in a very small change in the volume, and it certainly wouldn't change the mass of the water, which is probably more important most of the time.

With a bulk modulus of about 2 GPa (20,000 atmospheres), you would need 200 bar (200 atmospheres) roughly to change the volume by even 1%.

Instead, you may be mixing two different things. Instead of water, it is the oxygen which is shipped to the ISS highly compressed.

You can read more about the NORS system for transport of compressed oxygen to the ISS in these questions or their answers and in the links they contain:

It is possible to get one's wires crossed between water and oxygen, since in an emergency (or perhaps not even) water can be electrolyzed to make oxygen. See:

Also, if there is a source of hydrogen, it can potentially be combined with oxygen to make water, though you would not keep hydrogen around the ISS in any significant quantity since it is explosive. See Why vent CO2 and H2 waste products to space on ISS?


This is a misconception. What is done is to instead of bringing up gaseous oxygen which is bulky or need high pressures bringing up water which is then electrolyzed to separate the oxygen from the hydrogen. The oxygen is then used in the lifesuport system and hydrogen dumped overboard.

This is a method of compressing oxygen by adding a small bit of hydrogen and transporting it as water. This avoids high pressure tanks which are heavy and thus saves on mass.

This use of electrolyzis is for example mentioned here https://science.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1

  • $\begingroup$ @William well it's the cheapest way. Sending heavy pressurized tanks every few months is much more expensive than sending a few additional solar panels once plus simple and light water tanks. $\endgroup$
    – asdfex
    Aug 29, 2018 at 19:42
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    $\begingroup$ Downvoted till a reference is provided showing that water shipments are done in place of oxygen shipments. This reference shows the opposite: nasa.gov/content/… $\endgroup$ Aug 29, 2018 at 19:55
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    $\begingroup$ Here is a source science.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1 $\endgroup$
    – lijat
    Aug 29, 2018 at 22:32
  • $\begingroup$ They do need to send nitrogen, to replace that which escapes into space with venting the airlocks and general leakage. $\endgroup$
    – SF.
    Aug 30, 2018 at 0:00
  • $\begingroup$ That reference says nothing about where the water comes from. $\endgroup$ Aug 30, 2018 at 2:51

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