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Will we be able to take the amount of water that will be needed for this 18 month trip, or will they have to recycle the water that they take with them? I imagine that this will be quite a challenge because water is very heavy?

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It's likely that a Mars mission will take a total of 32 months, not 18; the launch window for a fuel-efficient return journey requires a wait of over a year after arrival.

Humans need 2-4 liters of water per day. Assume a crew of 5; that works out to about 10-20 tons of water if you don't recycle it. That's a significant chunk of the overall mass requirement of the mission (NASA thinks the crew habitat and command module for a Mars mission will mass about 50 tons), so it's obviously attractive to consider a small water supply and a good recycling system.

However, one of the major concerns for a manned Mars mission is radiation exposure over the journey, and water makes a decent radiation shield. If you have to carry 20 tons of shielding anyway, you could do worse than carrying it in the form of water.

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  • $\begingroup$ One year ??? I always thought that they would only be on Mars for a few days and then return here.So they will have to take a tremendous amount of food and water and probably even medicine. What about air, can they take a three year supply of that ? How will their body react to the lower gravity and also the radiation on Mars ? What is the longest period of time that humans have spent in space before this and did they suffer any ill effects from it ? $\endgroup$
    – Peter U
    Dec 13, 2014 at 22:27
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    $\begingroup$ @PeterU That's a whole bunch of questions, most of which are probably on Space Exploration. $\endgroup$
    – HDE 226868
    Dec 14, 2014 at 16:53
  • $\begingroup$ Several humans have spent hundreds of days in space in a single stretch (en.wikipedia.org/wiki/…) with minor ill effects -- bone loss and muscular atrophy. Radiation exposure is definitely the biggest concern. $\endgroup$
    – Russell Borogove
    Dec 14, 2014 at 17:28
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    $\begingroup$ Another issue I think is how to deal with fatalities and manage corpses. What about ethics envolved? And relationships? And stress? The negative impact on humans are huge but still are worth to try. Nevertheless, it is a one way trip. And to protect from radiation water barrier and a litte artificial magnetosphere would be a good start... $\endgroup$
    – Claudia
    Dec 15, 2014 at 19:05
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    $\begingroup$ Irradiated, yes, dangerous no. The radiation we're concerned with consists fast-moving particles - protons and small atomic nuclei. When they hit water, they are mostly scattered and slowed down (making them less harmful). Various nuclear reactions might heat up and ionize the water, but that's harmless. You might get a few heavier isotopes of hydrogen and oxygen than you started with, but those isotopes are mostly fairly stable - they emit radiation very slowly if at all. I'm not a nuclear physicist, but I think it's possible to get some fluorine as an output -- free fluoridated water? :) $\endgroup$
    – Russell Borogove
    Dec 17, 2014 at 4:38

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