Alternatively you can bring up 1g of hydrogen from a planetary gravity well. Even if you're near the galactic core with 1000 hydrogen atoms per cm^3, you'd need to scoop up all hydrogen atoms from 6910 km^3 of space for 1 gram of hydrogen. Anyway, there's virtually no oxygen in space, so you have to bring it from a planet.

And one oxygen molecule that is much heavier, but let's disregard that for the moment. There are 2 hydrogen atoms in a water molecule. There is on average 1 hydrogen atom per cm^3, with a range of 0.1 to 1000. The most common particle in the universe (>90%) are atomic hydrogen iones, not H2. Yes, these vast amounts are there - but they are spread over such huge distances that it's not worth collecting! Don't trust all those science articles that tell of the vast amounts of hydrogen that can be found in space. Space is mostly a vacuum, and in fact a better vacuum than anything we can create on Earth!. Click to expand.Because there is so little of it that not even current space faring tech has considered it a good alternative to bringing water along for e.

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