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While I am no expert in chemistry he mentioned that one of the benefits of using water was the fact that it is in a liquid state and thus you could effectively store more energy within a finite amount of space. At least one of the noble gases (argon) does not compress down to a liquefied state, so perhaps that applies to the others? Large compression tanks are also quite heavy, so perhaps that is a consideration too.

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The page is not very clear. The energy comes from the solar cells. They are using the water to store mass, no energy. It’s (probably) easier to store a lot of mass of water as a liquid than a lot of mass of Xenon because you must compress it a lot.

The idea is that to accelerate in one direction, you must push something in the opposite direction. They will convert the water into plasma (imagine very hot ionized water vapor) and push it using the energy from the solar cells.

It’s different from a normal rocket, because in a normal rocket the energy and the mass come both from the fuel and oxidizer.

The other benefit is that you can find more water in space - it's harder to find argon and harder still to put it in a usable form.

They're almost certainly thinking of what you could with ion thrusters, the sun (or plutonium) and some of the icy bodies in the outer solar system.

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