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I can imagine a plasma shield for interplanetary spacecraft would require a lot of power to run, which would then require even more equipment to radiate away the waste heat. That seems like it would come with a lot of potential failure points.

I wonder if storing water in a double hull would offer the same protection for less equipment/weight/risk?

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I think you underestimate the sheer mass of water.

Well, if you've got people on board, you're going to need lots of water anyway. I guess it depends on the cost of lifting mass out of Earth's gravity well compared to the cost of making a highly reliable plasma shield.

It's not just the lifting out of Earth's gravity; mass is a liability in all forms of space travel. For any sort of momentum transfer drive (i.e. all of them, barring a scifi breakthrough like reactionless or warp), adding mass will eat into your delta-vee.

That's a good point, but system complexity and failure risk is also a liability that needs to be considered. A few meters of ice around your ship will be pretty much 100% reliable and require zero power, but for the considerable extra fuel required to achieve your required delta-vee. An ice-hull will also stop micrometeorites which I'd imagine a plasma shield would not be able to deflect.

Space is hard!

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