You'll need a ton (well, probably several tons) of water. https://space.stackexchange.com/questions/1336/what-thickness-depth-of-water-would-be-required-to-pro

You'll need a ton (well, probably several tons) of water. https://space.stackexchange.com/questions/1336/what-thickness-depth-of-water-would-be-required-to-provide-radiation-shielding-i https://space.stackexchange.com/questions/1336/what-thicknes... ^^ Interesting read on this very topic.