I feel that this is something that was never really covered in the games, and that it might make an interesting question!! :)
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This belongs in the Universe forum, but on topic, you can't actually walk off the edge of a Halo ring, as there are enormous walls on the edges, which I'm guessing are used to keep in the atmosphere of the ring. [Edited on 12.09.2012 8:52 AM PST]