As this is an online game, think of the saves as something more akin to what WoW does. It doesn't save at all, at least not in the traditional sense. You die, you rez somewhere nearby, or someone rezzes you right there. As for how rezzing works while playing solo, we don' know yet. I would imagine you rezz somewhere nearby. As for saving your progress, the servers probably keep track much as they do in a traditional MMO. I doubt you'll be able to backtrack your progress if you screw up, as all things happen in real time. Remember you aren't running the game on your system as you would in a regular offline game. You are accessing the Destiny world on a server. This is all educated guesses, mind you, as the topic hasn't been discussed by Bungie at all. But I would bet I'm pretty close to being correct.

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Since the game is always-online all your progress is saved in real time presumably to the b.net servers; the exception to this would be rollbacks, which would reset any character progress in the scenario in which Bungie somehow goofs an update.