There was a grenade that was shoved into his armor, and he was pushed into a beam that was disintegrating everything. And don't forget about the fall all the way to Earth.
He must be some invincible mother-blam!- if he lived all that.
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Whos to say super powerful ancient forerunner war armour cant survive that? Master chief himself fell from space and survived. I think the blast must have just sent him miles away alive, could he have been talking to the precursors at the end?