I seriously believe the enemy is mainly going to be ONI aka Office on Naval Intelligence.Because they are known to set up at installations like these and known to sacrifice UNSC personnel.Think about it.It definitely shows in the books,games,and minifilms from waypoint.
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Confirmed enemies are Covenant Pirates and Prometheans, along with a host of unannounced enemies under the control of the Prometheans, (Ex. the animal looking things and the small floating probes that you saw in the gameplay video at E3, AND more yet to be announced). Does no one keep up with 343? Jeez. [Edited on 07.10.2012 5:34 AM PDT]