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You will have to leave it behind but if you do it right it should kill the elite on the other side of the door as soon as you get it open. If you don't want to do that then head back out (taking the warthog with you) and move down the beach. If you are fast enough you can drive your warthog into the door and it will get stuck. Once they are down you find the door locked and no way to get through. Go on down the ramp (leaving the warthog if you brought it in) and kill the group of Covenant at the bottom by the door. Head in (I suggest taking the warthog into the first area to make it easier on the higher difficulties) and eliminate all enemy presence. This is the entrance to the map room Cortana has been talking about. Keep going and you'll fight a pack of Elites and a bunch of Grunts at the entrance to the building here. You'll see a steep hill ahead and when you get to the top you see a Covenant dropship fly off.
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