Shuttle bay door

The shuttle bay door is a type of energy barrier that controls dropships' access to the shuttle bay of a large Covenant ship, such as the Truth and Reconciliation. It is likely that this mechanism also keeps the shuttle bays air-tight.

Appearances
In Halo: Combat Evolved, it is possible to operate the shuttle bay doors. On the level The Truth and Reconciliation, the player is able to access the doors by heading to the third floor of the shuttle bay and pressing X on the control panel. This is required to complete the mission, so you can escape in a Spirit with Keyes.

In Halo: Reach, on the Covenant corvette Ardent Prayer, the shields seemed to work differently: after Noble Six pressed a button, the shields still remained in place over the bay, but ships could now go in and out of them. Presumably they were simply lowered to a point things could travel through them while remaining in place to hold the atmosphere in the bay as the ship was in space.