Should the need arise, you can pause your mission by pressing the Navigation Dial.
When you're ready to continue playing, turn the dial until you hear Resume, then press the dial again to confirm your selection. You will then be placed back into your active mission.
If you need to exit the mission altogether, turn the Navigation Dial until you hear Exit, then press the dial again to confirm your selection. After exiting a mission, you will be placed back into Exploration Mode.
NOTE: When you're in the Travel or Armory Menu, you can hold down the triangular deploy button to exit the menu at any time.