For those who need to travel outside of U.S. boundaries for an extended amount of time, there is a re-entry permit (Form I-131). This form should be filled out prior to leaving the U.S., but does not need to be approved before exiting the states.
Reentry permits are issued every two year and cannot be renewed. If the person needs to travel for longer than this time period, they must return to the U.S. and apply for a new permit. Second permits can also have a time period of up to two years, but any additional permits will only be approved if they are one year at a time.