First, so you can accept that amazing offer to travel to Paris for the weekend or the exciting trip you just won for hitting your sales numbers. But all kidding aside, even when you take a cruise that starts and ends in the US, called a "closed loop voyage" you are at risk when you travel without a passport.
Let’s say your ship gets diverted to a non-US port or you have an emergency that requires you to depart at the nearest port, without a passport you are stuck.
Many people look at getting the US Passport "card" as an alternative which works great unless you need to fly. Only the US Passport "book" allows a US citizen to enter by air! The valuable lesson is to plan for any possibility and have a passport. Besides you never know where that next great vacation might take you.
March 27th is Passport Day where all offices will be open and will not require an appointment. For a list of offices, go to http://travel.state.gov/passport/passport_4312.html. Another point to note is that passport fees are going up in April.
Do you have a valid passport?
