John Allen Jr. offers a wealth of historical information in his posting of Trivial Pursuit about Papal Travel in America (NCRcafe.org March 29, 2008). Did you know that Benedict XVI will become the third pope to visit the United States, but he will actually be the fourth to set foot on American territory?
... In 1849, Pope Pius IX was received aboard the USS Constitution, which happened to be anchored in Gaeta, Italy, where the pope had taken refuge after a popular uprising chased him out of Rome. Allowing the pope to come aboard was a diplomatic gaffe, since the United States was officially neutral in the struggle over the Papal States between Pius IX and Italian nationalists. Nevertheless, the captain of the Constitution, John Gwinn, invited the pontiff aboard when he arrived with King Ferdinand of Sicily for a tour.Find out more.
Pius IX distributed rosaries to the Catholic sailors and gave blessings. He became seasick and took a brief rest in the captain’s chambers, then disembarked to a 21-cannon salute. Gwinn was later threatened with court-martial for the breach of protocol, but died before a trial could be held. (I owe this nugget to the book The United States and the Holy See: The Long Road, by former U.S. Ambassador to the Holy See James Nicholson.)