Thursday evening, Pope Benedict XVI will step onto a white helicopter at the Vatican and fly off to the summer papal residence, Castel Gandolfo, and into retirement. At 8 p.m. sharp, Rome time, he will cease being Pope Benedict, and become pope emeritus (or Roman Pontiff Emeritus). This is virtually uncharted territory for the Roman Catholic Church — the last pope to abdicate the Chair of St. Peter was Gregory XII in 1415. So what happens to a retired pope and the scandal-tinged church he leaves without a leader?

Early Thursday, the pope held a final meeting with the College of Cardinals, urging them to work together "like an orchestra" to harmoniously pick his successor. He greeted each cardinal individually and said he'd pray for them as they deliberate during the upcoming conclave, adding, "Among you is also the future pope, whom I promise my unconditional reverence and obedience." At about 5 p.m. local time, Pope Benedict will meet with a small group of staff and members of the Swiss Guards who have protected him over the years, then board his Vatican helicopter.

When he arrives at Castel Gandolfo, in the hills outside Rome, the pope will make one final public appearance on his balcony, then, "having greeted those gathered below, he will step back inside and begin his life of seclusion," says CNN's Laura Smith-Spark. The pope emeritus is expected to spend anywhere from a few weeks to a few months in Castel Gandolfo, starting what has been described as a quiet life of prayer and scholarship. Then he will return to the Vatican, to live in a converted convent.

There's an air of improvisation to this whole transition. Benedict, by his own choice, will keep his white garments, his title of "His Holiness," and the name Benedict XVI. He will share the services of his trusted secretary, Archbishop Georg Gänswein, with the next pope. He is giving up his Twitter account, his Swiss Guard protection — he'll be guarded instead by Vatican police — and his famous red shoes, a papal symbol representing the blood of martyrs. (He will wear brown artisan-made shoes given to him in Mexico, the Vatican says.)

Benedict has said he will remain "hidden from the world" in his Vatican retirement, but nobody quite knows what will happen with two popes within shouting distance of each other, and plenty of Catholics are worried. The discomfort is especially acute among cardinals and more conservative Catholics, says Michael McGough at The Los Angeles Times. "Although a papal resignation is provided for in church law, Benedict's decision undermines the mystique of papal uniqueness," reducing the papacy to just another bishopric. (The pope is also the bishop of Rome, and Vatican officials had suggested Benedict would return to black clerical clothes and use the title Bishop of Rome Emeritus.)

Liberal Catholics long have emphasized that fact, sometimes referring to the pope as the head of the "college of bishops." Conservative Catholics prefer the pre-Vatican II view of the pope as the source of all human authority in the church, akin to a king. Indeed, it is conservative Catholics who have been insisting that the proper term for Benedict's decision is "abdication," not "resignation." An abdication is seen as a rupture is the natural order of monarchy, even when it is necessary. But the less seen of the former king, the better. Britain's Edward VIII became the Duke of Windsor after his retirement, not "king emeritus."... Granted that there is only one actual pope at a time (and only one who can make infallible pronouncements on matters of faith and morals), the existence of two men who are addressed as "your holiness" changes the ecclesiastical atmospherics. For a lot of traditionalist Catholics, two "popes" (even if one is emeritus) is one pope too many. [Los Angeles Times]

Not that all of Benedict's more liberal critics are happy with his resignation choices. Swiss theologian Hans Küng, a former longtime colleague of the pope who became one of his harshest critics, tells Germany's Der Spiegel that "with Benedict XVI, there is a risk of a shadow pope who has abdicated but can still indirectly exert influence." No one likes to have his predecessor right next door, and "even for the bishop of Rome, it is not pleasant if his predecessor constantly has an eye on him."

Andrew Sullivan is even more offended. "If you were trying to avoid any hint of meddling, of a Deng Xiao Peng-type figure pulling strings behind the scenes, you would not be doing this," he says at The Dish. And the fact that Benedict's longtime personal secretary will live in the convent with him? "This is not the Vatican. It's Melrose Place."

Other Vatican-watchers are more sanguine, of course. "I was somewhat surprised that Benedict would still be called 'His Holiness' and would wear white, but it's akin to the former U.S. presidents being addressed as 'Mr. President,'" Fr. James Martin tells The Associated Press. "It's a mark of respect for the former office he once held."

The next pope will likely be elected sometime in March, as soon as the 115 cardinal electors agree on one and send up the white smoke from their Sistine Chapel conclave. What happens to the Catholic Church in the meantime? "Vatican operations essentially go as still as the characters in Sleeping Beauty — frozen in time as of 8 p.m.," says Cathy Lynn Grossman in USA Today. Essentially, the Vatican will act as if the pope is dead.

All the arrangements are set by the camerlengo (chamberlain of the Church) chosen by the pope. Benedict chose Cardinal Tarcisio Bertone, Vatican secretary of State, for that role but there's little for him to do with no funeral to arrange. All the cardinals and archbishops in the curia, the bureaucracy of the Church, lose their jobs on Feb. 28. It's a bit like all the U.S. president's cabinet resigning after a presidential election so the new head of state can name his team. But in the Holy See, many cardinals expect they'll be asked to stay on in the next papacy.... Vatican offices will be run by secretaries who handle ordinary, minor duties. All serious or controversial matters await the next pontiff.... Only three major officials keep their posts in the period between Benedict's resignation and a successor elected: The vicar of the diocese of Rome who cares for the city's pastoral needs; the major penitentiary who deals with the Holy See's confessional needs so there is always access to forgiveness; and the camerlengo, Bertone, who will deal with property and financial decisions for the Vatican for the time being. [USA Today]

With nobody really in charge, nothing much will happen in the Vatican until at least Monday, says Grossman. In fact, "for now, the busiest people in Rome may be the tailors at Gammarelli ecclesiastical tailoring shop. They're stitching up the vestments for the new pope in small, medium, and large sizes so that whoever is chosen will fit right in."

Sources: AFP, The Associated Press (2), CNNThe Dish, The Los Angeles TimesThe New York Times, The Washington Post, USA Today