The would-be assassin of Benedict XVI's Polish predecessor has been in jail, in Italy and Turkey, for a total of 19 years since shooting John Paul II at point-blank range outside St Peter's in 1981. Earlier this month he asked to be released, saying he wanted to meet Benedict. But the Turkish authorities were firm and Agca will remain in prison until 2010.

In every other respect, however, the pope's four-day Turkish trip is beginning to look like the pontifical voyage from hell. Or to it.

It was conceived as the relatively cosy and fraternal visit of the head of one part of Christendom to the head of another which, though infinitely reduced in numbers, is Roman Catholicism's equal in age and venerability. The Orthodox Patriarch of Constantinople extended the invitation, which was seen as a way for the pope to encourage and cheer Turkey's small and embattled Christian community, about 150,000 people in an overwhelmingly Muslim nation of 80 million.

The idea was mooted soon after the pope's election last year, but the Turkish authorities pointed out frostily that it was their prerogative to invite a head of state, and squelched it. Benedict had already made himself unpopular in Turkey when, as Cardinal Joseph Ratzinger, he commented in 2004 that Turkey had always been "in permanent contrast to Europe" and that for Turkey to join the EU would be a mistake.

On his election, Turkish newspapers screamed that the new pope was anti-Turkish.

But after rejecting the idea in 2005, the Turkish government then revived it, and the Vatican agreed.

Benedict would become the third pope to visit the country after Paul VI , in 1967, broken the ice between the Catholic and Orthodox churches that had lasted nearly a millennium. John Paul followed in 1979.

No pope gets the sort of welcome from Turkey that he can expect in a Catholic country, of course: the country's secular constitution requires that he be received, starchily, as a head of state rather than a religious figure.

But Benedict made it all the more difficult for himself in September when, in a lecture delivered in Regensburg, Germany, he quoted a 14th-century Byzantine emperor describing Islam as "evil and inhuman". It was an extraordinary gaffe, comparable to his failure when visiting Auschwitz months before to acknowledge the guilt of Germans like himself (once a member of Hitler Youth) for the Holocaust.

After Regensburg, as after Auschwitz, he most unpapistically tried to make amends by expressing regret. But in both cases the damage was done.

The Turkey trip, however, was now set in stone.

"The Turkish government wasn't in a position to call it off, because of their EU ambitions, " noted a Vatican insider. "The Vatican couldn't call it off, or it would be seen to be capitulating to those who attacked him." The Turkish government has promised to protect him as zealously as it would an American president. It may need to.