Wouldn’t want to the be the one in charge of watching over those who watch over him….

Those planning security for the New York City leg of the upcoming United States visit of Pope Francis have circled Sept. 25 on their calendars as the perfect storm.

That day, Francis is to address world leaders at the United Nations, preside over a service at the 9/11 memorial, do a procession through Central Park and celebrate Mass at Madison Square Garden. The agenda, combined with the popular pope’s habit of going off-script and mixing with crowds, has created an “unprecedented challenge” for law enforcement, New York Police Commissioner William Bratton said.

Bratton isn’t alone in his concerns about protecting the pontiff and the throngs expected to turn out to greet him. Officials in Washington, D.C., and Philadelphia, the pope’s other stops, have their own worries.

The U.S. Secret Service, working with the FBI and local authorities, has spent months coordinating the massive security effort. Secret Service officials have met multiple times with Vatican security officials both in Washington and in Rome to learn more about how the pope interacts with crowds. Francis travels with his own security detail and his Popemobile, the bulletproof vehicle he once called a “sardine can” that walls him off from people.

Many of the particulars on security haven’t been disclosed, but here’s a look at some of the variables: