There could be as many as 700,000 people gathered to witness space shuttle Endeavour's final mission in Central Florida on Friday afternoon.
"I'm thinking this is like the world's largest tailgate party," said Rob Varley, the executive director of the Space Coast Office of Tourism.
The launch will come at 3:47 p.m. ET.
Crowds are already pouring into the beachside towns surrounding the Kennedy Space Center, Varley said. Tickets for specialized tours of the Kennedy Space Center Visitor Complex are sold out, but general tours are still available.
One of the most high-profile spectators will be U.S. Rep. Gabrielle Giffords, wife of Endeavour commander Mark Kelly. Giffords is recovering from a gunshot to her head after a man attacked her and killed several others at a January political event in Arizona. She departed a Houston, Texas, rehabilitation facility Wednesday on her way to Florida.
President Obama is also expected to attend the launch.
By comparison, February's launch of the shuttle Discovery on its final mission drew about 450,000 spectators.
Anyone still toying with the idea of traveling to the area to see the event should know roadways will be crowded. Come and camp out early, Varley suggested.
Though Friday will mark the last time Endeavor flies, there will be one more shuttle flight scheduled before the program ends this summer.