UPDATE: Today's shuttle launch has been scrubbed because of a concern associated with Auxiliary Power Unit 1 heaters. NASA will hold a press conference to address the issue.
With up to 750,000 spectators expected to converge around the Kennedy Space Center area Friday, NASA's launch team has an added concern if weather or technical problems delay today's launch of shuttle Endeavour -- traffic.
"Driving home is going to be a challenge and that's going to factor into our decision this time more so than ever in the past," launch director Mike Leinbach told reporters.
Liftoff is slated for 3:47 p.m. EDT. Meteorologists on Thursday predicted a 30 percent chance the weather could be a problem for launch. Typically, NASA would try again 24 hours later. This time, however, 48 hours might be needed to allow workers enough time to get home, get rested and return to the space center to try again.
With just two shuttle flights remaining before NASA ends its 30-year- old program, crowds along Florida's central Atlantic coast have been growing on launch days. Discovery's liftoff in February drew about 400,000 visitors to the area, known as the Space Coast. Officials were expecting between 500,000 and 750,000 for Endeavour's launch on a Friday afternoon.
"What local law enforcement has told us is that whatever drive delay we experienced for (Discovery's) launch, it's going to be about 50 percent longer this time going home," Leinbach said. "But when you think about why the people are coming, to experience something that's uniquely American and be able to see one of the last two flights, that gives me a lot of pride."
The launch guests include President Barack Obama, who is due to arrive at the Kennedy Space Center on Friday with his family, as well as U.S. Rep. Gabrielle Giffords, the wife of Endeavour commander Mark Kelly. Giffords, an Arizona Democrat, is recovering from a near-fatal assassination attempt on Jan. 8. She arrived at the spaceport on Wednesday.
"I'm really happy that she's here," NASA administrator Charlie Bolden told Discovery News. "It's a triumph of good over evil."
"It's like the space program," Bolden said. "It's a story of being able to come back from adversity. We suffered real losses with Apollo 1 and a near loss with Apollo 13 and real losses again with Challenger and Columbia, and yet, we're doing what we do today in tribute to those people."
Three Apollo astronauts died in a launch pad fire in 1967, and the Apollo 13 crew nearly died on the way to the moon in 1970. Two fatal shuttle accidents, in 1986 and 2003, killed 14 astronauts.
"It could have been very, very tragic if we had lost Gabby," Bolden said. "We lost people in her office, but they didn't die in vain because of who she is and what she does."
Giffords was shot through the head as she met with constituents outside a Tucson, Ariz., grocery store. Six people were killed, including a nine-year-old girl, a federal judge, and a member of Giffords' staff. Thirteen, including Giffords, were injured.
Kelly, who was in training for Endeavour's mission, left work to oversee his wife’s care. He later decided to rejoin his crew after her condition stabilized and she was transferred to a Houston hospital for rehabilitation.
Bolden described Giffords, who has not been seen publicly since the attack, as "fired up."
"She's a feisty person," he said.
Kelly and his crewmates are scheduled to spend up to 16 days in orbit. Their primary job is to deliver a particle physics experiment and a pallet of spare parts to the International Space Station.
Four spacewalks are planned during Endeavour's stay at the station to help prepare the outpost for operations after the shuttles are retired.
NASA's last flight, scheduled for launch on June 28, will deliver a year's worth of supplies to the station to tide over the crew until U.S. commercial cargo ships begin flying later this year.