The president was on his way. Space shuttle Endeavour's astronauts were riding out to the launch pad in a van. And a wounded Rep. Gabrielle Giffords had flown in from her Houston rehab hospital to watch her husband blast off Friday on the historic, next-to-last shuttle mission.
Then it all came to a sudden stop.
Without warning, a faulty heater part forced NASA to scrub the launch and slam the brakes on the space agency's biggest event in years, a flight made more fascinating to many by the plight of Giffords and her husband, Mark Kelly, the mission commander.
Endeavour's flight was delayed until at least Monday.
"Bummed about the scrub!! But important to make sure everything on shuttle is working properly," Giffords' staff tweeted.
A post on Giffords' Facebook page said her "travel plans at this time are undetermined." However, a close friend of the Arizona congresswoman, who is still recovering from a gunshot wound to the head from an assassination attempt in January, said Giffords' family indicated she would stay in Florida at least until Monday. The friend asked not to be identified because he was not authorized to speak for the family.
Giffords' staff had no immediate comment on her travel plans.
President Barack Obama and his family came to Cape Canaveral anyway, and he met with Giffords for about 10 minutes. No details of that visit were given.
Her husband greeted Obama in a corridor, saying: "I bet you were hoping to see a rocket launch today."
Obama replied: "We were hoping to see you."
The two men shook hands and embraced.
The president told the Endeavour's six astronauts he is still hoping to get back to Florida for a shuttle launch.
"One more chance, we may be able to get down here," Obama said.
"It's a priority for us," Michelle Obama added.
As many as 700,000 tailgaters and other spectators had been expected to pour into the seaside area for the liftoff, one of the biggest launch-day crowds in decades. It would have been the first time in NASA history that a president and his family witnessed a launch.
Giffords arrived on Wednesday, nearly four months after the shooting in her hometown of Tuscon, but the 40-year-old congresswoman hasn't been seen in public. She has difficulty walking and talking and wears a helmet because doctors removed a large piece of skull to allow for swelling of her brain.
She had planned to watch the launch from a private VIP viewing area along with the other astronauts' families before the countdown was halted about 3½ hours short of the 3:47 p.m. liftoff. NASA's silver-colored astrovan did a U-turn and brought the astronauts back to their crew quarters.
Giffords' Houston doctors declined to say whether a prolonged stay away from her rehab center would cause problems. It was unclear whether she is continuing her therapy while in Florida; her staff would provide no details.
Dr. Jaime Levine, a brain-injury expert at Rusk Rehab at NYU Langone Medical Center in New York, said she had no reason to think that Giffords' recovery would be harmed by the time away from her rehab center or the psychological stress of the launch cancellation. But Levine said she did not know Giffords' situation.
Kennedy Space Center Director Robert Cabana said that while he didn't meet with Giffords, "she's enjoying her time in Florida, she's enjoying her time with Mark and she's doing well."
Engineers aren't certain what part on the heating system — needed for launch and landing — needs to be replaced. To fix the heater, technicians will have to crawl into the engine compartment. If it is a simple fix, NASA could make another launch attempt as early as Monday, Tuesday or Wednesday, but if not, the flight could be delayed to May 8 or later, said launch director Mike Leinbach.
If NASA tries on Monday, the president can't make it, Cabana said.
After Endeavour, there is only one more shuttle flight — by Atlantis — before NASA ends the 30-year-old program and the orbiters become museum pieces.
When Endeavour finally flies, it will be the last mission in its 19-year history. It will deliver a $2 billion instrument that will search the universe for antimatter and dark energy.
Tammi Flythe, among the thousands gathered across the Indian River in Titusville with her two children, was disappointed at the postponement. They had driven from Tampa, about 130 miles across the state.
"I really wanted my son to experience this," she said.
At the space center, astronaut Clayton Anderson was typically stoic.
"Of course, it's always disappointing, especially for the crew," he said. "NASA has a great safety record and they're going to do it the right way."
AP writers Erica Werner and Kyle Hightower in Florida, Malcolm Ritter in New York and Ramit Plushnick-Masti in Houston contributed to this report.