NASA has called off Friday's launch of space shuttle Endeavour because of a heater failure.
Commander Mark Kelly and his crew were on their way to the launch pad, when NASA halted the countdown. The astronauts' van did a U-turn, and returned the astronauts to crew quarters.
NASA spokesman George Dillard says the next try will be Sunday at the earliest.
NASA reported earlier that two heaters on an auxiliary power unit were not working. Engineers could not understand the problem, and the launch was halted.
Kelly is married to wounded congresswoman Gabrielle Giffords. She is already in Cape Canaveral. President Barack Obama planned to watch the launch but had not yet arrived.
Just a few hours from liftoff, NASA fueled space shuttle Endeavour for one last ride into orbit Friday as hundreds of thousands of visitors converged on the coast for prime viewing spots.
The launch team began loading more than a half-million gallons of fuel into Endeavour at dawn, moments after Prince William and Kate Middleton exchanged wedding vows across the ocean in London. Three hours later, the tank was full and NASA was keeping a close watch on a nearby storm.
Commanding Endeavour on NASA's next-to-last shuttle flight is Navy Capt. Mark Kelly, who is married to Rep. Gabrielle Giffords. She planned to watch the launch from Kennedy Space Center. Giffords was shot in the head in January and left rehab in Houston behind to attend the afternoon liftoff.
"Gabrielle is just as excited as all of you!" her staff said in a Twitter update late Thursday.
She's being accompanied by her husband's identical twin, Scott, also a space shuttle commander.
"Ready if replacement is required," Scott joked in a tweet.
President Barack Obama was supposed to attend with his wife and two daughters - the first time in NASA history that a sitting president and his family would have witnessed a launch.
A storm pushed through the area after daybreak, but NASA expected it to be gone by launch time at 3:47 p.m. Low clouds and stiff crosswind, however, remained a concern. Launch controllers quickly resolved a minor shuttle problem: elevated pressure in a fuel tank for on-board thrusters.
Endeavour is bound for the International Space Station.
For its last hurrah, it's carrying one of the most expensive payloads in NASA's 30-year shuttle history: a $2 billion particle physics detector that will seek out antimatter and dark energy across the universe.
Kelly and his all-male crew - all six of them space veterans - saw their families for the last time Thursday. Four of them went for a 3-mile early morning run Friday on the beach, including astronaut Mike Fincke, who thanked the Lord for his family and "the chance to fly in space again."
"Please don't let me mess it up!" he said in a tweet.
As the sun rose, recreational vehicles already lined the Banana River to the south, with a wide open view of the launch site.
As many as 700,000 people were expected to crowd nearby coastal communities. For days, police have been warning of massive traffic delays.
In Titusville - a prime viewing location - shuttle watchers lined up three rows deep along the Indian River more than five hours before show time. Parking spots went for as much $30 a shot, and businesses, churches and vendors raked in the money.
Corrine Summers was hawking T-shirts that read "Godspeed Endeavour on her final mission" made by her husband's print shop. She watched the royal wedding, then hit the street.
"It's a chance to make a few extra bucks," she said. "You won't always have this opportunity with so many people here."
The space center itself was bracing for 45,000 guests, including more than three dozen members of Congress, at least two former NASA administrators, and a score of high-level academic and space industry officials. The California Science Center in Los Angeles - Endeavour's retirement home - also was going to be represented.
NASA is ending the shuttle program this summer, after one last trip by Atlantis. Obama has put the space agency on a path to asteroids and Mars, ultimately, while encouraging private companies to take over Earth-to-orbit operations.
In the meantime, U.S. astronauts will keep using Russian Soyuz rockets to get to the space station.
Once Atlantis flies, it will be at least three years before America launches astronauts from their home soil again. Some fear it could drag on for a full decade.
AP writers Seth Borenstein and Kyle Hightower contributed to this report.
© 2011 The Associated Press.