The final mission of Nasa's Endeavour shuttle has been delayed by at least 72 hours because of a technical problem. The orbiter is being held on the ground while engineers investigate a failure in a unit that powers the vehicle's hydraulics.
America's youngest reusable spaceplane is set to deliver a $2bn (£1.2bn) particle physics experiment to the International Space Station (ISS).
US President Barack Obama had planned to attend the lift-off in Florida. He took a tour of the Kennedy Space Center (KSC) with his family instead.
Also present at the spaceport to see the ascent was Endeavour Commander Mark Kelly's wife, Gabrielle Giffords, the Arizona congresswoman shot in the head by a gunman in January. She had a 10-minute private meeting with the president.
It was not immediately clear, however, whether she would stay to see her husband's next attempt to get into space or return to Houston, Texas, where she has been undergoing rehabilitation.
SPACE SHUTTLE ENDEAVOUR
Endeavour was the last orbiter to be built and flew its maiden voyage on 7 May, 1992
It is named after the ship commanded by the British explorer James Cook from 1769 to 1771
Total space time to date: 280 days; Total Earth orbits: 4,429; Individual crew members: 133
Made the first American ISS construction flight, delivering the Unity Module
Carried out the mission to correct the Hubble Space Telescope's flawed vision
Its radar map of the planet is one of the most used Earth-observation data-sets ever acquired
Atlas complication
The lift-off had been targeted for 1547 local time (1947 GMT; 2047 BST), but was postponed just after midday.
Kelly and his crew had already suited up and were making their way to the shuttle. By tradition, their "Astrovan" would normally stop briefly outside Kennedy's launch control centre before moving on down to the pad, but on this occasion the vehicle pulled in front of the building and parked. The "scrub" was called moments later.
"A disappointing day for Team Endeavour and the astronauts but as we always say in this business, 'we will not fly this machine until it's ready' and today it was not ready," explained shuttle launch director Mike Leinbach.
Managers and engineers are now trying to establish how long a turn-around is needed to deal with the problem in one of the three auxiliary power units in Endeavour.
A heater that prevents freezing in a hydrazine fuel line leading to the unit stopped working. Another heater was also displaying unusual behaviour.
Nasa managers say it could be a simple thermostat failure but may indicate a more deep-seated issue in an electrical switchbox. If the former, Endeavour could make another launch attempt on Monday; if the latter, Endeavour is looking at a much longer delay.
Complicating matters is the launch of an Atlas rocket on the nearby Cape Canaveral Air Force Station scheduled for 6 May. A lengthy repair job in Endeavour could see the shuttle mission slip to perhaps 9 or even 10 May.
"We're set up to try for Monday. We can try all the way through Wednesday and then we have to stand down for the Atlas," said Mr Leinbach. "If we have to change out that box - the load control assembly - then that's a significant deal, and the re-test on that box is quite extensive. A full two days."
Local police were expecting up to 750,000 people to gather near the Kennedy spaceport to watch Endeavour's ascent. The impeding retirement of the shuttle programme has drawn huge interest in the final flights of the orbiters.
Atlantis underbelly
The Discovery ship was the first to bow out with a mission in February/March. It is now being dismantled and made safe for museum display.
Endeavour is next. After its 14-day trip to the space station, it will be prepared for exhibition at a science centre in California.
The Atlantis orbiter will end the shuttle programme with a mission in June or July.
Nasa hopes by the middle of the decade to be buying crew transportation services from a range of commercial providers.
President Obama was making his way to Florida from a tour of the tornado-ravaged State of Alabama when Endeavour's lift-off was called off.
But he decided to continue on to Kennedy anyway, taking time to talk with staff and look around the facilities. Mr Obama, first lady Michelle Obama, and daughters Sasha and Malia were shown the Atlantis shuttle in the building where it is being processed for flight.
They also met US Navy Captain Kelly and his crew, who by then had changed out of the orange escape suits worn during a shuttle launch.
"They were just talking about the mission and what they were going to do," Bob Cabana, the director of KSC, told reporters.
"The girls asked some great questions when they were touring Atlantis of [astronaut] Janet Kavandi. They were really enthused, as were the president and the first lady."
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