Nasa postpones Endeavour mission
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 said the next try will be on 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.
Mr Kelly is married to wounded congresswoman Gabrielle Giffords, who is in Cape Canaveral to watch the launch after making a remarkable recovery since she was shot at a constituency event in Arizona in January.
President Barack Obama planned to watch the launch but had not yet arrived. Mr Obama and his family still plan to visit the space centre for a tour.
Launch director Mike Leinbach said the next try will be on Monday at the earliest - but hinted at an even longer delay. Technicians will have to crawl into the engine compartment to track a suspected electrical short in a power distribution box, and that will take time.
Mr Leinbach said the delay was "unfortunate" for Commander Kelly and his crew. The six astronauts were en route to the launch pad when the countdown was halted. Nasa's silver-coloured van did a U-turn at the launch control centre and returned them to crew quarters.
"It's the nature of our business," Mr Leinbach said. "We'll fly no orbiter before its time, and today she just wasn't ready to go."