Endeavour's crew head back to their quarters in the Astrovan after the postponement
Nasa has delayed the launch of its Endeavour shuttle by at least 48 hours because of technical problems in the last hours before lift-off.
The orbiter is being held on the ground while engineers investigate a hitch with the vehicle's onboard heaters.
The youngest of America's reusable spaceplanes is set to deliver a $2bn (1.2bn) particle physics experiment to the International Space Station (ISS).
US President Barack Obama was due to attend the lift-off in Florida.
Also present at the Kennedy Space Center is Endeavour commander Mark Kelly's wife, Gabrielle Giffords, the Arizona politician shot in the head by a gunman in January.
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 suited up and were making their way to the shuttle. By tradition, their "Astrovan" would 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.
Managers and engineers are now discussing how long a turn-around they need to deal with the two heaters associated with shuttle Endeavour's Auxiliary Power Unit 1. More than 48 hours may be required.