The final launch of the space shuttle Endeavor hit yet another snag as engineers battle to resolve an electrical problem that scuttled Friday's launch attempt.
As late as Saturday astronauts and families were hoping for a launch attempt as early as Monday, but more extensive work is still needed to fix a faulty heater system in the engine compartment.
The heaters keep fuel from freezing in the line, critical in preventing it from rupturing in the cold vacuum of space.
Things happen fast. We are now all aboard (a plane) for return to Houston. Be back in a few days, said Endeavour's pilot, Gregory Johnson on Twitter.
The NASA space shuttle was scheduled for lift-off on Friday afternoon in its historic final flight, to deliver the $2bn Alpha Magnetic Spectrometer particle detector to the International Space Station.
Officials had expected up to 500,000 people to descend on the area for the event, including President Barack Obama and his family.
Endeavour is the second-to-last shuttle mission for the 30-year US program. Atlantis will mark the end to the program in in June and July.
NASA is winding down its space shuttle operations this summer as it tries to save money, which will leave American and European astronauts with only Russian rockets as options for going into space.
In February, the space shuttle Discovery, NASA's oldest and most travelled spacecraft, made its final voyage into space.
The country is focusing on inspiring 3rd part companies to put astronauts in orbit using privately run launch, transport and services companies.
NASA will now focus its resources on deep space exploration, such as potential landings on asteroids and, eventually, Mars.