The crew of the shuttle Discovery emerged in bright sunlight for Tuesday's much-anticipated mission.
Security was tight as the astronauts made their way to the launch pad.
The shuttle flight would be only the second since the Columbia disaster three years ago.
Cameras were at the ready as crowds gathered to watch lift-off after the all-clear was given.
Safety fears had grounded Discovery but engineers were confident a crack in the insulation system was not a cause for concern.
Two attempts at the weekend had been scrapped because of bad weather but Tuesday's conditions were perfect.
The crew will carry out safety inspections and deliver supplies to the International Space Station.
Discovery is not expected to return to Earth until 16 July.