The launch of Nasa's Discovery shuttle has been put back until the end of May because scientists want more time to carry out final safety checks.
The American space agency's shuttle fleet has been out of action since February 2003, when Columbia broke up, killing all on board.
The lift-off has been delayed until 22 May - a week later than planned.
Scientists have spent the past two years working on improving the safety of the vehicles.
One of the team said: " I'm very comfortable and very confident in all the changes we have made."
Columbia disintegrated when returning to earth, killing seven astronauts.
There have been 113 shuttle missions. Columbia was the second fatal accident after Challenger blew up in 1986.