NASA postponed the launch of space shuttle Discovery until Sunday at the earliest due to possible lightning and thunderstorms near the Florida launch pad on Saturday.
"It's not a good day to launch the shuttle so we're going to try again tomorrow," launch director Mr Mike Leinbach said, with the countdown clock stopped at nine minutes.
Weather was a persistent worry leading up to Saturday's scheduled launch, with storm clouds hovering over Kennedy Space Centre. The forecast for Sunday is no better, with a 60 percent chance that storms could force a delay of that launch too.
Sunday's preferred launch time is 3:26 p.m. (8:26 p.m. British time).
This is only the second shuttle mission since the fatal Columbia accident in 2003, and is a critical turning point for NASA.
If Discovery is critically damaged on launch or afterward, and astronauts cannot fix the problem in orbit, the seven crew members would stay on the Space Station and wait for rescue, but NASA chief Mr Mike Griffin has said it could be the end of the shuttle program.
That would leave the $100 billion International Space Station incomplete and would also mean the United States has no way to carry humans to space.
Mr Griffin decided to launch the shuttle over the objections of the US space agency's head of safety and its top engineer, who wanted the mission delayed to allow more work on the fuel tank and its insulating foam.