Nasa counted down to the launch of space shuttle Discovery today, hoping to fly a crucial mission whose failure could ground the shuttle fleet permanently and leave the International Space Station unfinished.
Discovery is scheduled to lift off from the Kennedy Space Centre in Florida at 8pm Irish time on a voyage to the space station that will test repairs to the shuttle's troublesome fuel tank, which triggered the destruction of shuttle Columbia and the deaths of seven astronauts in 2003.
NASA weather officials predicted a 60 per cent chance the flight will launch as scheduled, an improvement over Friday's estimate. Lightning near the launch site is a potential hazard, but forecasters early Saturday reckoned storms would stay to the south of Cape Canaveral at launch time.
Space center workers began the process of loading super-cold fuel - liquid hydrogen and liquid oxygen - into Discovery's tank a few minutes behind schedule, but NASA said this would not delay the flight.
"We're looking forward to good weather and when we have it were going to be ready to go," Nasa Administrator Michael Griffin said at a news briefing.
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.
He has said if the shuttle encounters another major problem, he would likely shut down the programme.
Nasa has spent $1.3 billion on repairs and safety upgrades to the shuttle fleet since Columbia disintegrated over Texas in February 2003. But the work failed to completely resolve a problem with insulating foam on the fuel tank breaking off during launch.
The foam prevents the build-up of potentially damaging ice when the tank is filled with fuel. A piece of tumbling foam punched a hole in Columbia's wing on launch, allowing superheated gases to rip the shuttle apart on re-entry into Earth's atmosphere 16 days later.
The fuel tank shed foam again when Discovery was launched a year ago and NASA engineers went back to the drawing board.
"Foam is a concern. But I very strongly feel we are not risking crew for foam in this case or I wouldn't feel comfortable launching," Mr Griffin said of Discovery's current mission.
Nasa plans to house the shuttle crew on the space station to await rescue if the craft is damaged during launch.
In addition to testing the redesigned fuel tank, Discovery will tote more than 2,268 kilogrammes of equipment and supplies to the orbiting outpost. Astronauts will make two spacewalks during the mission, one to test a new 515-metre extension to the shuttle's robot arm.