The Atlantis shuttle is set for the first major International Space Station construction mission in nearly four years, but NASA kept a close eye on Saturday on storms that could delay Sunday's launch.
Atlantis will carry a new 16-tonne segment with two huge solar panels to the ISS on the first of 16 flights planned to complete assembly of the half-finished space station by 2010, when the shuttle fleet is set to retire.
The Columbia shuttle disaster in February 2003 forced a halt in the orbiting laboratory's construction.
But after two Discovery shuttle flights in the past two years aimed at improving safety, NASA declared it was ready to resume construction of the station, which is key to US ambitions of sending humans to Mars.
Safety remains a priority, however, and NASA will not take any chances, as it will employ the same thorough shuttle inspection techniques used in the Discovery missions.
Atlantis is scheduled to launch with six astronauts on board at 4:30 pm (2030 GMT) from the Kennedy Space Center on Florida's Atlantic coast, but the shuttle has only a 40 per cent chance of launching as scheduled due to thunderstorms.
Still, NASA was hopeful.
{{/usCountry}}Still, NASA was hopeful.
{{/usCountry}}"I'm pleased to announce that our vehicle, the launch team and hopefully the weather are ready for tomorrow's launch," NASA Test Director Jeff Spaulding told reporters in Florida.
The weather would improve on Monday and Tuesday, with an 80 per cent chance the shuttle could launch. The shuttle's launch window closes on September 13.
Officials were also monitoring Tropical Storm Ernesto in the Caribbean, which was moving toward the Gulf of Mexico, but it did not threaten liftoff.
NASA officials said the Atlantis mission was a critical first step in the ISS's assembly.
"This flight has to work for the next flight to occur, and the next flight to occur, and the next flight to occur," ISS program manager Mike Suffredini emphasized to reporters on Friday.
The installation of the solar panels, which will eventually provide a quarter of the station's power, is among the most complex parts of the ISS assembly sequence.
Three spacewalks are planned during the 11-day mission, which will be followed by another shuttle flight planned for December, for another assembly sequence that officials said would be even trickier.
The ISS currently weighs 197 tonnes (434,000 pounds) and will mushroom to a massive, 454-tonne (one-million-pound) structure once it is completed.
The station, launched in November 1998, allows scientists to understand the effects of living in zero gravity.
NASA Administrator Michael Griffin warned prior to Discovery's last mission that failure to complete the ISS would hurt US credibility as it attempts to convince its international partners to share its goal of sending humans to Mars.
The United States, Russia, Japan, Canada, Brazil and 11 countries from the European Space Agency are involved in the ISS.
The shuttle launch was cleared unanimously by NASA officials this month, although two of them urged the space agency to redesign foam insulation on the external fuel tank as soon as possible.
NASA has modified several foam sections to limit the amount and size of debris that falls during liftoff since a large chunk pierced Columbia's heat shield, dooming its return home.
NASA officials have said that the tank's performance has improved. Smaller pieces of foam fell off during Discovery's last launch.
The Atlantis crew will be commanded by Brent Jett; Chris Ferguson will be the co-pilot, and the four mission specialists will be Steve MacLean of the Canadian Space Agency, Dan Burbank, Heidemarie Stefanyshyn-Piper and Joe Tanner.