The Atlantis was set to blast off Monday with vital supplies and spare parts for the International Space Station to extend its life past the 2010 retirement of the aging shuttle fleet.
Atlantis is due to launch at 2:28 pm (1928 GMT) from the Kennedy Space Center near Florida's Cape Canaveral carrying six astronauts and some 27,000 pounds (12,300 kilos) of gyroscopes, ammonia tanks and other equipment.
"There is a 70-percent chance of favorable weather for launch. The primary concern at this time is for low cloud ceilings," a NASA statement said.
Space agency officials said the mission was crucial as just five more shuttle launches remain before the planned September 2010 retirement of the fleet and the spare parts will mean spare years on the station's life.
"You'll see this theme in some of the flights that are going to come after ours as well," said mission director Brian Smith. "This flight is all about spares, basically, we're getting them up there while we still can."
Led by Marine Corps colonel Charlie Hobaugh, the all-male crew arrived Thursday at Kennedy Space Center from Houston, Texas, where the astronauts are based.
The 11-day space outing, to be the fifth and final shuttle mission for 2009, is set to include three space walks aimed at storing hardware on the exterior of the International Space Station (ISS).
Atlantis will also bring back US astronaut Nicole Stott, who has served as flight engineer on the ISS since August.
The launch follows Friday's dramatic revelation that a "significant amount" of frozen water has been found on the moon, a discovery hailed by NASA as heralding a giant leap forward in space exploration.
Preliminary data, uncovered after NASA sent two spacecraft crashing into the lunar surface last month, indicated the discovery of some two-dozen gallons of water in a permanently shadowed lunar crater.
Scientists had previously theorized that, except for the possibility of ice at the bottom of craters, the moon was totally dry.
Only 12 men, all Americans, have ever walked on the moon, and the last to set foot there were in 1972, at the end of the Apollo missions.
NASA has ambitious plans to put US astronauts back on the moon by 2020 to establish manned lunar bases for further exploration to Mars under a project called Constellation.
But NASA's budget is currently too small to pay for Constellation's Orion capsule, a more advanced and spacious version of the Apollo lunar module, as well as the Ares I and Ares V launchers needed to put the craft in orbit.
A key review panel appointed by President Barack Obama said existing budgets are not large enough to fund a return mission before 2020.
The White House could still decide to extend the shuttle program through 2011 to reduce American reliance on Russia's Soyuz craft for transporting astronauts to the ISS.
Ahead of the decision on the future of manned space flight, NASA is undertaking numerous efforts to broaden its public appeal, including hosting about 100 Twitter followers Monday at the Kennedy Space Center for a "tweet-up" -- the first ever at a shuttle launch.
Tweets from blast-off can be found at .nasatweetup.
Last month NASA successfully launched the prototype Ares I-X as part of its effort to build a new generation of space rocket to transport the Orion capsule in the post-shuttle era, but Orion will not be ready until at least 2015.
Source: AFP American Edition
