The National Aeronautics and Space Administration on Tuesday began the countdown for Friday's launch, scheduled for 19:38 pm (2338 GMT) at the Kennedy Space Center in Cape Canaveral on Florida's east coast.

NASA officials said the shuttle was in tip-top shape, while forecasts gave a 70 percent chance of favorable weather as winds were expected to nudge clouds away to give Atlantis an open sky for the launch.

"The team is ready to go and we're just really excited to be at this point after a very long and arduous spring and a lot of really hard work by the entire team," LeRoy Cain, launch integration manager for the Space Shuttle Program, told a news conference Wednesday.

The seven shuttle astronauts -- all American men -- arrived in Cape Canaveral Monday evening from their base at the Johnson Space Center in Houston, Texas, as they prepare for the 21st shuttle mission to the International Space Station (ISS).

With the loading of the shuttle's external fuel tank scheduled to begin at about 9:30 am (1330 GMT), launch was set for 7:38 pm (2338 GMT) Friday.

"All our systems right now are in great shape -- we're tracking no constraints that are show-stoppers," NASA Test Director Steve Payne told reporters.

"Teams are ready and everybody's eager to launch -- we're looking forward to a successful assembly mission," he said.

If a technical glitch or nasty weather force a delay, NASA has until June 12 to launch the shuttle, but Kathy Winters, the launch weather officer, was optimistic Wednesday. There was a slight risk of thunderstorms in the area, but they should blow further west and clear by launch time on Friday, she said.

After June 12, NASA would have to wait a few more days to blast off to allow the launch of a military satellite from the Cape Canaveral military base. NASA and the Air Force share the same radars to track shuttle and rocket launches.

Atlantis will be the fifth shuttle mission since the Columbia tragedy of February 2003, when the orbiter disintegrated as it re-entered the atmosphere, killing its seven astronauts.

The shuttle returned to space in July 2005 with a Discovery mission focused on making space flight safer. A second safety mission in July last year proved successful, paving the way for the resumption of ISS assembly missions.

After that, NASA launched two complex construction missions in September and December that the space agency hailed as major successes.

During their 11-day mission, the Atlantis astronauts will install a new, 16-tonne truss segment on the ISS and deliver a third set of solar panels as well as batteries for the orbiting laboratory.

Three spacewalks lasting six and a half hours each are planned on the fourth, sixth and eighth days of the mission.

NASA plans at least 13 shuttle missions to finish the 100-billion-dollar station by 2010, when the US space agency retires its three-shuttle fleet. The delay of the Atlantis launch forced NASA to cut back its 2007 shuttle flight program from five to four launches.

After Endeavour and Discovery shuttle missions in August and October, Atlantis will fly back to space in December to deliver Europe's Columbus laboratory, which will be attached to the ISS.

The mission flying Friday is led by Commander Frederick Sturckow, a marine colonel, who will be joined in the cockpit by co-pilot Lee Archambault, an air force colonel. The crew includes mission specialists James Reilly, Patrick Forrester, Steven Swanson and John Olivas.

The seventh passenger, Clayton Anderson, will stay behind at the ISS while Atlantis brings back to Earth flight engineer Sunita Williams, who has been working in the space station since December.

earlier related report
Atlantis is 'Go' for Launch
At today's Launch Readiness News Conference, the Mission Management Team announced that they were in agreement that Space Shuttle Atlantis is ready to fly on Friday.

"The team is ready to go and we're just really excited to be at this point after a very long and arduous spring and a lot of really hard work by the entire team," said LeRoy Cain, launch integration manager for the Space Shuttle Program. "I'm very proud of the team."

NASA Launch Director Mike Leinbach reported that the countdown is going fine and there no significant issues to report.

The forecast for launch day, according to U.S. Air Force Lt. Col. Pat Barrett, weather officer from the 45th squadron, remains at a 30-percent chance of violating launch constraints. A high-pressure ridge is expected to move to the north which could help clear out any thunderstorm activity over Kennedy Space Center.

At 10:30 p.m. EDT Thursday, the protective rotating service structure will be rolled away from the space shuttle. External tank fuel loading is scheduled to begin at around 9:30 a.m. EDT Friday.

The launch countdown for Atlantis officially began at 9 p.m. EDT Tuesday, at T-43 hours. Included in the countdown is nearly 28 hours of built-in hold time prior to a targeted 7:38 p.m. EDT launch on Friday.

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