Countdown Begins for Sunday's Shuttle Launch

Atlantis' six astronauts arrived at the Kennedy Space Center on Thursday as the countdown clock began ticking toward a Sunday launch of the space shuttle.

"We have a saying back in Texas, 'It's time to walk the walk,'" said Brent Jett, Atlantis' commander, after arriving from Houston by training jet. "We are ready for the challenge ... All we need is a little good weather on Sunday and we'll be out of here."

And good weather appeared likely. There was only a 30 percent chance it would prevent a liftoff around 4:30 p.m. Sunday.

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If the launch is delayed that day, for weather or any other reason, the space agency would keep trying over several more days.

The weather forecast actually improves on Monday and Tuesday as a high pressure ridge moves north. Forecasters were keeping an eye on a tropical wave that could develop into a tropical depression by the Lesser Antilles, said Kathy Winters, shuttle weather officer.

"Overall the weather is looking pretty good," Winters said.

Launch managers foresaw no technical problems, said Steve Payne, NASA test director. "Atlantis is in excellent shape," he said.

This mission is the start of a renewed effort to finish building the international space station before the cargo-carrying shuttles are retired in 2010.

Construction has been delayed since the Columbia accident in 2003, which killed seven astronauts. The two space missions since that time have been focused on testing safety improvements on the spacecraft.

Atlantis will carry a 17½-ton addition, costing $372 million, from which two solar wings will be opened up. The solar arrays eventually will provide a quarter of the space station's power when it is finished.

The addition arrived at the Kennedy Space Center almost seven years ago, said payload manager Robbie Ashley.

"It has been a long time coming," he said.

Atlantis' crew, who have been training together for more than four years, includes pilot Chris Ferguson and mission specialists Joe Tanner, Dan Burbank, Heidemarie Piper and Steve MacLean of the Canadian Space Agency.

Crew members will take three spacewalks during the 11-day mission.

"Walking out to the pad on Sunday will be much like walking into an Olympic stadium for your athletic event," said MacLean, who was on the Canadian national gymnastics team three decades ago. "With a team like this ... I promise you we'll bring home a gold medal."