Launch Countdown Begins for Atlantis Shuttle Flight

CAPECANAVERAL, Fla. - The astronauts are here and the clock is ticking for NASA'splanned launch of the spaceshuttle Atlantis on Sunday.

Atlantisand its six-astronaut crew are poised to launch toward the International SpaceStation (ISS) at 4:30 p.m. EDT (2030 GMT) on NASA's first majorconstruction flight since 2002. With current weather forecasts predicting a 70percent chance of clear conditions on launch day, Atlantis' STS-115 astronautshad reason to smile as they arrived here at NASA's Kennedy Space Center (KSC) today.

"I hope youcan tell by the smiles on our faces that we're very, very happy to be here inFlorida to start the launch countdown," shuttlecommander Brent Jett told reporters at the Shuttle Landing Facility afterthe crew flew over the airstrip in formation twice in their T-38 NASA jets."There's been a lot of talk in the press lately about NASA being back, and Ithink we would all certainly agree with that talk. But we have a saying in Texas, 'It's time to walk the walk.'"

Jett,Atlantis pilot ChrisFerguson and mission specialists JosephTanner, DanielBurbank, HeidemarieStefanyshyn-Piper and StevenMacLean, of the Canadian Space Agency, will ride Atlantis to the ISS anddeliver twomassive trusses and a pair of solar arrays to the orbital outpost.

"When weclimb aboard Atlantis on Sunday we do so with confidence that the vehicle isready, prepared by the best technicians, engineers and managers in thebusiness," Tanner said, thanking the teams of shuttle workers to prepare hiscrew's spaceship. "We just thank you for allowing us to play a little part inyour life's work."

NASA launchcontrollers began counting down toward Atlantis' STS-115 liftoff at 12:00 p.m.EDT (1600 GMT) today, about six hours earlier than planned to allow extra timeto load the shuttle's fuel cell propellant, space agency officials said.

At aboutthat same time, pad workers were expected to close Atlantis' cargo bay doorsfor the final time after one last battery charge for its 17.5-ton solar arrayand ISS truss payload.

"Thevehicle, the launch and the flight teams, and the flight crew are all ready forlaunch," NASA test director Steven Payne said in a status briefing.

Atlantishas a 12-day launch window that closeson Sept. 7 to allow a buffer between the shuttle's flight and the planedSept. 14 launch of a Russian Soyuz spacecraft ferrying the next station crew -Expedition 14 commanded by NASA astronaut Michael Lopez-Alegria - and a spacetourist to the orbital outpost.

While thereis only a 30 percent chance of poor weather preventing a Sunday launch, thatpercentage drops to just 20 percent on subsequent days, shuttle weather officerKathy Winters said.

"Overall,the weather looks pretty good for launch day," Winters added.

The 11-daymission features threespacewalks to install the Port 3/Port 4 (P3/P4) truss segments and newsolar arrays, a $371.8 million addition to the ISS that will double its poweroutput once fully activated.

Thespaceflight is NASA's third shuttle mission since the 2003 Columbia accident, butthe first ISS construction flight since the November2002 launch of STS-113aboard Endeavour.

"All weneed is a little good weather on Sunday and we'll be out of here," Jett said.

• VIDEO: First Tasks of NASA's STS-115 Mission
• Gallery: Prepping Atlantis
• The International Space Station So Far: Five Years of Service, But Incomplete
• Complete Space Shuttle Mission Coverage
• NASA's STS-115: Shuttle Atlantis to Jump Start ISS Construction
• The Great Space Quiz: Space Shuttle Countdown