Shuttle readied for liftoff despite worries

One postponement already
Discovery's 12-day mission, which will be only the second shuttle flight since the Columbia accident, already was postponed once, from May to July. A faulty fuel tank sensor was blamed _ much like it was last summer when a similar problem forced NASA to delay launching Discovery by several weeks.

Since Discovery's flight last year, technicians also have replaced or removed almost a third of the shuttle's 16,000 gap fillers. During last year's mission, two of these heat-resistant strips came loose, jutting from the shuttle's belly, and an astronaut had to remove them in a high-stakes spacewalk to avoid any harm to the shuttle on its return flight.

Discovery also has stronger insulation tiles around the vulnerable spot of the nose landing gear door, a sturdier tire and wheel system and new cameras attached to the solid rocket boosters that can capture more images of falling foam or other dangers to the shuttle and crew.

Efforts to get the shuttles flying again weren't just hampered by technical problems but by Mother Nature and a series of worker accidents.

Hurricane Katrina caused almost half a billion dollars in damage to two southern facilities where rockets are tested and designed. At Kennedy Space Center, Discovery's robotic arm was bumped by a platform.

Discovery's mission, like the previous one, is considered a test flight. Astronauts will be loaded up with experimental tasks, trying different methods of inspecting the vehicle for damage. Any missions that follow this will be dedicated to finishing construction of the space station before the shuttle is retired — with the possible exception of that mission to service the Hubble.

Space station chores
Once at the station, Discovery will be leaving one of the seven astronauts behind. The European Space Agency's Thomas Reiter will return the international space station to a three-man crew for the first time since early 2003.

The shuttle crew also will deliver 5,100 pounds  (2,300 kilograms) of cargo, including an oxygen generation system that can support a space station crew of six, and a laboratory freezer. They will haul back 4,700 pounds (2,100 kilograms) of cargo, including lots of trash.

"We have a lot of stuff onboard the space station that we need to get rid of," said U.S. flight engineer Jeff Williams, who currently lives on the station along with Russian commander Pavel Vinogradov.

Astronauts Piers Sellers and Mike Fossum will make two spacewalks, with a third one possible, adding an extra day to the mission, to test repair techniques on the space shuttle's thermal protection system. It will be Fossum's first spacewalk.

The spacewalkers will replace a cable connecting the space station's external railroad car and test the stability of a boom for use as a platform for shuttle repairs. The 50-foot (15-meter) boom, hooked to Discovery's 50-foot robotic arm, will swing the astronauts to various parts of the station and shuttle.

"You're standing at the end of it at night, so you'll feel like you're standing on a diving board or standing at the top of a telephone pole or hanging down from a ceiling," Sellers said. "It's disorienting, there's no question ... You won't always know which way is up."

Backup plans in place
Discovery's astronauts also will gather 3-dimensional images of their ship's wings and nose cap using a laser imager at the end of the boom. From the space station, Williams will photograph images of the shuttle's underside as it does a flip before docking.

If something looks suspicious, the astronauts can try to repair any damage during a spacewalk. In a worst-case scenario, the astronauts could stay at the space station for 81 days until a rescue shuttle is sent up to bring the crew back. Space shuttle Atlantis, the potential rescue vehicle, is scheduled to be moved to the launch pad in late July.

NASA managers said they'll only breathe easily once Discovery returns safely to Kennedy Space Center.

"We are home free only when the wheels stop rolling on the runway," Hale said.

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