Countdown begins for milestone launch

Changed mission
Originally, Collins and her crew were to fly right after Columbia, in March 2003, ferrying a fresh crew to the space station. Now the mission has changed.

Discovery will still bring tons of supplies to the station. Spacewalkers will replace a balky station gyroscope and install a storage platform on the station's hull. But the mission's biggest job is to test the methods for inspecting the shuttle and fixing cracks in the shuttle's protective outer skin.

For the first time, the shuttle's 50-foot-long robot arm will stretch out a newly designed extension boom that's just as long, using sensors and cameras to check the orbiter's nose and edges of the wings.

The on-orbit inspection, along with the unprecedented imagery of the launch and readings from temperature sensors on the shuttle orbiter and fuel tank, would have detected the kind of damage that felled Columbia, NASA says.

Discovery's crew will try out three techniques for sealing cracks in the shuttle's thermal protection tiles and its reinforced wing panels: a glorified caulking gun, a daub-on adhesive applicator and screw-in plugs. But even the space agency admits that astronauts could not have fixed a Columbia-size hole.

In that event, Discovery's astronauts would have to take shelter on the space station and wait for a rescue mission. Spaulding said the next shuttle in line, Atlantis, could be launched as early as Aug. 13 if needed to retrieve a marooned crew.

Many of the shuttle's upgrades are aimed at avoiding damage in the first place: The external fuel tank and its fittings have been redesigned to minimize the risk of flying foam, and new, improved heaters have been installed on the tank to keep hazardous ice from building up.

Last month, a task force monitoring NASA's return to flight reported that the space agency still hadn't fully addressed the risks from foam and ice, but members of the task force also said that "incomplete" mark shouldn't stand in the way of this week's scheduled launch.

Then and now
Discovery's astronauts have voiced more anticipation than trepidation during the final days before flight.

"It's been two and a half years since a crew stood here before you, and that's way too long," Australian-American spacewalker Andrew Thomas told reporters Saturday night after he and the rest of the crew arrived at Kennedy Space Center. "And it is definitely time that we went back to flight and back to space."

Hauck said that sentiment isn't surprising, based on his own return-to-flight experience. "You find yourself saying, 'Let's get on board this ship and head out to space,'" he said.

Hauck recalled that he breathed a big sigh of relief after Discovery's launch in 1988, but he guessed that Collins and her crew members would have an additional nagging thought in the back of their minds all the way until landing. "You know you've got to come back home, and that's when your friends were killed," he said.

But an awareness of the risks comes with the territory. It is a part of every spaceflight, Hauck said.

"If you don't have some fears and some issues to deal with, then you probably should not be in this," he said.

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