Shuttle set for crucial test, then launch

The safety issues are particularly sensitive this time around because of all the changes in safety procedures made in the wake of the Columbia tragedy in 2003. Just after the launch of that mission, a piece of foam insulation flew off the fuel tank and hit the shuttle's left wing. At the time, engineers thought no damage was done. But after the catastrophic breakup of the shuttle — in which Columbia's seven astronauts lost their lives — investigators determined that the debris cracked a hole in the wing, allowing hot atmospheric gases to enter and destroy the craft from within.

FREE VIDEO Astronaut supports launch decision July 25: Astronaut Pamela Melroy talks to MSNBC-TV's Chris Jansing about the NASA's decision to launch Discovery Tuesday. MSNBC

NASA has made dozens of upgrades in the shuttle and the fuel tank since then, and has also taken measures to reform its "safety culture." More than 100 cameras will record Discovery's launch, and engineers have developed a new extension boom for inspecting the shuttle in orbit. During the 12-day mission's first of three spacewalks, astronauts will test techniques that could eventually be used to repair damage to the shuttle during this mission.

However, a task force monitoring the return to flight determined that NASA has not yet met three of the Columbia Accident Investigation Board's recommendations for heading off potential damage from ice or foam flying from the fuel tank, or for repairing such damage. The prospect of launching with a known defect could add to the perception that NASA is potentially cutting corners on safety.

When asked about such perceptions, NASA Administrator Mike Griffin said that he felt "comfortable" with the process mission managers have gone through, and that he was more concerned about "doing what's right, not what necessarily is obvious or popular."

An unexpected glitch isn't the only thing that could delay launch: Anvil clouds or storms in the vicinity of Kennedy Space Center also could force a scrub. Launch weather officer Kathy Winters put the chances of acceptable weather for liftoff at 60 percent.

If the countdown is halted Tuesday, mission managers could try again Wednesday. In all, four launch attempts could be made between now and the end of the month — with a possible extension of the launch window several days into August. After that point, NASA would have to wait until September, due to the lighting conditions and the position of the international space station.

In addition to testing safety, inspection and repair procedures, Discovery's crew is due to bring up tons of supplies to the station. Spacewalkers are to install a replacement guidance gyroscope on the station, as well as a storage platform for future construction jobs.

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