On Wednesday, barring weather delays or a mechanical problem, NASA's long wait will end, and the space shuttle Discovery will roar toward orbit once again.
The launching comes two and a half years after the loss of the shuttle Columbia and its crew of seven astronauts. Since then, the National Aeronautics and Space Administration has struggled on the one hand to reduce risks like that of the falling debris that doomed the Columbia, and on the other to accept the risks it cannot eliminate.
The agency's administrator, Michael D. Griffin, recently told NASA employees "this is a very risky venture," but says it is time to fly again. And outside experts who studied the Columbia disaster agree, giving an even greater impression of cognitive dissonance.
"We think that there is no reason they shouldn't fly," Harold W. Gehman Jr., the retired admiral who headed the board investigating the Columbia accident, said in a recent conference call with reporters.
NASA, Admiral Gehman said, has made great strides in meeting his board's safety recommendations. The space agency has significantly reduced the overall risk that falling foam will lead to disaster again, he said, even though a NASA panel that monitored the agency's progress in meeting those goals found that it had fallen short in three areas, including the prevention of all launching debris and the ability to repair damage in space.
"I'm sure that this next flight will be safer than the previous ones," Admiral Gehman said. But he added, "By any measure of 'safe,' this is not safe."
Steve B. Wallace, a member of the Gehman board who serves as director of the office of accident investigation for the Federal Aviation Administration, noted that if commercial air travel had the same accident rate as the shuttle program, 566 airliners would crash every day.
Yet, Mr. Wallace said, he considers himself a strong supporter of human spaceflight. Asked whether he would get on the shuttle, given the chance, he replied, "Absolutely."
The members of the shuttle crew have repeatedly said that they are comfortable with the safety measures NASA has taken and that they are eager to begin their mission, which will involve resupplying the International Space Station, replacing a broken station gyroscope and testing in-space inspection and repair techniques. The crew is made up of the mission commander, Col. Eileen M. Collins; its pilot, James Kelly; Stephen Robinson; Soichi Noguchi; Andrew Thomas; Wendy Lawrence; and Charles J. Camarda.
Within NASA, officials have spent a great deal of time and effort rallying the troops to accept what they call reasonable risks. Last October, N. Wayne Hale, the deputy manager of the shuttle program, sent an open memorandum "from my heart," explaining that some risk will always remain.
"There is nobody in the shuttle program management, or the agency management that has any delusion that we can reach perfection," he wrote.
"Our collective job is to understand the risk, mitigate it as much as possible, communicate accurately all round about the risk remaining, and then decide if we can go on with that risk."
In April, Mr. Hale returned to the topic with another memorandum about "the American character," which he described as "an innate optimism and the bold willingness to take on risks if they hold the promise of a better tomorrow."
Accepting risk is a mental calculus that balances danger against benefit, said Gerald J. S. Wilde, a professor of psychology at Queen's University in Kingston, Ontario.
"The task of an individual or group is not to minimize risk, but to optimize it," Professor Wilde said.
The argument that the inherent risks of shuttle flight must be accepted as part of the overall risk of spaceflight, however, sits poorly with those who believe the shuttle system is uniquely dangerous and that shuttles should no longer be flown.
Alex Roland, a professor of history at Duke University and a former historian at NASA, said that the shuttle is, simply, an "inherently flawed" vehicle.
Professor Roland predicted that the next few flights would be safe, but that "safety cannot be sustained over time" because problems build up and NASA tends to relax its vigilance.
"Eventually the law of averages catches up with them," he said.
John Pike, the director of GlobalSecurity.org, a private research group, said that after the Columbia disaster, "I thought there was a case for not returning to flight" with the shuttle at all.
Unexpected risks, Mr. Pike said, will always be there. But thanks to safety measures after previous disasters, he said, "I can guarantee you that it's not going to kill anybody the way it killed the Challenger crew, and it's unlikely to kill anybody the way it killed the Columbia crew."
But the shuttle, Mr. Pike added, is "perfectly capable of figuring out a new way of killing them."
No one understands that better than NASA. But officials say that they have made the shuttle as safe as they can, and that the risk of backing down is greater than the risk of launching again.
In his April memorandum, Mr. Hale asked, "Do we have the character to dare great deeds?"
He added, "History is watching."