It's been more than three years since the space shuttle Columbia broke apart over Texas, killing its crew of seven.
In the time since, several commissions have recommended changes and billions of dollars have been spent on shuttle modifications.
As NASA prepares to launch another shuttle this weekend, however, the agency is still struggling to rebound from the Columbia tragedy.
There are internal conflicts at all levels at NASA over the safety of flying a space shuttle again. The shuttles were never meant to fly for more than 10 years, but replacements were never built as NASA funding got bogged down in the policy debate over the U.S. role in space.
Discovery first launched in 1984 - 22 years ago.
The intensity of the latest debate surfaced at the flight readiness review two weeks ago, when Bryan O'Connor, NASA's top safety officer, and chief engineer Chris Scolese both voted to scrub the shuttle mission because they were concerned about foam coming off the tank and hitting the shuttle.
Who would have thought a four-letter word - foam - would become such a problem for NASA?
Dangers caused by dislodged foam have plagued recent shuttle missions, and some believe there is still reason for concern.
Foam from an external tank punched the hole in Columbia's wing, causing it to disintegrate over Texas. A hat-shaped piece of foam came off during Discovery's launch last year. It missed the shuttle, but caused the shuttle fleet to be grounded for a year until NASA could come up with a solution.
NASA administrator Mike Griffin believes the agency has solved the problem.
"The piece of foam that brought down Columbia is no longer there. It doesn't exist on the machine. The piece of foam that did not harm Discovery last August - but which did come off - is no longer there," he said.
Griffin believes this shuttle is much safer than any previously flown, and he recognizes there may be no more chances if problems continue.
"Given our history now with this shuttle, the future of the shuttle depends on every single flight. We cannot have any more bad flights on the shuttle," he said.
Chance for Another Columbia Disaster?
Some questions remain, as the preparations continue for Discovery's planned Saturday launch in Florida.
The safety debate surrounding the latest planned launch focuses on the external tank once again - specifically, on the foam covering the brackets on the external tank, which attach pressure lines to the tank on the shuttle's ice frost ramps.
NASA's research shows foam has come off these ice frost ramps for at least 20 percent of previous shuttle flights.
"We did what we call a worst analysis. We assumed a piece came off at the worst possible time. We assumed that we put it on the transport line that took it to the worst possible place, and we assumed it hit the weakest place," said space shuttle program manager Wayne Hale.
"These assumptions drive you to a risk that is on the order of one in 75 chance of having another Columbia, which is not good," Hale said.
Hale says foam will come off this external tank during this next launch, but he believes none of the pieces will be large enough to hurt Discovery.
This will be the fourth flight for Cmdr. Steve Lindsey, and he says he understands the risk.
"We've always understood the risks. We can't be perfectly safe. It's a dangerous business. But we have made the vehicle as safe as it can possibly be. I have told my family that if I ever felt that was not case, I would walk away from the flight," he said.
NASA will launch in a gentler mode on this next mission to baby the tank during the extreme aerodynamics of launch.
Ascent flight director Steve Stich says it's a technique - called a "Low Q" launch - that NASA has used before.
"We typically throttle the engines back to 72 percent on a nominal mission. For 'Low Q,' we throttle back to 67 percent, and we go through the atmosphere a little bit slower. So the loads on the vehicle are a little bit less," Stich said.
"If you imagine you are in a car and you stick your hand out the window of a car, if you are going real slow you don't feel much load on your hand, as you go faster and faster if you stick your hand out you feel a lot more pressure on your hand," Stich said.
Conserving Power to Extend Mission
The Discovery is scheduled for a 12-day mission. If the crew can conserve enough power, it will stay in space for 13 days.
How does the crew do that? By turning the lights off every 45 minutes when the space shuttle moves into daylight as it orbits Earth.
Astronaut Piers Sellers is one of the two spacewalkers on this flight. He remembers his first shuttle launch.
"It was a surprise. Nothing really prepares you for it," he said. "I thought the vehicle was coming apart. When the solid rocket boosters went off, I thought there had been an explosion. It really is. ... It's like you are being kicked off the face of the planet like a soccer ball."