The Columbia Accident Investigation Board made 15 recommendations for safety improvements to the shuttle, including reducing the use of foam on the external fuel tank.
The space agency has sought to reduce debris created at liftoff and to detect any damage before reentry.
The foam-insulating ramp that failed on Columbia has been replaced with a heated plate that prevents ice buildup. Bolts on the external tank have also been reversed and sealed to reduce gas leaks that had weakened insulating foam.
A multitude of cameras have also been installed on the ground to allow engineers to monitor any possible problems with the shuttle as it takes off. And once in orbit, astronauts can scan the craft's exterior with a new camera and laser-powered measuring device attached to a 100-foot-long (30-meter-long) robotic arm that can reach around the spacecraft.
"We have gone far beyond the recommendations made by the investigation board," said Melissa Mathews, a NASA spokesperson in Washington, D.C.
Not 100 Percent Safe
Some critics have suggested that the space agency may be playing down the risks of damage from debris in order to resume flight quickly, a charge Mathews denies.
"We are obviously very committed to doing this correctly, and the launch window has been adjusted many times to make sure people have [had] the time to get it right," she said.
Still, Mathews admits that the risks of shuttle missions can never be entirely eliminated.
"We're saying at every opportunity that space flight inherently has some risk in it," she said. "I would imagine the public understands that. Certainly the people who are dealing with this on a daily basis are keenly aware of that."
Stanford University physicist Douglas Osheroff, a member of the Columbia investigation panel, underscored this point.
"Correcting the problems that led to the loss of Columbia and Challenger will make the shuttle system safer, but there are a lot of potential problems remaining," he said. "The shuttle will never be absolutely safe, just as we are never absolutely safe in an automobile."
Even with the recent improvements, the space shuttle's days are numbered. As part of NASA's new focus on space exploration, rather than space science, the shuttle will be retired in 2010 and replaced with a new Crew Exploration Vehicle.
The White House says it wants the U.S. to set up a station on the moon and use it as a base for travel to Mars.
Louis Friedman, executive director of the Pasadena, California-based nonprofit research group The Planetary Society, welcomes the change in direction.
"The idea that astronauts were going up there to do what we already do on Earth never really held up," he said. "The real justification is the exploration of other worlds. We have to get the shuttle flying again because it has jobs to complete, but then it has to be retired. I don't regard the shuttle itself as important in the future."
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