"Every time that I strapped on the shuttle, I felt confident everybody had done what I thought was everything possible to make sure we were ready to go," said astronaut Scott Altman.
Altman said he wouldn't hesitate to fly on the shuttle again. Neither would former astronaut Sally Ride, who has served on the boards that probed both the Columbia and Challenger accidents.
James Voss, who retired this month after four flights and a nearly six-month stay on the international space station, said decisions about the Columbia flight were those "any reasonable person would have made with the information that we had," he said.
In recent years, Altman said, the space agency has done a good job of addressing problems it uncovered, noting last year's grounding of the fleet after hairline cracks were found in fuel lines. Flights resumed after the cracks were welded.
However, the probe of the Columbia accident revealed NASA's failure to see the dangers of fuel-tank foam insulation flying off and hitting the shuttle — the leading theory for the Feb. 1 disaster that killed seven astronauts.
"Maybe we just haven't imagined enough what could go wrong," said Altman, who commanded Columbia on its last mission before the accident. "As you try to make safety improvements, you go at what you perceive are the biggest risks. Was it a bad decision to go after things that were more of a threat before? I'm not sure I could say it was."
Just last week, new fears arose over giant bolts designed to break off when the rocket boosters separate from the shuttle's external fuel tank. Investigators found that pieces of the 2-foot-long bolts might not land in the canisters designed to catch them.
The bolts are not considered a cause of the Columbia disaster, but could threaten future shuttles, investigators said. NASA is now working to fix that problem along with the breakaway foam.
Altman said that returning to space will involve managing risk but acknowledging it will always be there. He predicted changes from the Columbia investigation will probably be "incremental rather than revolutionary."
The investigative board has said it wants to improve hands-on shuttle inspections, which decreased due to greater reliance on contractors. Until the mid-1990s, NASA itself did more than 40,000 inspections before a shuttle flight, compared with 8,500 right before the latest accident.
But former astronaut Tom Henricks, who commanded two shuttle missions and was a pilot on two others, said more inspections aren't necessarily the answer.
"Safety and reliability have to be designed into systems. If that's done properly it reduces the requirement of inspections," he said. "Every time you touch a system like that you are opening yourself up (to) breaking the components. You're better off just leaving it alone. One reason you saw the number of inspections decrease over time was because things were designed better."
However, Henricks, who flew twice on Columbia, said he felt NASA began to accept too much risk not long after the 1986 Challenger accident.
"The pendulum (after Challenger) had swung to as conservative as they could make it," he said. "But then that pendulum started swinging back almost immediately and it was very prevalent by the time we were going to (the Russian space station) Mir. We were still sending Americans to Mir after a fire and a collision. Near the post-Challenger time frame, that wouldn't have happened.
"If leadership will accept foam coming off, then why would somebody lower in the organization get all that concerned about it? Culture is driven top-down," he said.
Henricks retired in 1997, turning down one last mission, to Mir, in part because of safety concerns with the Russian space station and the risks that come with any shuttle flight.
"We can analyze it, spend billions of dollars before we fly again and it will not be safer," he said. "To get more reliable we have to move on to new designs, processes. The whole thing needs to be redone instead of marching down the path we did after Challenger."
Retired astronaut Charles Bolden, who was chief of safety at Johnson Space Center after Challenger, said people will continue to second-guess what happened with Columbia. But he said managers and others at NASA made the best decisions possible.
"I think we tend to be much harder on ourselves than people from the outside tend to be. We take a lot of blame on that maybe we should not and we beat ourselves up when maybe we should not," said Bolden.
By Juan A. Lozano