As NASA looks toward its next shuttle mission — one that is critical for the agency’s human spaceflight program — agency employees and many others involved in the program are stopping to mark the anniversaries of NASA’s three greatest tragedies.

Those anniversaries of the Apollo 1 fire that killed three astronauts and the two shuttle accidents that claimed the lives of a total of 17 astronauts occur within just six days of each other.

On Jan. 27, 1967, astronauts Roger Chaffee, Virgil (Gus) Grissom and Ed White perished in a fire that consumed their Apollo 1 spacecraft while it sat atop its launch pad as NASA worked feverishly to send Americans to the Moon.

Twenty years ago on Jan. 28, 1986, the Space Shuttle Challenger was destroyed just after liftoff when a rocket booster seal failed, causing an explosion that ripped the shuttle apart, killing all seven astronauts aboard.

The Columbia orbiter broke apart during re-entry Feb. 1, 2003, after a successful 16-day science mission. Wing damage sustained during launch by a chunk of fuel tank insulation was later cited as the accident cause.

“This is a time to think about those kinds of losses,” NASA Administrator Mike Griffin said in a news conference last week. “Spaceflight is the most technically challenging thing nations do … it is difficult, it is dangerous and it is expensive, given the technology we have today.”

Each fatal accident grounded NASA’s human spaceflight program as the agency rooted out the causes and dealt out new safety plans before again launching astronauts into space. It took more than two years following both the Challenger and Columbia accidents before NASA launched another shuttle — most recently with last year’s STS-114 flight aboard Discovery on a test flight, which proved that still more work was needed to prevent fuel tank debris at liftoff.

“The anniversaries remind us that we can never be complacent about anything,” astronaut Steven Lindsey, commander of NASA’s next shuttle flight STS-121, said in an interview. “[They] help us remind each other, each year, to refocus … because the next several years, that’s all we’re going to think about, but what about 10 years from now? If we’ve been successful for 10 years and haven’t had an accident, that’s what you worry about. We’ve got to pay attention to the past so that we don’t repeat it.”

Grissom, White and Chaffee died aboard Apollo 1 in 1967 during a routine training test overseen by flight controllers. But the 1986 loss of Challenger and its crew occurred on national television and in full view of spectators who turned out for the launch.

In addition to schoolteacher Christa McAuliffe, Challenger’s STS-51L mission commander Francis “Dick” Scobee, pilot Michael Smith, mission specialists Judith Resnick, Ellison Onizuka, Ronald McNair and payload specialist Gregory Jarvis were killed in the accident.

“When you look back at all these accident anniversaries coming within a few days of each other, they’ve had a cumulative effect that suggests how important a well-designed crew-carrying vehicle is,” said John Logsdon, director of the Space Policy Institute at George Washington University in Washington, in a telephone interview. “The point that’s been made over and over again is that the shuttle will always be an experimental vehicle.”

NASA first learned that lesson after the Challenger accident, Logsdon said, but then had to relearn it after the loss of Columbia.

The agency now is developing a new capsule-based spacecraft — the Crew Exploration Vehicle — to launch atop a shuttle booster-derived rocket. A separate cargo launcher is expected to carry heavy payloads into orbit.

“It unfortunately took two shuttle accidents to get NASA away from its dependence on the shuttle for future human transportation,” said Logsdon, who also served on the accident investigation board following the Columbia accident.

On Feb. 1, 2003, Columbia’s seven STS-107 astronauts already had accomplished a whirlwind science mission that kept them working in round-the-clock shifts when tragedy struck during re-entry. STS-107 commander Rick Husband, pilot William McCool and mission specialists Kalpana Chawla, Laurel Clark, David Brown, payload commander Michael Anderson and payload specialist Ilan Ramon — Israel’s first astronaut — were lost in the accident.

“There’s not a day that goes by that I don’t think about the Columbia crew,” Lindsey said.

The Challenger and Columbia accidents were devastating losses and the lessons learned from them — both mechanically and culturally — came at great cost, said Tony Ceccacci, who served as an ascent and re-entry flight controller during Challenger’s ill-fated final flight and now is lead shuttle flight director for NASA’s STS-121 mission.

“Even after Challenger, I was never afraid to step up in a meeting,” Ceccacci said. “But you can see now that people understand that there’s a lot more urgency.”

The very public loss of Challenger and Columbia were vivid reminders of the risks inherent to human spaceflight, astronauts said.

“There’s been a perception for as long as I’ve been in the program until this recent accident that spaceflight’s routine, that’s the public perception,” said Lindsey, who joined NASA’s astronaut corps in 1995. “It wasn’t until I came here and started getting involved that I realized how close to the edge we always are when we fly this, and recognize the inherent danger in what we do. It’s not routine.”

But the results, including scientific research, unexpected spin-offs and pushing the boundaries of human exploration, are worth the risk, he said.

Some space experts believe that, statistically, another spaceflight accident will occur in the future, forcing NASA or another space agency to once again take a close look at the processes and the risks involved in human spaceflight. Griffin also said that the progress of human spaceflight will likely suffer painful setbacks, much like the early air industry, adding that the lessons learned from each experience will lead to safer craft.

“I know that in the course of this, there will be other opportunities to learn, and they will be sober opportunities surrounded with black crepe,” Griffin said. “But we will learn in the same way that the nation and the world learned how to do air transport, and it will be difficult.”

Risk will always go hand-in-hand with human spaceflight, Lindsey added. “If we want a completely safe program, then we shouldn’t fly at all,” the shuttle commander said. “Because there’s no such thing.”