Friday marks the beginning of a somber time of year for NASA, commemorating the first of three spaceflight disasters that have claimed the lives of 17 astronauts over the last 40 years.

NASA will commemorate the 20th anniversary of the Challenger accident during a live NASA TV broadcast on Jan. 28 at 10:00 a.m. EST. Click here.

Saturday also marks the 20^th anniversary of the space shuttle Challenger accident. The orbiter was destroyed 73 seconds after liftoff on Jan. 28, 1986 when rocket booster seal failed, leading to a subsequent fireball and the deaths of all seven astronauts aboard - including Christa McAuliffe, the first school teacher to launch spaceward. 

NASA will also honor the seven STS-107 shuttle astronauts lost in the 2003 Columbia accident next week. The Columbia orbiter broke apart during reentry on 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 chief Michael Griffin said in a news conference last week. "Spaceflight is the most technically challenging things nations do...it is difficult, it is dangerous and it is expensive, given the technology we have today."

NASA held an agency-wide Day of Remembrance on Jan. 26 for all three accidents.

Each fatal accident grounded NASA spacecraft as the agency rooted out their 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 accident 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.

STS-121 Commander Looks Toward Launch

"We've got to pay attention to the past so that we don't repeat it," Lindsey said.

Lindsey's STS-121 mission, currently set to launch no early than May 3, will mark NASA's second shuttle flight since the Columbia disaster and complete a series tests designed to increase shuttle safety.

Remembering the fallen

Astronauts Gus Grissom, Ed White and Roger 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.

As it happened... To hear Mission Control transmissions of the countdown, launch and announcement of the chilling disaster, choose .AIFF or .WAV format (1.8 MB). The audio runs four minutes, beginning from 30 seconds before liftoff, and continuing through confirmation of the explosion.

"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, D.C., 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, but then had to relearn it after the loss of Columbia, Logsdon said.

The agency is now 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 investigation board following the Columbia accident.

On Feb. 1, 2003, Columbia's seven STS-107 astronauts had already accomplished a whirlwind science mission that kept them working in round-the-clock shifts when tragedy struck during reentry. 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 reentry flight controller during Challenger's ill-fated final flight and is now lead shuttle flight director for NASA's STS-121 mission.

"Even after Challenger, I was never afraid to step up in a meeting," Ceccacci told SPACE.com. "But you can see now that people understand that there's a lot more urgency."

Pushing forward

In Their Own Words... Remembering Columbia's Crew A SPACE.com TV exclusive > Click to Watch

"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, the astronaut added.

"I think that you could wake up in the morning, and until you go to bed at night, and even while you sleep, wherever you are, you could look at multiple things that came out of the space program," Lindsey said. "It impacts everything that we do."

Some space experts believe that, statistically, another spaceflight accident will occur in the future, forcing NASA or other space agency to once again take a close look at the processes and the risks involved in human spaceflight. NASA's chief 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."