Johnson Space Center (search) workers wiped away tears during an emotional tribute Friday to the seven astronauts who died a year ago during the space shuttle Columbia's (search) final mission.

"To this day, I have this lump in my gut," center director Jefferson Davis Howell Jr. told about 1,000 workers who gathered under cloudy skies. He said he still asks himself, "How did you let this happen? What could you have done to prevent this?"

A chunk of foam the size of a suitcase tore a hole in Columbia's left wing 82 seconds after liftoff. The gap let in the searing gases of re-entry two weeks later as the orbiter returned home. The spaceship broke apart over Texas on Feb. 1, killing all aboard.

"I, like you, went through the terrible myriad of emotions: denial, horror, terrible grief, frustration, anger, guilt," Howell said.

Despite those feelings, Howell said workers did what had to be done, buried the dead and got back to work.

"This is exactly what this crew would have wanted us to do," he said. "Wouldn't they be upset with us if we quit? Wouldn't it be an insult to them if we gave up?"

Workers at the center, where Mission Control is located, should use the crew's example of "incredible courage, teamwork, total integrity, professional excellence, commitment to their purpose," as they continue their work, Howell said.

"That is a way we can honor them," he said. "Crew of Columbia, you will never be forgotten. Crew of Columbia, we honor you with our actions and our success."

Workers bowed their heads during a moment of silence and a recording of bells and the astronaut's names were played: "Rick Husband ... William McCool ... Kalpana Chawla ... David Brown ... Michael Anderson ... Laurel Clark ... Ilan Ramon."

On Thursday, NASA (search) employees throughout the country paused to remember the 17 astronauts who died in three separate tragedies over the years.

The Apollo 1 spacecraft fire on the launch pad killed three on Jan. 27, 1967. The Challenger launch explosion killed seven on Jan. 28, 1986. The Columbia disintegration happened Feb. 1.

On Sunday, the anniversary of the Columbia disaster, a ceremony at Florida's Cape Canaveral will honor all astronauts whose lives have been lost in the line of duty. On Monday, NASA chief Sean O'Keefe will dedicate a Columbia memorial at Arlington National Cemetery in Virginia.