CAPE CANAVERAL, FLA. -- NASA marked the 20th anniversary of the space shuttle Columbia tragedy with somber ceremonies and remembrances during its annual tribute to fallen astronauts on Thursday.

More than 100 people gathered under a gray sky at Kennedy Space Center to remember not only Columbia's crew of seven, but the 18 other astronauts killed in the line of duty. NASA's two shuttle accidents account for more than half of the names carved into the black granite of the Space Mirror Memorial; plane crashes are to blame for the rest.

None of the Columbia astronaut family members attended the morning ceremony. But Zvi Konikov, a local rabbi, recalled how Israel's first astronaut, Ilan Ramon, asked him before the flight how to observe the Sabbath during two weeks in orbit with multiple sunsets a day.

"Ilan taught us a powerful message. No matter how fast we're going, no matter how important our work, we must pause and think about why we're here on Earth, and that's what we're doing today. We pause to recall the memory of all those courageous souls," said Konikov.

Columbia was destroyed during reentry on Feb. 1, 2003, after a piece of fuel-tank foam came off and punctured the left wing during liftoff 16 days earlier. The shuttle broke apart over Texas, just 16 minutes from its planned Florida touchdown.

NASA managers dismissed the impact during the flight despite the concerns of others. That same kind of cultural blunder led to the loss of shuttle Challenger during liftoff on Jan. 28, 1986, killing all seven aboard, including schoolteacher Christa McAuliffe.

The Apollo 1 launch pad fire claimed three astronauts' lives on Jan. 27, 1967.

Because of the clustering of these three dates, NASA sets aside the last Thursday of every January to commemorate its fallen astronauts. At space centers across the country, flags were lowered to half-staff, with ceremonies held along with spaceflight safety discussions.

Like NASA's earlier tragedies, Columbia's loss was avoidable, said former shuttle commander Bob Cabana, now NASA's associate administrator.

"When we look back, why do we have to keep repeating the same hard lessons?" he said. "I don't ever want to have to go through another Columbia."

Besides Ramon, Columbia's last crew included commander Rick Husband, pilot Willie McCool, Michael Anderson, Kalpana Chawla, David Brown and Laurel Clark.

A ship's bell pealed after each of the 25 names were read as the ceremony drew to a close.

Bob and Diane Kalander interrupted their sailing trip from their home in Jamestown, Rhode Island, to Florida's Key West to honor the lost shuttle crews. Their daughter and her boyfriend joined them at Kennedy.

"It's fading from people's memory," Diane Kalander said. "There's been a de-emphasis on space because people say, 'Let's worry about problems on Earth as opposed to the future.' We've got to look toward the future."

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