Flags around NASAs central Florida spaceport were flying at half staff, and both NASA and contractor workers observed a voluntarily moment of silence to honor the Challenger astronauts, who were killed after the shuttle exploded 73.6 seconds into the ill-fated flight.

"Everybody who was here at the time remembers theres no question about that," said Jack King, a spokesman for NASAs prime shuttle contractor, United Space Alliance. "And many of them remember every time we launch to make sure it never happens again."

With five men and two women onboard, Challenger was engulfed in a giant fireball after a seal on its right hand solid rocket booster failed, allowing hot gasses to escape its metal casing.

The nose of the 149-foot (45.2-meter) booster pivoted into the shuttles fuel-filled external tank, triggering an explosion that led to the aerodynamic break-up of the vehicle.

Killed were mission commander Francis "Dick" Scobee, rookie pilot Michael Smith, mission specialists Ronald McNair, Ellison Onizuka and Judy Resnik, and two payload specialists: Gregory Jarvis and Christa McAuliffe.

An educator from New Hampshire, McAuliffe had been selected to become NASAs first "Teacher-In-Space."

NASAs hushed tribute came as 200 city officials and residence gathered in nearby Titusville to honor the Challenger crew and the Apollo 1 astronauts, the latter of whom died 35 years ago Sunday in a launch-pad fire at Cape Canaveral Air Force Station.

Virgil "Gus" Grissom, Ed White and Roger Chaffee perished when a flash fire swept through their Apollo 1 spacecraft during what was supposed to be a routine practice countdown.

"It was a tragic accident, and a severe blow to the U.S. space programs race to beat the Soviet Union to the moon," former shuttle payload specialist Sam Durrance told the crowd at a memorial plaza built to honor the two lost crews.

But "Americas political leaders, NASA and its contractors reacted with tenacity and dedication to find the problems and correct them, to keep the dream of spaceflight alive," he added.

And two-and-a-half years later, Durrance said, "the world watched in awe as two American astronauts walked on the surface of the moon."

The nation responded in a like manger to Challenger disaster, Durrance said.

"It was a tragic accident, and a severe blow to the U.S. space program. (But) once again, Americas political leaders, NASA and its contractors reacted with tenacity and dedication, to find the problems and correct them, to keep the dream of spaceflight alive," he said.

And then in late September 1988, shuttle Discovery set sail on a mission that returned the nation to space once again.

Durrance, who now heads up a state of Florida space research organization, said the same type of resolve now should be applied toward NASAs troubled International Space Station project, which is facing an anticipated $4.8 billion cost overrun.

The projected shortfall has prompted NASA to at least temporarily shelve plans for a U.S. habitation module and an American crew rescue vehicle components considered key to expanding station staffing and carrying out a robust research program.

"The International Space Station is undoubtedly the largest, most complex international engineering and scientific project ever undertaken," Durrance said.

In the span of the past 18 months, he noted that NASA and its international partners have taken a once-vacant station and added crew quarters, a $600 million U.S. power tower, a 1.4 billion U.S. laboratory, a $600 million Canadian robot arm and two airlocks.

Science research began in earnest last year, and full-time crews have continuously occupied the station since November 2000.

"We should be celebrating these remarkable achievements," Durrance said.

But instead, he noted that the centerpiece of Americas human space flight program has encountered severe cost and management difficulties that could damage its value as a platform for conducting world-class science the purported goal of the project.

"Now this of course is not a dramatic, tragic event like Apollo 1 or the Challenger accident," Durrance said. "But I believe it is again time for Americas political leaders, NASA and its contractors, to react with tenacity and dedication, to find the problems, correct them and keep the dream of spaceflight alive."

Held annual at a city park with a view of NASAs shuttle launch pad, the astronaut memorial event is staged annually by the city of Titusville.

A church choir sang patriotic songs, and local veterans groups presented the colors. School children read biographies they had written about the fallen astronauts, and bouquets were placed on plaques honoring each of the Apollo 1 and Challenger crewmembers.

An apple and flowers were placed on the plaque that pays tribute to McAuliffe. An oversized American flag was lowered to half-mast and taps were played at the end of the ceremony.

Columbia, meanwhile, will be launched next month on NASAs 108^th shuttle mission. The flight will be the agencys 83^rd shuttle mission since the Challenger accident.