CAPE CANAVERAL - NASA halted shuttle launch preparations at Kennedy Space Center on Thursday so workers could contemplate a rash of recent accidents, including one that the center director said could have been disastrous.

In a videotaped message to spaceport workers, KSC Director Jim Kennedy ordered a two-hour "safety stand-down" that stopped work on NASA's three-orbiter shuttle fleet and International Space Station components.

One recent close call could have done "major damage" to Endeavour, and another incident -- a small fire at the Vehicle Assembly Building -- "could have been catastrophic," Kennedy said.

"We must stop, in their tracks, the chain of events that led me to call for this safety stand-down," he added.

Last week, workers inadvertently started a small fire during roof repairs to the 52-story assembly building. Two fully fueled shuttle solid rocket boosters were stacked on a mobile launcher platform in the building at the time.

Had the fire reached fully fueled rocket segments it could have prompted a major explosion.

Among other recent incidents:

• Endeavour escaped damage in January when workers were transferring the weight of the 75-ton spaceship between floor jacks that support the orbiter in its processing hangar.

Workers had not locked down the orbiter's nosewheel landing gear, and the ship pitched forward when the weight was shifted. The movement was stopped before serious damage could be done to the underside of the orbiter, which is covered with fragile heat-shield tiles.

• A cooling system pump package on Atlantis had to be replaced, and another had to be repaired, after workers inadvertently overpressurized a coolant line in early February.
• Discovery's robot arm was damaged March 4 when a bridge bucket carrying technicians from a payload bay cleanup site struck the crane-like lifting device. The 50-foot arm will be shipped back to its Canadian manufacturer for repairs.
• The technicians had been cleaning up glass shards from an incident that occurred on Discovery on March 3. A technician working on thermal tiles knocked a heat lamp into a handrail. The lamp broke and sprayed glass shards into the payload bay.

NASA has found no common cause for the incidents.

"However, I considered all a threat to successfully accomplishing our mission -- the launching of astronauts and payloads into space," Kennedy said.

Kennedy noted KSC workers are responsible for the safety of fellow employees as well as astronaut crews, and he made reference to the 1967 Apollo 1 fire and the shuttle Challenger and Columbia accidents in 1986 and 2003, respectively. Seventeen astronauts died in the accidents.

He also reminded workers that KSC is the only U.S. government spaceport capable of launching humans into orbit, and its facilities are critical to the future of American human spaceflight.

"We understand that incidents may occur, but a major mishap could result in losing the confidence of the American people," Kennedy said.

And that, he added, "could derail NASA's plans to complete the International Space Station and begin exploration of the moon, Mars and beyond."