Only sheer chance averted another shuttle catastrophe, NASA acknowledged yesterday amid disclosures that at least two shuttles flew repeatedly with components so defective they could have ignited another explosion.
"We dodged a bullet on that," said Dan Germany, a shuttle project manager for the National Aeronautics and Space Administration.
Temperature sensors designed to protect shuttles actually endangered them during many, if not all, of the fleet's recent missions.
In addition, a paperwork mix-up contributed to an eight-month delay in NASA's learning about the risk: one or more of the sensors' eventually breaking during blastoff and being sucked into an engine, probably destroying the shuttle and its crew.
At least one faulty sensor was aboard Columbia for seven flights, and at least two were on Discovery during recent missions, NASA said.
Now, the entire fleet is being inspected.
NASA also disclosed that a contractor sent initial evidence of the defect to the wrong place for analysis in September.
So it was not until 30 hours before Columbia's scheduled liftoff with seven astronauts Wednesday that NASA learned of the dimensions of the problem. That launch was scrubbed until at least June 1 to change the shuttle's sensors and make other repairs.
NASA said the flawed sensors already found on Columbia and Discovery, as well as possibly others now being removed and inspected from those shuttles and Atlantis, had severe cracks in crucial welds.
Mr. Germany said a sensor removed quite by chance from Columbia in September contained a weld that "was cracked all the way around. The inside was cracked, as well. It was a matter of time before that tip would have broken off."
When it did, it would have dropped into the shuttle's fuel line, clogged a fuel pump and caused engine failure and probably an explosion, the agency said.
The sensors are pencil-size rods of stainless steel that monitor the temperature of fuel rushing to the shuttle's three main engines. Each shuttle has nine sensors screwed into its fuel lines.
Technicians eventually determined that the problem appeared to be common in sensors produced after the design was modified in 1981 to solve electrical problems. They believe that the cracks are caused by weaker welds and by the extremely cold temperatures of the fuel.
So far, no cracks have been found in older sensors, and those are now being used to replace the sensors removed from Columbia, NASA said.