CAPE CANAVERAL, Fla. -- Discovery's astronauts got a green light Monday for a planned launch to the International Space Station after an engineering analysis showed that a suspect fuel injector within one of the shuttle's solid rocket boosters is safe to fly.
A four-man space taxi crew, consequently, remains scheduled to blast off at 5:38 p.m. EDT (2138 GMT) Thursday on a mission to ferry a new crew to the station and then return home with a Russian and two Americans now on the outpost.
A three-day launch countdown began on schedule at 5 p.m. EDT (2100 GMT) Monday.
"After an extraordinarily thorough analysis," said NASA spokesman Rob Navias, "the entire shuttle program community has an extraordinarily high confidence in not only the reliability of this component but its margins for safety."
Plans to launch NASA's fifth flight to the station this year hit a roadblock last week when engineers discovered five tiny cracks within a spare fuel injector that last flew on a 1997 shuttle mission to Russia's Mir space station.
The thin, stem-like injectors -- which are about 12 inches (30 centimeters) long -- play a key role in the operation of a shuttle booster's steering system. They regulate the flow of rocket fuel to a hydraulic power unit that enables a booster nozzle to be swiveled in flight.
A leaky injector could diminish hydraulic power unit performance, prompting booster steering problems in flight. That prospect raised concerns about the fuel injector within the hydraulic power unit within Discovery's left-hand booster, which came from the same manufacturing lot as the cracked part.
Senior NASA managers, consequently, ordered special inspections to all readily available fuel injectors in the agency's inventory.
Thirty-eight other injectors, including 20 from the suspect manufacturing lot, were examined with inspection tools that enable engineers to image the interior of the tube-like devices.
No damage was detected in any of the other parts, which represent 60 percent of the agency's inventory of hydraulic power unit injectors. The remaining injectors all are housed within assembled hydraulic power units that would have had to be taken apart for inspections.
Manufacturing, launch processing and inspection records, meanwhile, show that the suspect injector on Discovery meets all applicable NASA safety criteria.
"So the mission reliability for this particular unit is extraordinarily high, and as a result, (engineers) concluded that this unit is safe to fly," Navias said.
What's more, a weekend engineering analysis showed that even the damaged injector could be exposed to launch vibrations without triggering fuel leaks that could hamper booster steering.
"That unit would be good to fly under its current condition because those cracks could not propagate," Navias said.
Engineers now think the one faulty injector likely was bent during repeated booster splashdowns in the Atlantic Ocean and then further damaged during routine ground processing.
Shuttle boosters are jettisoned two minutes into flight and then fall into the ocean, where they are recovered and refurbished for future flights.
With the booster problem resolved, only the weather stands as a potential showstopper for the Discovery launch.
Shuttle meteorologists say there is a 60 percent chance conditions will be acceptable for launch Thursday, but the rain showers and thunderstorms are expected to sweep into the Central Florida area both Friday and Saturday.
An on-time launch would lead to a shuttle docking at the International Space Station Saturday.
Incoming station commander Frank Culbertson and two Russian cosmonauts -- Vladimir Dezhurov and Mikhail Turin -- then would take the helm of the outpost Sunday.
After a weeklong stay at the station, Discovery would depart the station Aug. 19 with current outpost commander Yuri Usachev and two U.S. astronauts -- Susan Helms and Jim Voss. Landing back here at Kennedy Space Center is scheduled for Aug. 21.
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