CAPE CANAVERAL, Fla. A stray metal pin that prompted NASA to scrub the planned launch Tuesday of shuttle Discovery could have slammed into the ships three powerful main engines at ignition, potentially triggering disaster.

That simple but serious concern, meanwhile, sparked an overnight push to fetch the loose pin and then press toward yet another attempt to launch NASAs 100th shuttle mission Wednesday.

As it stands, Discovery and its International Space Station construction crew which includes six U.S. astronauts and a Japanese mission specialist are tentatively scheduled to blast off at 7:17 p.m. Eastern Daylight Time (23:17 GMT) Wednesday.

But NASA officials say technicians will be racing the clock between now and then.

"Its a tight schedule to meet tomorrow night," said Dave King, NASAs director of shuttle launch operations and management at Kennedy Space Center. "But we think we can make it."

Heres the situation:

During a routine final inspection just three hours before a planned launch Tuesday night, sharp-eyed engineers wielding binoculars spotted a stray metal pin wedged between Discovery and its 15-story external liquid-fuel tank.

The loose T-shaped pin -- which weighs about a half pound (0.23 kilograms) -- was lodged in a critical area where propellant lines run between the shuttles engines and its fuel tank.

Engineers, meanwhile, were concerned that the pin might break free during ignition, fall 40 feet (12 meters) to the deck of the shuttles launch platform and then bounce back up and strike one of the $35 million engines.

"In this particular instance, we could not get comfortable with the fact that it would fall to the deck and not represent some risk of rebounding up into an engine," said NASA shuttle program manager Ron Dittemore.

"And impact to an engine or a critical part of the orbiter would not be a good day so were not going to launch when we have that sort of risk [and] we decided to scrub."

NASA TV picture of the offending pin

The so-called "pip pin" normally is used to piece together temporary work platforms and handrails at the launch pad. It apparently dropped into its precarious position during the routine tear down of ground support equipment prior to launch. But it was not reported missing during a 6 a.m. EDT (10:00 GMT) inspection Tuesday.

Dittemore and other NASA officials said "pip pins" normally are cataloged as technicians tear down work platforms prior to launch, and that it was "disconcerting" that the stray tool was not reported missing prior to its 11th-hour detection by the final prelaunch inspection team.

"Certainly Im not pleased to find out that we get this late in the countdown and find out that we have a tool resting in a critical area in our orbiter," said Dittemore.

"On the positive side, though, our [final] inspection team spotted this problem," he added. "They do an excellent job of looking at the vehicle. This was spotted from some distance away using binoculars, so its not easy to do these inspections."

Both Dittemore and King said an investigation will be carried out to determine how the pin got lodged between the shuttle and its tank, why it wasnt reported missing and why the problem wasnt spotted earlier.

The two, however, stopped short of blaming the problem on human error or lapses in quality control at the space center.

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"I dont want to sugarcoat anything here. There was a mistake made, but well withhold judgment until we get to the root cause," Dittemore said.

The launch scrub prompted an overnight scramble to swing a 100-foot- (30-meter-) tall service tower back around Discovery so that the pin can be dislodged and removed from its resting place between the shuttle and its fuel tank.

NASA managers will meet 9 a.m. EDT (13:00 GMT) Wednesday to decide whether to fuel Discovery for another launch attempt Wednesday night.

The weather forecast for a Wednesday launch attempt, meanwhile, calls for favorable conditions. Meteorologists say there is a 70-percent chance the weather will be acceptable for flight.

Tuesdays delay was the latest in a string of problems that have kept Discovery and its six-man, one-woman crew grounded.

An initial launch attempt last Thursday was scrapped when a potential problem cropped up with a suspect bolt on the shuttles external tank. The need to replace a balky main propulsion system valve prompted NASA to push the planned flight back to Monday.

High winds at KSC, however, prevented NASA from finishing up key prelaunch preparations in time for fuel-loading operations, forcing yet another delay Monday.

Once airborne, Discovery and its crew will set sail on one of the most ambitious space construction missions of all time.

During a planned 11-day flight, the astronauts are to mount the first piece of the International Space Stations metal backbone and add a new shuttle docking port to the outpost. The job calls for the crew to carry out a quartet of spacewalks on four consecutive days.