Shuttle Program Manager holds up a pin similar to the one found on the Discovery. Photo: NASA TV/Spaceflight Now

Clearly frustrated, NASA managers today ordered a fourth launch delay for the hard-luck shuttle Discovery after engineers spotted a small but potentially dangerous metal pin lodged in an inaccessible area at the base of the ship's external fuel tank.

See our graphic of the pin's location.

How the eight-ounce "pip pin," used to secure tools and access platforms at the launch pad, got lodged in the strut between the shuttle and the external tank is not yet known.

But engineers were worried it could have fallen free during ascent, possibly hitting the spacecraft with catastrophic results.

"We tried to analyze to the best of our ability where the pip pin would go," said shuttle program manager Ronald Dittemore. "And after all the talking and the meetings were over, we came to the conclusion that we just didn't have a good handle on where this thing would fall.

"The damage to an engine or the body flap could be severe and because we did not want to take that risk, we decided to scrub today and go retrieve the pip pin."

The final inspection team were able to photograph the pin lodged in the strut between the shuttle and the fuel tank. See our graphic of the pin's location. Photo: NASA

As a result, launch of a fully fueled 4.5-million-pound spacecraft was blocked by an eight-ounce piece of debris that Dittemore said should have been spotted earlier.

"It's disconcerting to all of us that we didn't spot it until T-minus three hours," he said. "So we're going to go back and look at our inspection points, we're going to look at how we do them and when we do them."

The scrub was particularly frustrating because after three previous delays and an initially bleak forecast today, the weather had improved dramatically and launch managers were optimistic about finally getting the 100th shuttle flight off the ground.

But it was not to be.

Launch director Michael Leinbach officially called off the countdown at 6:30 p.m. and re-targeted launch for 7:17 p.m. Wednesday. Forecasters are calling for a 60 percent chance of acceptable conditions Wednesday evening.

Discovery's crew - commander Brian Duffy, pilot Pamela Melroy, Japanese robot arm operator Koichi Wakata and spacewalkers Leroy Chiao, William McArthur, Peter "Jeff" Wisoff and Michael Lopez-Alegria - took the delay in stride and climbed out of the orbiter around 7 p.m.

The offending pip pin was noticed earlier today by a technician using binoculars during a routine post-fueling ice and debris inspection.

Members of the Final Inspection Team on the mobile launcher platform near the tail of Discovery. Photo: NASA TV/Spaceflight Now

While the pin was clearly visible, the inspection team had no way to reach the area - 40 feet above the pad deck - without special access platforms.

Such pins are "used all over the pad in many, many locations," said David King, a senior shuttle manager at the Kennedy Space Center. "We obviously don't know exactly where this particular one came from. We will be doing an inventory to figure that out."

After draining the shuttle's half-million-gallon load of oxygen and hydrogen rocket fuel, engineers will move a rotating service gantry back in place around the shuttle to gain access to the area where the pip pin is located.

Once it is removed, the area will be examined for any damage and the launch team will prepare Discovery for its fourth launch try.

While the pin and a 12-inch long section of plastic-wrapped tether did not appear particularly threatening, engineers could not predict what might happen if it fell off at launch or later in the ascent.

If the pin fell off during engine ignition, it possibly could bounce off the shuttle's mobile launch platform and hit one of the orbiter's engines, the tank or boosters.

The tether from the pin hangs down as seen by a launch pad camera. Photo: NASA video/Spaceflight Now

"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 back up into an engine," Dittemore said. "And impact to an engine or a critical part of the orbiter would not be a good day."

And even if the pin stayed in place during the engine start sequence, there was no way to predict what might happen if it fell off later.

A quick analysis indicated the pin likely would move toward the external tank's aft dome at velocities below about Mach 2.5 - two-and-a-half times the speed of sound.

At speeds higher than Mach 2.5, the analysis indicated the pin likely would move back toward the orbiter's body flap area.

Engineers discussed using high-pressure water or compressed air to dislodge the pin, but in the end decided against either option.

And with just a three-and-a-half-minute launch window, there was no time to erect a platform or ladders to reach the debris in time for Discovery's planned 7:40 p.m. liftoff.

Discovery on the launch pad earlier this week with flood lights brightly illuminating the shuttle. Photo: NASA-KSC

As a result, Leinbach had no choice but to call off the countdown. While King and Dittemore agreed the scrub was probably due to human error, both refused to place any blame. Yet.

"We could have a hole in our process, too," King said. "You cannot point the finger at any individual.

"What we try to do here is we try to put processes in place that will allow us multiple opportunities to find these kinds of things and for some reason, this one got through at least one inspection that we probably should have found it prior to this one. But even that's speculation at this point."

Said Dittemore, "Certainly, I'm not pleased to get this late in the count and find out we have a tool resting on a critical area in our orbiter, in our stack. So I'm disappointed in that sense."

"On the positive side," he said, "our inspection team spotted this problem. That is a very thorough inspection at T-minus three hours and perhaps we need to have the same folks do it earlier. I'm not sure yet."

The goal of the 100th shuttle mission is to install a new docking port on the space station and to attach an 18,300-pound truss to the U.S. Unity module that will serve as a temporary mounting point for a huge set of solar arrays scheduled for installation in December.

The Z1 truss also houses four stabilizing gyroscopes and the station's main KU-band and S-band communications antennas. Operating Discovery's 50-foot-long robot arm, Wakata plans to mount the truss atop the Unity module Friday afternoon.

That will set the stage for four back-to-back spacewalks Saturday through Tuesday to electrically hook up the Z1 truss and pressurized mating adapter No. 3, which will serve as an alternate docking port for upcoming assembly flights.