Although the repair method of welding the cracks was selected for Atlantis and Endeavour, the faulty parts inside are made of a different type of metal and Dittemore is not yet convinced the same repair method will be used. Columbia was NASA's first space-worthy orbiter. Additional tests and analysis might force that mission to be further delayed, he said.

"We're going to ask them to be patient and wait a little while longer," Dittemore said of the Columbia mission team.

Discovery is in the midst of an overhaul and is not scheduled to fly until 2004.

The welding team that will do the job were hand picked during a "Superbowl of welders" for the critical job that requires the steady hands of a surgeon, Dittemore said. It will take about three to five days to make the welds, polish the area and check the repairs.

The method was selected over others being considered because it was the most simple. It restored the plumbing to its original state and the repair itself was reversible should a better method or other work be required in the months and years to come, Dittemore said.

While all of this is going on, the shuttle engineering community will continue to study why the cracks occured and determine the best way to keep the shuttle's plumbing safe to fly for another 20 years, Dittemore said.

The tiny cracks were found on the surface of metal liners that are inside liquid hydrogen pipes located within each shuttle's rear engine compartment. The flaws are not within the Rocketdyne main engines themselves.

The liners help direct the flow of propellant through the plumbing. The concern is that if the cracks were to grow during a launch and a piece of metal were to break free, the debris would be sent into the engine where it could cause a potentially catastrophic shutdown.

David Strait, a United Space Alliance technician at the Kennedy Space Center, is credited with making the discovery and Dittemore had nothing but high praise for his attention to detail and his attitude toward safety.

"With this particular finding (Strait)'s a star and deserves a medal," Dittemore said.

Engineers are confident the 11 cracks found inside NASA's four-orbiter fleet were caused by high-cycle fatigue -- a phenomona where the metal rapidly flexes back and forth thousands of times and then fails by cracking.

By comparison, Dittemore said an example of low-cycle fatigue is when someone opens a can of pop and bends the tab back and forth just a few times before it breaks. Essentially the same thing is happening inside the shuttle's plumbing, only it's taking tens of thousands of much tinier movements before something breaks.

What's causing the metal to flex back and forth remains the mystery. Suspects include random vibrations at launch, the supercold temperature of liquid hydrogen, the way the liquid flows over the area or the method used in manufacturing the flow liners in the first place.

"We are not yet at the point that we an identify the root cause," Dittemore said. He noted that he has put in place a test program to find out because the answer will be critical to NASA's ability to continue flying shuttles for the next 20 years.

The shuttle official could not identify how much additional money NASA has spent to solve this problem, saying it's "not that large" compared to the program's annual $3 billion budget and the nearly $10 billion in assets it takes to run a human spaceflight program.

"It's insignificant compared to the long-term goals we have to open up the space frontier," Dittemore said.