Update for July 10: Engineers inspecting the shuttle Endeavour's main propulsion system plumbing have found at least one crack in a fuel flow liner leading to one of the ship's main engines. The defect is similar to small cracks discovered earlier in NASA's other three space shuttles. A telephone news conference with shuttle program manager Ronald Dittemore is planned for Friday morning.

Tim Appleby, with United Space Alliance, inspects the flow liner on Columbia after the engines were removed. Photo: NASA

NASA managers and contractors are continuing to work through an exhaustive series of inspections and tests to determine what caused small but potentially dangerous cracks in shuttle fuel line baffles that have now been found in three of the agency's four orbiters. The main propulsion system in the fourth shuttle - Endeavour - is being inspected this week. Until engineers complete their analysis and determine what, if anything, needs to be done to fix the propellant flow liner cracks, the next shuttle flight is on hold.

But contrary to widespread news accounts saying the shuttle fleet is "grounded," only one flight has actually be delayed at this point: Launch of the shuttle Columbia on a dual-shift 16-day microgravity research mission featuring the first Israeli astronaut. That flight, STS-107, originally was scheduled for takeoff July 19. But on June 24, NASA managers put pre-launch processing on hold pending the outcome of the flow liner crack investigation. At this point, the earliest Columbia could be launched, assuming processing resumed right away, is mid to late August.

In the meantime, engineers are continuing to process the shuttle Atlantis for launch Aug. 22, as scheduled, on the next space station assembly mission. That flight, in theory at least, could still be launched on schedule if the crack issue is quickly resolved. The year's final flight, launch of the shuttle Endeavour on a mission to deliver a fresh crew to the station and to bring the lab's current occupants back to Earth, is targeted for launch in mid October.

While NASA is in no undue rush to resolve the flow liner crack issue, the station crew's time in orbit is a definite concern. The Expedition 5 increment is targeted to last about four-and-a-half months. But commander Valeri Korzun, Sergei Treschev and NASA astronaut Peggy Whitson have more than enough on-board supplies to remain aboard the lab complex well beyond Endeavour's current launch date. And if all else fails, they could always return to Earth aboard the station's Soyuz lifeboat. But that possibility is so remote at this point that "worst-case scenario" does not really apply. The worst-case scenario at this point is simply that Expedition 5's time aloft could be extended.

"Before we launched, I assumed I would be here at least six months," Whitson told an interviewer. "So I think I could handle up to six months with no problem."

These images show a crack found on a liner in shuttle Discovery's main propulsion system. The top photo is magnified 30 times and the bottom photo is magnified 100 times. Photo: NASA

The cracks in question have now been found in flow liners inside the hydrogen fuel lines aboard Columbia, Atlantis and Discovery. The flow liners, mounted inside the big propellant lines that feed the shuttle's main engines, are designed to ensure incoming liquid hydrogen enters each engine in a uniform manner. Should even a small part of a liner break off, it could be carried downstream and "ingested" by a main engine hydrogen turbopump, possibly triggering a catastrophic shutdown.

The cracks detected in Atlantis and Discovery are located where the flow liners cross over the gimbal joints closest to the point where the incoming 12-inch hydrogen lines enter the main engines. On Atlantis, there are three small cracks in the flow liner leading to main engine No. 1. Two of these are circumferential cracks while one is axial. On Discovery, three axial cracks were found. Data on the locations of several cracks found in Columbia's flow liners are not yet available and inspections of Endeavour are not yet complete.

"These cracks may pose a safety concern and we have teams at work investigating all aspects of the situation," shuttle program manager Ronald Dittemore said June 24. "This is a very complex issue and it is early in the analysis. Right now, there are more questions than answers."

NASA's immediate concern, he said, was to "inspect the hardware to identify cracks that exist, understand what has caused them and quantify the risk."

"I am confident the team will fully resolve this issue, but it may take some time," Dittemore said. "Until we have a better understanding, we will not move forward with the launch of STS-107."

Another flow liner is inspected on Columbia by Boeing's A.J. Koshti (left) and United Space Alliance's Ken Tauer (right). Photo: NASA

If repairs are required, engineers likely will attempt to weld or drill out the cracks with the flow liners still installed in each shuttle. Removing the liners for repairs is a worst-case scenario that could delay the next shuttle flight to late this year at the earliest. But most engineers and managers believe whatever repairs may be required can be done with the flow liners in place.

And at this point, it's not clear any repairs will be needed. One school of thought holds the cracks probably developed many years ago, possibly during the first few times each orbiter's propulsion system was exposed to supercold liquid oxygen and hydrogen. Under that theory, the cracks pose no safety threat and shuttles can be safely launched as is.

Another school of thought holds that areas where the cracks were found should be strengthened to make absolutely sure the cracks cannot grow. That possibly could be done by drilling small holes at the ends of each crack to stop potential growth or by employing high-tech welding techniques. But in either case, engineers need to understand the issue well enough to know whether additional cracks might develop later.

Dittemore is not expected to make any decisions until after Endeavour's flow liners are inspected this week and the results of a detailed engineer analysis are complete.

In the Orbiter Processing Facility, Columbia's main engine no. 2 is removed from the shuttle so inspections could be performed. Similar work is underway on Endeavour. Photo: NASA