Imagess of a crack found on a metal liner used to direct flow in shuttle Discovery's main propulsion system propellant lines. The top photo is magnified 30 times and the bottom photo is magnified 100 times. Photo: NASA

At issue is the health of metal liners mounted inside the propellant lines that feel supercold liquid oxygen and liquid hydrogen to a shuttle's three main engines.

Recent inspections of the fuel flow liners aboard Atlantis and Discovery revealed cracks measuring up to three-tenths of an inch long. Should a liner rupture or break apart during ascent, the high-speed flow of propellants would be disrupted at the very least. Whether any fragments could be ingested by an engine, possibly with catastrophic results, is not yet clear.

But the cracks clearly pose a safety risk and shuttle program manager Ronald Dittemore wants to find out whether the flow liners aboard Columbia have experienced similar problems. At the same time, engineers are assessing what must be done to fix the already discovered cracks.

NASA had planned to haul Columbia to the launch pad in the next few days to begin final preparations for launch July 19. But work to ready the ship for rollover from its hangar to the Vehicle Assembly Building for attachment to a pair of boosters and its external tank was put on hold over the weekend.

Atlantis is scheduled for launch Aug. 22 on the next space station assembly flight. Depending on how the flow liner issue plays out, Atlantis could end up launching before Columbia. But at this point, trying to guess how the launch schedule might work out is pretty much an exercise in futility.

Here is the text of a NASA statement on the issue:

NASA managers today temporarily suspended launch preparations for Space Shuttle Columbia until they have a better understanding of several small cracks found in metal liners used to direct the flow inside main propulsion-system propellant lines on other orbiters in the fleet.

Columbia's launch on STS-107, previously planned for July 19, will be delayed a few weeks to allow inspections of its flow liners as part of an intensive analysis that is under way.

Recent inspections of Space Shuttle Atlantis and Space Shuttle Discovery found cracks, measuring one-tenth to three- tenths of an inch, in one flow liner on each of those vehicles. Some of the cracks were not identifiable using standard visual inspections and were only discovered using more intensive inspection techniques.

"These cracks may pose a safety concern and we have teams at work investigating all aspects of the situation," said Space Shuttle Program Manager Ron Dittemore. "This is a very complex issue and it is early in the analysis. Right now there are more questions than answers. Our immediate interests are 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. Until we have a better understanding, we will not move forward with the launch of STS-107."

The impact of the investigation on other upcoming space shuttle launches has not been determined.