Columbia is scheduled for liftoff Thursday on a 16-day science research mission. Photo: NASA-KSC

"It is all coming together, except that we just weren't able to pull all the data and finish all of the testing in time to support this readiness review, which we had scheduled for today," said shuttle program manager Ronald Dittemore. "We're optimistic. ... That's why we targeted our launch date (for Jan. 16 as planned) and that's why we are putting the crew in (medical) quarantine. We don't have any real indications that look like this is going to be a show stopper."

Regardless of the outcome of the teleconference Sunday, NASA managers likely will not formally put the matter to rest until holding their standard launch-minus-two-days mission management team meeting Tuesday.

There are no known problems with any of the 16 cue-ball-size bearings in Columbia's fuel lines. But a crack found in a bearing in the shuttle Discovery last month - and recent tests to determine what might happen should a bearing crack and fragment during engine operation - have raised safety questions that must be resolved. At issue is debris getting sucked into one of the shuttle's three main engines, possibly with catastrophic results.

Bearings in NASA's other shuttles have been inspected and found to be free of any visible defects. No such inspections have been made with Columbia, however. Standing vertically at the launch pad, attached to an external fuel tank with three engines installed, only a handful of the 16 bearings are accessible for boroscope inspection.

The 2.25-inch-wide bearings are part of a system that allows propellant line bellows inside the shuttle's engine compartment to flex slightly while maintaining their structural integrity. Eight such bearings are present in the liquid oxygen lines and eight more in the ship's liquid hydrogen feed system.

Inside a given bellows section, two sets of struts forming interlocking three-legged pyramids are hooked together under compression. The bearing in question serves as a common apex of sorts for the interlocking struts. Should a bearing crack apart in flight, when propellant is rushing by at high velocity, debris could get sucked into an engine.

During recent tests, engineers were able to roughly duplicate the crack found in Discovery's bearing, but the nature of the "foreign object debris," or FOD, was not what they expected. Engineers currently are reviewing data and continuing their analysis to build a rationale for flight even though Columbia's bearings cannot all be inspected at the pad.

This issue is complex because there is no easy way to make repairs, should any be needed. The bearings are spread out in the engine compartment's maze of plumbing. To replace one, the propellant line in question would pretty much have to be disassembled, a job that would cause lengthy processing delays.

In the meantime, engineers at the Kennedy Space Center are continuing to ready Columbia for launch Thursday on the year's first shuttle flight, one of the last missions on NASA's manifest that does not involve the international space station. The ship's crew, including Israeli astronaut Ilan Ramon, is expected to arrive Sunday.

Liftoff is targeted for sometime between 10 a.m. and 2 p.m. Thursday. The exact launch time will be revealed 24 hours before launch as part of NASA's post Sept. 11 security policy. Once in orbit, the crew will work in two shifts, around the clock, to carry out dozens of microgravity experiments. Assuming an on-time launch, Columbia will return to Earth Feb. 1.