Countdown clocks began ticking late Sunday for launch of the shuttle Columbia Thursday on a 16-day microgravity research mission featuring dozens of high-tech experiments and a seven-member crew that includes the first Israeli astronaut. There are no technical problems at pad 39A and forecasters are predicting a 95 percent chance of good weather during the shuttle's morning launch period. The odds drop to 40 percent "go" Friday when rain and high winds are expected.

"The STS-107 launch countdown has begun and things are proceeding well," said NASA test director Jeff Spaulding. "The shuttle Columbia is in excellent shape and we're continuing our preparations for Thursday's launch of the Spacehab research double module."

Liftoff is targeted for sometime between 10 a.m. and 2 p.m. Under NASA's post Sept. 11 security policy, the exact time will not be announced until the day before launch.

The only wild card in all of this is ongoing work to determine the health of large bearings in the shuttle's propellant feed lines after a crack was found last month in a bearing aboard the shuttle Discovery.

Each shuttle is equipped with 16 such bearings, each the size of a cue ball, which anchor flexible bellows assemblies that allow the propellant lines to bend slightly when subjected to the vibrations of launch. The concern is that a bearing might crack and break apart during flight, allowing metal debris to get sucked into a main engine with possibly catastrophic results.

Only one such crack has been found - aboard Discovery - but Columbia's bearings cannot be inspected at the launch pad. Engineers have been reviewing data and subjecting similar bearings to various stresses to develop a rationale for launching Columbia as is.

During a teleconference Sunday, NASA managers gave the launch team clearance to begin Columbia's countdown while a final few tests are carried out. Senior NASA managers will review the analysis Tuesday and, barring some unexpected development, they are expected to clear Columbia for launch Thursday.

"That analysis thus far has gone very well and has given us enough confidence to begin the launch countdown," Spaulding said. "We do expect to have final flight rationale presented tomorrow afternoon."

Columbia is carrying scores of experiments into orbit, many of them commercially funded, for 16 days of around-the-clock research. Because many of the experiment subjects and samples have a short shelf life, Columbia's seven-member crew will have just two days to get off the ground before standing down for four days to refurbish the science hardware and reload fuel for the shuttle's electrical generators.

In this case, however, four days beyond Friday would put the shuttle flight in conflict with launch of an Air Force Titan 4B rocket carrying a high priority military satellite. The Titan is scheduled for liftoff from the Cape Canaveral Air Force Station on Jan. 21 and the Air Force will get at least two days to get the rocket off the ground. Engineers then would need two more days to reconfigure radar tracking systems to support the shuttle. As a result, if Columbia is not off the ground by Friday, launch likely will slip to Jan. 25. And that assumes a small NASA satellite already scheduled for launch Jan. 25 on an air-launched Pegasus rocket slips a few days to make room.

Columbia originally was scheduled for liftoff last July, but the shuttle fleet was grounded by work to repair small cracks in fuel line baffles. Columbia ended up slipping behind two space station assembly missions, pushing the flight to this month.

"It has been a long road for us," flight engineer Kalpana Chawla said after flying to Florida Sunday evening. "I'm very, very happy to be here. I think we're going to go."