Shuttle Delayed Again; Unit for Navigation Fails : Space: The problem is found shortly before blastoff. Scrubbing the launch comes at an awkward time as hearings on NASA start this week.

The long-delayed launching of the space shuttle Columbia was scrubbed again Saturday morning when a critical component of the spacecraft’s navigation system failed less than a hour before the scheduled blastoff.

“We’re very disappointed this happened,” said Bryan D. O’Connor, the Columbia’s commander. “We came back here thinking the orbiter was ready to go. I think we all realize that there are millions of parts involved in this vehicle and that it’s a miracle when we do launch.”

“Disappointment is something we have to deal with,” launch director Robert Sieck said after halting the countdown, the second time the liftoff has been postponed in as many weeks.

The launching of the Columbia on the first dedicated medical research mission in shuttle history is now tentatively set for Wednesday.

From a political standpoint, the delay could not have come at a worse time. The National Aeronautics and Space Administration’s proposal to build a permanently manned space station is in serious jeopardy in Congress, and key hearings will begin Tuesday in Washington and continue throughout the week. Many critics believe the shuttle, which would have to make at least 24 flights to build the space station, is not up to the task, and the delay means the Columbia will be on the ground instead of in orbit when Congress takes up the issue.

“It would be more positive to be flying” as Congress debates the fate of the international space station, said Robert L. Crippen, director of the space shuttle program. He said, however, that “we could stand up to six months without a launch” during the construction phase of the station, so a few more days now does not mean the shuttle could not perform satisfactorily.

The latest delay in the launching of the aging Columbia, a veteran of 10 other missions, came when one of three “inertial measurement units” began “behaving erratically” during pre-launch testing, Crippen said.

The units measure movement of the shuttle “so the vehicle knows where it is at,” Crippen said, and they are crucial to navigation. Although the shuttle can operate with only one unit, NASA insists on full redundancy before launching.

Sieck said it would take four days to replace the faulty unit because special equipment will have to be loaded aboard the Columbia to lift the unit out.

The delay was a disappointment to NASA, but it won a brief reprieve for 30 rats that were to be sacrificed for medical research. Scientists planned to kill the rats after the mission to study the effects of weightlessness on their bodies. The 30 that were aboard the shuttle waiting for launching will be replaced by a new group.

About 2,500 jellyfish will also have to be replaced. The jellyfish are to be used to study how a simple organism uses gravity to maintain balance.

The shuttle’s crew includes three medical doctors who are to carry out 10 experiments on the astronauts during the nine-day mission. Seven experiments are to be performed on the rats after the mission ends.

The crew includes Bryan D. O’Connor, 44, the Columbia’s commander; pilot Sidney M. Gutierrez, 39; Drs. James P. Bagian, 39, Margaret Rhea Seddon, 43, and Francis Andrew Gaffney, 44, and mission specialists Tamara E. Jernigan, 32, and Millie Hughes-Fulford, 46.