Atlantis' speed brake is split open in the Vehicle Assembly Building last month when workers replaced one of the main engines. Photo: NASA/KSC

NASA officials today are expected to decide how engineers should go about repairing a hydraulic problem in shuttle Atlantis' steering system used during landing.

Senior shuttle managers on Friday were still hoping to replace the suspect Power Drive Unit, or PDU, located inside Atlantis' tail at launch pad 39A. But the unprecedented job will require workers to manhandle the 340-pound units with the shuttle sitting vertically at the seaside pad and call upon a new technique to ensure additional harm is not done to the hydraulic system.

Last week engineers studying results from a test conducted March 28 discovered a pressure measurement three times higher than normal in the PDU. The PDU is a critical item on each space shuttle because it contains six hydraulic motors that drive the rudder/speed brake located on the shuttle's vertical tail stabilizer.

Without the PDU operating properly, the rudder/speed brake might not work to steer the shuttle during final approach to the runway, then slow the craft down during landing.

See an illustration of the suspect Power Drive Unit and its location aboard the space shuttle.

NASA still does not know why the PDU behaved the way it did during the test last month, though a failure inside the unit is suspected of causing the problem.

The launch pad is the place NASA wants to complete the repair. However, if the obstacles of doing the work there become too great to overcome, officials will have no choice but to roll Atlantis from the pad back to Kennedy Space Center's mammoth Vehicle Assembly Building.

Inside the 52-story structure, the spaceplane would be detached from the external fuel tank and booster rockets. Atlantis could then return to a nearby processing hangar, allowing workers to complete the repairs with the shuttle sitting horizontally.

NASA says if the launch pad option works, Atlantis should be able to make its planned April 24 liftoff on a 10-day maintenance mission to the International Space Station. But if a rollback is ordered, the launch will be put off at least several weeks.

Shuttle Atlantis approaches pad 39A during a Saturday morning rollout on March 25. Photo: NASA/KSC

Managers met on Friday to discuss the PDU replacement plan, but deferred a decision until Monday so further engineering analysis could be performing, including tests of a system that would use liquid nitrogen to freeze the hydraulic fluid within the plumbing leading to the unit.

Freezing the fluid, engineers believe, will prevent air bubbles from forming inside the lines. If air were to enter the shuttle's hydraulic system, the astronauts could have trouble controlling the $2 billion spaceship during its high-speed landing.

In addition, the freezing idea means workers would not have to drain any hydraulic fluid from the lines. Officials say the hydraulic system only can be refilled back in the processing hangar.

"There are still a lot of questions left," NASA spokesman George Diller said Friday. "There are a lot of unknowns, too, that we need to work through."

Meetings planned for today are aimed at sorting out the pros and cons of the launch pad and hangar options, as well as reviewing the additional analysis conducted since Friday afternoon.

Watch our Mission Status Center today for updates as news develops.

If the launch pad plan is attempted, the unit swap probably won't start until sometime Wednesday. Officials say it could take 12 hours to remove and replace the PDU, then another day to conduct tests to ensure the new unit is working and the problem has been fixed.

Since there are no spare PDUs, the unit aboard shuttle Columbia is being removed for installation into Atlantis. Columbia is currently undergoing a major overhaul at Boeing's shuttle assembly plant in Palmdale, Calif., and is not scheduled to fly again until late February 2001.

Racing against other launches
Should NASA be successful in keeping Atlantis on track for liftoff on April 24, the shuttle launch team will have just two further attempts to get the shuttle airborne this month if other problems arise.

If the launch does not occur on April 24, 25 or 26, NASA will likely have to wait until May 10 or 11 to reschedule the mission. The reason: the U.S. Air Force-controlled Eastern Range that governs all launches from the Cape needs time between flights of different space vehicles and there is a logjam of rocket liftoffs and tests slated through May 8.

Zarya and Unity modules of ISS orbiting Earth. Photo: NASA

Officials want Atlantis to reach the International Space Station as soon as possible because the 16-month old fledgling outpost is losing about 1.5-miles of altitude per week. The current orbit is 232 by 215 miles. An increase in solar activity prompted by the nearing peak of the Sun's 11-year cycle is causing the Earth's atmosphere to bulge, creating additional drag on ISS.

Atlantis will reboost the ISS orbit about 19 miles during three engine firings consuming 1,200 pounds of shuttle fuel. That should keep ISS safely aloft until the Russian-built Zvezda service module arrives in July, which is designed to maintain the station's altitude.

If Atlantis is delayed or does not dock to ISS for some reason, the station's Zarya module can use some of its fuel to raise the orbit. However, station officials don't want to use Zarya's limited fuel supply because it is the safety reserve to keep ISS in space until Zvezda arrives.

During the time docked to ISS, the seven Atlantis astronauts will replace faulty equipment inside the station and deliver about one ton of supplies that future crews will use.

One spacewalk is planned to install a new communications antenna assembly, add the rest of a Russian-built crane and reseat a U.S. crane that is not properly attached to the station's exterior.