Atlantis' rudder/speed brake is seen outlined with black thermal tiles on the back half of the vertical stabilizer. Photo: NASA

NASA officials are hoping to keep space shuttle Atlantis on track for launch later this month to service the International Space Station despite uncovering a hydraulic problem in the ship's tail.

Engineers on Wednesday identified an unexplained pressure reading while reviewing data from a hydraulic system test conducted last Tuesday aboard Atlantis at Kennedy Space Center's pad 39A. The information indicated a pressure measurement three times higher than normal in the Power Drive Unit, or PDU, for the shuttle's rudder/speed brake.

The rudder/speed brake is the movable aerosurface on Atlantis' vertical tail stabilizer that is used to steer the shuttle and then to slow it down during landing. The aerosurface must operate properly for a safe touchdown.

Deciding what to do
Suspicion of what might have caused the problem is centering on the PDU and NASA managers are debating whether the unit should be replaced.

"They now need some time -- give them a day or two -- to collect as much engineering data as they can and apply some engineering analysis and thought to this," Atlantis commander Jim Halsell said Thursday during a break in training at Kennedy Space Center. "Then they'll be able to come up with the best way of fixing the problem."

If the PDU is deemed faulty and has to be swapped, the space agency isn't sure the work could be completed at the seaside launch pad. Such a job has never been attempt in the 19-year history of the shuttle program. Besides handling the 300-pound units, engineers are also concerned about air getting into Atlantis' hydraulic lines and creating even more troubles.

For now, officials say they think the replacement could be completed at pad 39A with no impact to the scheduled April 24 launch date.

Atlantis seen rolling out of the Vehicle Assembly Building on March 25 bound for pad 39A. The shuttle could be returned to the VAB for repairs. Photo: NASA

However, if NASA rules the job can't be done with Atlantis sitting vertically on the pad, the ship will have to be rolled back to the Vehicle Assembly Building, detached from its external fuel tank and solid rocket boosters and returned to a processing hangar. Such a move would delay the launch at least several weeks.

Whatever extra work is performed on the PDU likely won't start before next Monday, once a test of Atlantis' three Auxiliary Power Units is completed. The APUs provide the pressure needed to power the shuttle's hydraulic systems. Monday's test is a routine part of pre-launch preparations.

Just three April launch attempts
NASA has just three days in which to launch Atlantis this month -- April 24, 25 and 26 -- before having to wait until May 10 or 11 to reschedule the 10-day 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.

Officials want to dispatch Atlantis to 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.

Besides replacing broken equipment and delivering about one ton of supplies to the station, Atlantis will also reboost the ISS orbit about 19 miles during three engine firings consuming 1,200 pounds of fuel. That should keep ISS safely aloft until the Russian-built Zvezda service module arrives in July, which is designed to maintain the station altitude.

If Atlantis is delayed or does not dock the station for some reason, the station's Zarya module can use some of its fuel to raise the ISS 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.

Countdown dress rehearsal
The seven Atlantis astronauts will be awakened at about 6 a.m. EDT today to have breakfast and put on their spacesuits for a simulated launch countdown. They are slated to depart the crew quarters at about 7:45 a.m. EDT bound for pad 39A where personnel will help the astronauts to board Atlantis.

The seven Atlantis astronauts after arriving at KSC on Wednesday. From left: Susan Helms, Jim Halsell, Scott Horowitz, Jeff Williams, Jim Voss, Yuri Usachev and Mary Ellen Weber. Photo: NASA

The crew will remain inside the shuttle for the final three hours of the countdown dress rehearsal, which will culminate in a pretend ignition and emergency shutdown of Atlantis' three main engines at about 11 a.m. EDT.

Following the test, the astronauts will unstrap and exit the shuttle. They are scheduled to leave KSC at 2 p.m. EDT and return to Johnson Space Center in Houston for final pre-flight training.

If the April 24 launch date holds, Halsell and crew are slated to head back to KSC on April 21, and the real countdown should start at 7 p.m. EDT on that day.

The mission will be the 21st for Atlantis and its first in 2 1/2 years. The $2 billion spaceplane's last flight was to the Russian space station Mir in September 1997. Since then, the Atlantis underwent a major overhaul and inspection period at Boeing's shuttle assembly plant in Palmdale, Calif.

The shuttle was outfitted with a sophisticated "glass cockpit" that replaces the 1970s-era gauges and displays and other enhancements during the year-long, $74 million tune-up.