With coverings removed from a site near the tail of space shuttle Atlantis, Tod Biddle, a United Space Alliance (USA) technician, points to the power drive unit (PDU) inside. Photo: NASA/KSC

NASA workers will perform an unprecedented repair job on space shuttle Atlantis today, replacing a balky 300-pound hydraulic unit in the spaceship's tail while the winged-orbiter stands on its exposed seaside launch pad.

The complex repair has to go well if NASA is to keep the shuttle on track for an April 24 launch to the International Space Station. Atlantis is on an important mission to boost the orbit of the fledgling outpost and replace troublesome power batteries.

The shuttle's pre-launch preparations ran into trouble last week when the Power Drive Unit (PDU) for the rudder/speedbrake, needed to steer and slow the gliding spaceship during landing, failed a routine test.

For the past several days, engineers have been working on an ingenious plan to remove and replace the faulty device -- a task they have never before attempted at the launch pad.

The carefully coreographed, day-long repair job will get underway at about 9 a.m. EDT (1300 GMT) when the launch pads's protective gantry-like rotating service structure is swung away from the shuttle.

Coils of pipe flowing with supercold liquid nitrogen will be used to freeze the fluid in six titanium hydraulic lines going in and out of the PDU, plugging the lines and preventing the entry of air into the system.

Launch pad diagram illustrates the positions of cranes and cherry picker platforms to be used during today's repair.

Perched on cherry pickers, more than 100 feet above the launch pad flame trench, engineers will prise the PDU from its location at the base of the shuttle's tail.

A 250-ton capacity crane will lower the balky box to the ground and then hoist its replacement -- borrowed from shuttle Columbia -- into position. A smaller crane will be used to move the devices in and out of crates on the launch pad surface.

If all goes well, and the weather cooperates, the task should be completed by late afternoon. Watch our Mission Status Center today for updates on the repair work.

On Thursday and Friday, technicians will hook up electrical and mechanical connections between the PDU and orbiter, in readiness for a crucial test over the weekend to make sure the system is working.

Further tests, running up to the day before launch, will ensure the shuttle's hydraulic lines are free of air bubbles that could impair the system.

The PDU replacement will eat up all the extra padding NASA had built into the pre-launch schedule, leaving the space agency with no time to spare before the planned April 24 liftoff date. "We can still make the launch date, but we have no wiggle room," NASA spokesman Joel Wells said.

If NASA cannot get Atlantis off the ground by April 26 the mission is likely to wait two weeks, because of a logjam of rocket liftoffs and tests from Cape Canaveral slated through May 8.

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 as the Sun nears the peaks of its 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 will take over the job of maintaining 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.