That mission "is a complex flight involving four spacewalks," said Dina Barclay, a lead engineer with NASA's spacewalk projects office at Johnson Space Center in Houston. "And we want to make sure we put Quest through the paces before those spacewalks," all four of which will be staged from the U.S. airlock.

Shaped like a giant Genie bottle, Quest was delivered to the station last July. A host of relatively minor glitches, however, cropped up during an inaugural spacewalk from the airlock.

Chief among them were difficulties encountered during the depressurization of the airlock -- a job that took twice as long as originally anticipated. The problem ultimately was traced to a leaky valve fitting, and NASA engineers have put in place procedures to speed up the process.

Engineers, meanwhile, also want to the astronauts to step through a new medical protocol aimed at avoiding what scuba divers call "the bends," a potentially fatal type of decompression sickness that can endanger spacewalkers.

Like deep-sea divers, Walz and Bursch will be venturing outside the station to work in suits that are designed to operate at low internal pressures. Nitrogen bubbles, consequently, can lodge within the bloodstream, triggering acute pain in the limbs, joints and abdomen. And in severe cases, such decompression sickness can be deadly.

To combat "the bends," astronauts typically breathe pure oxygen to rid their bodies of nitrogen bubbles before setting out on a spacewalk. The new protocol calls for Walz and Bursch to pedal an exercise bike while breathing pure oxygen, accelerating the rate at which nitrogen is eliminated from the bloodstream.

Developed by NASA astronaut and former professional diver Michael Gernhardt, the novel modus operandi is enabling NASA to ditch previous plans to have suited astronauts "camp out" in the airlock for four hours prior to a spacewalk -- the time it otherwise would take to rid their bodies of nitrogen bubbles.

At the heart of NASA planning for almost a decade, such Quest "camp-outs" now are considered a serious safety hazard.

Stranding two astronauts behind a sealed hatch of a depressurized airlock -- and leaving a single crewmate alone inside the station -- could prove deadly during a fire, a serious cabin leak or any other emergency.

The new procedure provides astronauts with a way to prepare for spacewalks without having to isolate themselves within the confines of the Quest airlock.

At the same time, though, it also points up the extra preparatory overhead facing astronauts conducting spacewalks from Quest in U.S. spacesuits.

Russian spacesuits are designed to operate at a higher internal pressure, eliminating the need for such an elaborate "pre-breathe" procedure prior to excursions carried out from the station's Russian airlock.

"They don't have to go through quite as many hoops in order to stage a spacewalk out of the Russian side," Barclay said.

Spacewalks carried out from shuttle airlocks also are less difficult to stage.

Internal pressure within the shuttle's small cabin are routinely reduced prior to a spacewalk, significantly cutting down on the amount of time astronauts have to breathe pure oxygen prior to an excursion outside the ship.

That same mode of operation, however, isn't considered a viable option aboard the station. Its internal volume is simply too large, and limited reserves of precious gaseous oxygen would have to be dumped overboard to reduce atmospheric pressure within the entire complex.

Walz and Bursch, consequently, will don oxygen masks and ride an exercise bike in the station's Unity module before floating into open space through a Quest airlock hatch.

Once outside, the two astronauts aim to lighten the workload for colleagues who will be carrying out a time-critical spacewalk during the upcoming station construction mission, which now is scheduled to launch April 4.

That excursion will involve installing the station's central truss segment and then bringing it to life electrically -- work which must be done within about eight to nine hours at most.

Otherwise, the harsh thermal environment could begin damaging sensitive truss electronics and other components.

Walz and Bursch, as a result, plan to scurry around the exterior of the station, gathering up tools that visiting shuttle astronauts will need to do the truss installation job.

The pair also will test a set of back-up power cables that would be used to connect the truss segment to station power supplies if prime electrical lines for some reason fail. And they'll pack away thermal insulation blankets that have been tabbed for removal prior to the truss work.

Added up, the prep work is expected to take about three hours. And with crewmate Yuri Onufrienko watching on from inside the station, Walz and Bursch also plan to take time to snap pictures of small micrometeorite "hits" on two station radiations as well as two windows.

Two adapters that had been used to temporarily mount parts of a Russian crane in a stowage spot outside the station will be removed, and Walz and Bursch plan to secure loose latches on oxygen and nitrogen tanks that are attached to the outside of the Quest airlock.

Finally, the astronauts will take photos of a nearby materials science experiment package before heading back inside Quest.

The planned six-and-a-half hour spacewalk, meanwhile, will be the first to be staged from Quest without a visiting shuttle present -- a milestone that NASA officials are eager to surpass.

"The spacewalk is really a groundbreaking for many future U.S. spacewalks from the International Space Station," Barclay said.

"We wanted to show we could do a spacewalk without the shuttle present, because later on in the (station) assembly sequence, we do in fact have to do spacewalks after the shuttle departs," added NASA station flight director Sally Davis.

Launched Dec. 5 on shuttle Endeavour, Walz, Bursch and Onufrienko are in the midst of a five-and-a-half-month tour of duty aboard the station. Known as the Expedition Four crew, the trio remains scheduled to return to Earth in mid-May.