Space station commander Michael Lopez-Alegria and flight engineer Sunita Williams are gearing up for a critical spacewalk Wednesday, the first of two required to complete the activation of the lab complex's permanent ammonia cooling system. Cosmonaut Mikhail Tyurin will monitor the activity from inside the lab complex.

On guard against potentially toxic ammonia leaks, Lopez-Alegria and Williams plan to connect fluid lines to bypass the station's interim cooling system, used during the initial stages of assembly, and to connect the lab's heat exchangers to a pair of independent ammonia loops housed inside the station's main solar array truss.

Coolant loop A, which will service the station's life support systems, will be tied into the system during the spacewalk Wednesday while coolant loop B, which will cool the lab's computers and avionics gear, will be addressed in a separate excursion planned for Sunday morning. A third spacewalk will be staged Feb. 8 to remove thermal shrouds from a segment of the solar array truss on the left side of the station and to deploy two external cargo stowage platforms.

Kirk Shireman, deputy manager of the space station program at the Johnson Space Center in Houston, said the next 12 months will be the most challenging yet for space station assembly: Five Russian Progress supply ships are scheduled for launch, two Russian Soyuz crew ferry craft, six space shuttle missions, the first flight of a new European supply ship, 11 spacewalks by station crews and 12 excursions by visiting shuttle astronauts.

"So right now, that's a total of 23 EVAs between now and the beginning of next year," Shireman said. Throw in the arrival of European and Japanese research modules and "as you can see, lots and lots of activities going on."

During a shuttle visit in December, spacewalking astronauts and ground controllers carried out a complex mission to switch the station's electrical system from a temporary configuration, used during the initial stages of construction, to its permanent system. They also powered up two pumps in the main solar array truss and began circulating ammonia through coolant loops A and B.

Electrical components in the station are mounted on cold plates that use circulating water to carry heat away. Heat exchangers, in turn, transfer that heat to external ammonia coolant loops. Up until now, lab systems have been cooled by an interim cooling system, which used radiators on the P6 solar array truss, extending up from the Unity module, to dissipate heat to space.

During the spacewalks Wednesday and Sunday, Lopez-Alegria and Williams will make a variety of electrical and fluid line connections to tie the heat exchangers into coolant loops A and B. Most of the work will be carried out in the so-called "rat's nest," a confined area between the Destiny laboratory module, the Unity connecting module and the Z1 truss that extends upward and supports the P6 solar array that provided the station's interim power.

NASA planners initially considered having the shuttle Discovery's crew handle the required plumbing changes during the December mission to activate the station's permanent electrical system. But given the complexity of the electrical work, the cooling system re-plumbing was passed on to Williams and Lopez-Alegria.

"The first spacewalk will be one of the external power loops," Williams said in a NASA interview. "We call it loop A. We will switch it from using a radiator and cooling loop system on the P6 to its more permanent cooling system out on the truss. And that involves changing some electrical connectors in the 'rat's nest,' which is the area between Z1, S0 and the Lab. It's a small area that has a lot of electrical connections and a lot of fluid connections; it's this very tight space.

"As it's planned, both me and Michael Lopez-Alegria will be inboard in this small little area. On the first loop reconfiguration, I'll do the electrical connectors then he will follow that with the big fluid jumpers to switch the fluid lines from P6 to the (permanent system). Following that, we will be up on the P6 truss; they will retract the starboard radiator, which we've been using for the early external thermal control system. The ground will be doing that but our role in that is to cinch it down, because it needs to stay down and compacted so the radiators aren't moving around as the space station is rotating.

"He'll be up on the zenith side; I'll be on the nadir side of the radiator. We will wait and watch the radiator retract - potentially it could need a little bit of a push from us at the very end - and then we cinch bolts around the edge of it that we need to, simultaneously, connect to make sure that the radiator is contained nicely. That's the main portion of EVA number one."

The second spacewalk is virtually identical to the first, but focusing on making the required loop B fluid connections and retracting the aft radiator on P6.

As part of the work to switch the station from interim power and cooling to its permanent systems, the port wing of the P6 solar array was retracted during the December shuttle mission. The starboard wing is scheduled for retraction during the next shuttle visit in March and if all goes well, P6 will be unbolted from Z1 and moved to the left end of the station's main solar truss in September.

"This is really an exciting time for all of us in the EVA world," said Glenda Laws, lead Expedition 14 spacewalk officer at the Johnson Space Center. "This will be the first time we've executed more than a single stage EVA at a time. We're going to be knocking out three, one right after another, and that presents a lot of new challenges to our team.

"But we have a great team and they're up to the challenge. We've been training together with this crew for almost two years. We're well prepared. We're prepared for nominal stuff as well as contingencies, even an ammonia leak should one occur. We've been treating that almost as a nominal activity because it's become second nature to us to follow up on those kinds of contingencies. I think we're well prepared."

Going into Wednesday's spacewalk, 46 NASA astronauts, 13 Russians, two Canadians, a German, a Frenchman, a Japanese and a Swedish astronaut had logged 469 hours and 59 minutes of space station assembly and maintenance time during 77 spacewalks.

Lopez-Alegria currently ranks 13th in the world for cumulative spacewalk time with 39 hours and 36 minutes during six previous excursions. Williams has one spacewalk to her credit, an outing during the last shuttle mission lasting seven hours and 31 minutes.

Altogether, the three upcoming spacewalks are budgeted for 18 hours and 55 minutes (6:30, 5:55 and 6:30). Assuming they run full duration, Williams' total will climb to 26-and-a-half hours, the most for any female astronaut. Lopez-Alegria's time will climb to 58-and-a-half hours, putting him in a virtual tie with Jerry Ross for the No. 2 mark.

Lopez-Alegria will join Tyurin for a Russian spacewalk Feb. 22 that will put the American in sole possession of second place, behind cosmonaut Anatoly Solovyov who has more than 84 hours of spacewalk time over 16 outings.

The possibility of an ammonia leak during the spacewalks Wednesday and Sunday has prompted exhaustive analysis and simulations to make sure both astronauts know how to decontaminate their suits before re-entering the closed environment of the space station.

And the threat is real. During the activation of the interim cooling system in 2001, leaking ammonia contaminated the suit of spacewalker Robert Curbeam.

"Because of that leakage we experienced on the 5A mission, and because of subsequent ground testing, we have spent some time preparing ourselves and preparing the spacewalking crew for the potential we may have some ammonia leakage," said Derek Hassmann, chief spacewalk manager at the Johnson Space Center.

Engineering teams analyzed "all the possible leak points in the sequence of tasks the crew's going to work, what our reaction on the ground is to that leakage, what the crew's reaction is and then once we've completed a QD (quick-disconnect) operation, how we verify the crew's suits are indeed clean and it's safe to go back into the airlock," Hassmann said. "So that's the contingency that we've worked and we've planned for and we feel we're ready to address if it does come."

Laws said the first line of defense was to study the quick-disconnect fittings on the fluid lines "and to understand how to operate them to minimize risk of a leak. We've since been more clear about specifying the operation of the QDs and understanding what possible failure modes are."

The result was a crib sheet that "leads you through a sequence of steps that will help you stop a leak and secure the system," Laws said. "And in some cases, we have to intentionally leak off a portion of the ammonia so we can move on to the next step. For example, one of the QDs that's on the lab, what we refer to as the male portion of the QD, if one of the seals was to leak we would have to intentionally leak off all of the ammonia, so we would have the crew member open the valve and then move out of the way. We would then leak out the ammonia in just the lab portion of it. That would probably take less than 10 minutes or so.

"Then once that portion of the ammonia was leaked out, we could move the fluid line off to its new location and get back to normal operations. The more typical failure of these kinds of seals is that there is one set of seals for the valve-open position and a separate set of seals for the valve-closed position. So if they're going to leak, they're most likely only going to leak in one of those two positions. So we should be able to quickly isolate and continue on with our operations."

If a leak does occur, ammonia may contaminate one or both of the astronauts' spacesuits. But Laws said engineers are confident Lopez-Alegria and Williams can clean the suits up prior to re-entering the space station.

"When ammonia comes out of the fluid line, it immediately freezes and becomes little snow pellets," she said. "Those snow pellets themselves won't stick to the suit but the velocity of the pellet moving toward the suit itself, when you impact the suit the pellet breaks open and you get a bit of liquid. That liquid will wet the suit and it will stick.

"Our plan is, we'll wait until we completely finish all of our fluid QD operations and when we're done with all the possible actions that could contaminate the crew member, then we'll go off and clean up the suit. The way you clean off the suit, it doesn't brush off well but it does sublimate well when you hit it with something that's warm. We happen to be carrying with us several things that will work well for that, including the power tool that we're carrying as well as some of the contingency fluid QD tools that we'll have with us. ... You just lay the warm metal onto the suit and it sublimates off the ice that's formed. Just keep doing that, repeating until you get all of the ice off the suit."

Extensive testing has resulted in data tables that correlate sublimation rates with temperatures at various points around the station.

"We'll keep them outside until we believe it's all gone or until we no longer have enough consumables left (to continue the spacewalk)," Law said. Tests will be run in the airlock as it is re-pressurized and if ammonia readings are higher than a pre-set level, the astronauts will dump the air overboard and re-pressurize the airlock.

"One cycle will probably take care of it," Law said. Any residual ammonia detected after an airlock cycle will simply be dispersed in the station's air supply.

Wednesday's spacewalk is scheduled to begin around 10 a.m. After floating out of the Quest airlock module, the astronauts will make their way to the rat's nest between Z1, the node, the laboratory module and the S0 solar array truss segment.

The first item on the agenda is to make an initial electrical connection to support a new system that will feed station solar power to docked space shuttles starting this summer.

Once that work is complete, Lopez-Alegria will reposition two coolant system fluid lines to connect low-temperature life support equipment heat exchangers to Loop A while Williams makes required electrical connections. The spacewalkers then will stand by while ground commands retract the P6 starboard radiator that helped provide no-longer-needed interim cooling. Once the radiator is fully retracted, the astronauts will lock it down and mount a shroud to maintain proper temperatures.

The spacewalkers then will make preparations for the eventual jettison of an early ammonia servicer, a backup coolant reservoir that was in place to protect against a major leak in the interim cooling system. The servicer will be dumped overboard during a station spacewalk this summer.

If time is available, Lopez-Alegria and Williams also will photograph the P6-2B solar array, which extends to the right side of the station. The array will be retracted during a March shuttle flight and given problems retracting the port-side array last December, engineers want to characterize the 2B array's current condition.

Other get-ahead tasks that will be carried out Wednesday if time is available:

• Installation of a computer network cable on the Unity module that eventually will permit commanding of Russian systems from the U.S. segment of the station.

• Removal of a handrail on the Unity module to provide clearance when a new multi-hatch node is temporarily mounted on Unity during an upcoming assembly mission.