Image: Neutral Buoyancy Laboratory
In this 2008 photo, a scale model of the Orion spacecraft is lowered into the 6.2-million-gallon Neutral Buoyancy Laboratory at NASA's Johnson Space Center.
updated 8/4/2010 11:09:42 AM ET 2010-08-04T15:09:42

A malfunctioning pump in a vital cooling system onboard the International Space Station sent NASA scrambling to plan two emergency spacewalks this week to replace the faulty component module. At the center of that planning is a huge swimming pool in Texas.

But coordinating unplanned spacewalks, particularly in the face of a system breakdown, is an intricate process that requires balancing safety and efficiency. Station managers and engineers on Earth have been working around the clock to develop and choreograph plans for the two expeditions outside the International Space Station.

To plan and practice the spacewalks, other spacesuit-clad astronauts on Earth rehearse the maneuvers in the Neutral Buoyancy Laboratory, a huge underwater tank at Johnson Space Center in Houston. The NBL is immense: It is 202 feet long, 102 feet wide and 40 feet deep. Altogether it holds 6.2 million gallons of water.

It is in this massive pool that NASA is testing the best ways for American astronauts Douglas Wheelock and Tracy Caldwell Dyson to go about their spacewalk repairs. [Graphic: Space Station's Cooling System Problem Explained ]

Wheelock and Caldwell Dyson will replace a faulty ammonia cooling system pump that shut down last weekend. The first spacewalk begins Friday at 7 a.m. ET. The second will occur Monday.

The timing of the emergency spacewalk actually coincides with one that was previously planned, which helped to speed up the planning process, space station flight director Courtenay McMillan said in a Monday briefing.

The astronauts were initially scheduled to prepare the Russian Zarya module for future robotics work and prepare the space station for future upgrades.

Instead, the astronauts will focus on replacing the faulty pump with one of four spare pump modules stored on the space station's exterior.

Wet spacewalk rehearsals
Astronauts Catherine "Cady" Coleman and Sunita Williams are conducting sessions in the NBL to practice the upcoming spacewalks and identify challenges to be addressed when Wheelock and Caldwell Dyson replace the ammonia pump module.

"We have crewmembers in the NBL today doing what we call a development run," McMillan said. "Cady Coleman and Suni Williams were in the pool doing some of the tasks that we foresee on these two EVAs."

Practicing for spacewalks can sometimes be as tough as the real thing, too. Coleman and Williams are spending hours clad in spacesuits and submerged in water working on life-size replicas of space station hardware. NASA's spacewalks typically last about 6 1/2 hours during space station maintenance or repairs.

It helps that NASA has actually prepared for this specific type of pump failure. It is one of 14 major malfunctions the space agency has planned for in advance because of its severity. That preparation has helped speed spacewalk repair planning, station managers have said.

Still, it's expected to be challenging work. During their spacewalks, Wheelock and Caldwell Dyson will have to work together precisely to remove the bulky faulty pump module and install a new one.

On Monday, Caldwell Dyson described the ammonia pumps as "gigantic," comparing their size to that of a laundry dryer box. Each pump weighs 780 pounds and is 5 1/2 feet long by 4 feet wide. They are about 3 feet tall.

The faulty station cooling system pump is on a truss segment on the right side of the space station. Its replacement is attached to a storage shelf about 30 feet away. Because of their awkward size, moving the pumps back and forth will be tricky, McMillan said.

More planning time needed
Based on the results of Monday's NBL session, program managers decided to delay the first spacewalk to Friday, giving the team more time to analyze and refine the engineering requirements of the repair mission.

Space station managers pushed the start of the first spacewalk a day back from their original plan, which gives the engineering team more time to coordinate and plan the maneuvers needed to carry out the challenging repair. The two spacewalks are now targeted for Friday and Monday.

"This is a very aggressive timeline for us to get the procedures ready," McMillan said.

NASA will broadcast the International Space Station spacewalk repairs live from space on NASA TV, with the first spacewalk slated to begin Friday at 7 a.m. ET (1100 GMT). Click here for space station mission updates and's NASA TV feed.

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