Two Expedition 38 crew members are set to head out of the Quest airlock Tuesday for the second in a series of spacewalks to replace a degraded ammonia pump module on the International Space Station's starboard truss.

Flight Engineers Rick Mastracchio and Mike Hopkins, who completed a 5-hour, 28-minute spacewalk Saturday to remove the faulty pump module from the starboard truss, spent their Monday preparing for the Christmas eve excursion slated to begin at 7:10 a.m. EST.

The two spacewalkers will retrieve a spare pump module from an external stowage platform and install it in the currently vacated slot to restore full cooling capability for the complex. The pump module removed by Mastracchio and Hopkins Saturday encountered a problem Dec. 11 with its internal flow control valve, causing temperatures in the station's cooling lines to drop.

During repressurization of the Quest airlock at the conclusion of Saturday's spacewalk, a spacesuit configuration issue put Mastracchio's suit in question for the second spacewalk. A small amount of water entered the suit's sublimator -- the cooling system of the suit itself -- after the two astronauts had already hooked themselves up to airlock umbilicals. As a result, flight controllers decided to switch to a backup suit for Mastracchio for the next spacewalk.

This issue is not related to the spacesuit water leak that was seen during a July spacewalk by European Space Agency astronaut Luca Parmitano and NASA's Chris Cassidy. Both Mastracchio and Hopkins reported dry conditions repeatedly throughout Saturday's activities and the two were never in danger.

Mastracchio and Hopkins resized other suits in the Quest airlock Sunday while allowing the suit Mastracchio wore Saturday to dry out. That spacesuit will be returned to service in a future spacewalk.

After preparing spacewalking tools and configuring the crew equipment lock of the Quest airlock Monday, the two NASA astronauts joined up with Flight Engineer Koichi Wakata of the Japan Aerospace Exploration Agency to review the spacewalk's procedures. Wakata will again be in control of the station's robotic arm, Canadarm2, this time with Hopkins attached to the foot restraint at the end of the 57-foot arm.

During Tuesday's spacewalk Mastracchio and Hopkins will retrieve the spare pump module from External Stowage Platform 3 and install it in the slot that is now empty through the removal of the degraded pump Saturday. After the spacewalkers complete five electrical connections and four fluid connections, activation of the pump will restore full dual-loop cooling capability for the station.

Mastracchio will be wearing a suit marked with red stripes. Hopkins will wear a suit with no stripes. This will be the 176th spacewalk in support of space station assembly and maintenance. It also will be the second Christmas eve spacewalk conducted in history. Fourteen years ago, space shuttle Discovery astronauts Steve Smith and John Grunsfeld conducted an 8-hour, 8-minute spacewalk to install upgrades and new insulation on the Hubble Space Telescope.

If the spacewalk goes as expected Tuesday, all of the pump replacement work should be completed with just two spacewalks. The degraded pump, removed during Saturday's spacewalk, will remain on the Payload Orbital Replacement Unit Accommodation (POA) on the station's railcar, or Mobile Base System. According to flight controllers, the degraded pump can remain on the POA until at least next June, based on a thermal analysis by the ground control team.

The completion of the spacewalk will clear the way for Commander Oleg Kotov and Flight Engineer Sergey Ryazanskiy to conduct their own spacewalk Friday out of the Pirs docking compartment to install a pair of high-fidelity cameras on the Zvezda service module and to refresh several experiment packages on the exterior of the Russian segment of the station.

Flight Engineer Mikhail Tyurin assisted Kotov and Ryazanskiy on Monday with a "dry run" of the spacewalk as the two cosmonauts donned their Russian Orlan spacesuits and entered Pirs to test their suits.