"It's like Christmas morning opening up a little present here," Mastracchio said as he checked his toolkit.
Mission Control in Houston was in a festive mood during Tuesday's spacewalk. Tabletop Christmas trees, Santa dolls and red Santa caps decorated the desks.
"It is Christmas Eve ... and in this holiday way of giving, we're giving you a spacewalk today," said commentator Rob Navias.
NASA's only previous Christmas Eve spacewalk occurred in 1999 during a Hubble Space Telescope repair mission.
But NASA's most memorable Christmas Eve was back on Dec. 24, 1968. Apollo 8 astronauts read from Genesis, the first book of the Bible, as they orbited the moon on mankind's first lunar flight.
Space station managers considered waiting until January for the repair spacewalks, so an unmanned rocket could blast off with supplies from Virginia. But flight controllers were unable to patch the cooling line by remote control, and the orbiting outpost was considered in too vulnerable a state to put off the spacewalking repairs. The delivery mission was bumped, instead, to January.
A bad valve in the ammonia pump caused the breakdown.
Another team of spacewalking astronauts installed that pump just three years ago, and engineers are perplexed as to why it didn't last longer. NASA hopes to salvage it in the years ahead.
The 2010 replacement required three spacewalks because of the difficulty in removing pressurized ammonia fluid lines. This time, NASA reduced the pressure and the task was simplified, allowing the astronauts to get ahead Saturday. Although three spacewalks were scheduled this time around, Mastracchio and Hopkins' advance work allowed NASA to squeeze everything into two, barring any problems Tuesday.
The second spacewalk was supposed to take place Monday, but was delayed a day to give Mastracchio time to switch to another suit. He inadvertently hit a water switch in the air lock at the end of Saturday's excursion, and a bit of water entered his suit, making it unusable this week.
During the rest of the spacewalk, however, the suits remained dry. Last July, an astronaut almost drowned when water from his suit's cooling system flooded his helmet. Makeshift snorkels and absorbent pads had been installed in the suits as a precaution.
A Moscow-led spacewalk, meanwhile, is set for Friday. Two Russian crew members will install new cameras and fresh experiments outside.