Two NASA astronauts stepped out Tuesday on a rare Christmas Eve spacewalk to complete repairs at the International Space Station.

"The 10th spacewalk of the year at the International Space Station is now officially under way," a NASA commentator said at 6:53 am (1153 GMT), as astronauts made a second outing to replace an ammonia pump module whose internal control valve failed December 11.

The main task of the day is to retrieve a spare pump module from an external stowage platform and install it.

NASA astronauts Rick Mastracchio, 53, and Mike Hopkins, 44, made swift work of their first spacewalk on Saturday, disconnecting and pulling out the old cooling pump that regulates the temperature of equipment at the orbiting space lab.

They managed to complete almost two days' worth of work in a single outing that lasted just five and a half hours, ending an hour earlier than planned.

Flight Engineer Koichi Wakata of the Japan Aerospace Exploration Agency is operating the space station's 57-foot (17-meter) robotic arm, which will hoist Hopkins and the new, refrigerator-sized pump module from its stowage platform to the place it must be installed.

Wakata arrived at the space station in November for a half-year stay as part of the six-member international crew.

In March, he will become the first Japanese commander of the space station, NASA said.

Mastracchio is wearing a different spacesuit than he did Saturday, a backup that was stored at the station and was resized to fit him.

A "small amount of water" entered the suit's cooling system in the space station airlock after Mastracchio finished the last spacewalk, NASA said.

But the US space agency said the problem was not related to the water leak in a helmet that cut short Italian astronaut Luca Parmitano's spacewalk in July and risked drowning him.

NASA is still investigating what went wrong in that case. As a backup measure, the astronauts are now outfitted with emergency snorkels in their spacesuits and extra pads to absorb any leaking water in their helmets.

Hopkins and Mastracchio must complete five electrical connections and four fluid connections before the pump can be activated.

"If the spacewalk goes as expected Tuesday, all of the pump replacement work should be completed with just two spacewalks," NASA said.

NASA said tests will be done on the new pump later Tuesday to see how well it is working.

Tuesday's spacewalk is the second of Hopkins' career, and the eighth for Mastracchio.

NASA said the last time astronauts embarked on a Christmas Eve spacewalk was 14 years ago, when space shuttle Discovery astronauts Steve Smith and John Grunsfeld stepped out to install upgrades and new insulation on the Hubble Space Telescope.

Later this week, Russian cosmonauts Oleg Kotov and Sergey Ryazanskiy will make a spacewalk on December 27 to install a pair of high-fidelity cameras on the Zvezda service module and to do maintenance on the Russian segment of the station.