NASA astronauts step out on Christmas Eve spacewalk
"The 10th spacewalk of the year at the International Space Station is now officially under way," a NASA commentator said at 6:53 am (1723 IST), marking the start of the second of two outings 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 17-metre robotic arm, which will hoist Hopkins and the new, refrigerator-sized pump module from its stowage platform to the place it must be installed.
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.