By Irene Klotz
CAPE CANAVERAL, Fla., Dec 24 (Reuters) - Two NASA astronauts floated outside the International Space Station on Tuesday for a second and final spacewalk to fix the outpost's critical cooling system.
Flight engineers Rick Mastracchio and Mike Hopkins were slated to spend about 6 1/2 hours outside the station to install a new ammonia cooling system pump. A nonworking unit was removed during a spacewalk on Saturday.
"Let's get to work," Hopkins said as left the station's Quest airlock just before 7 a.m. EST (1200 GMT) as it soared 260 miles (418 km) over Mongolia.
The spacewalk was broadcast live on NASA Television.
Hopkins, in his second spacewalk, was due to attach himself to the end of the station's 58-foot-long (17.7-meter) robotic arm for a ride over to where the spare pump is located.
Flying the arm from a control post inside the station's Destiny laboratory was Japanese astronaut Koichi Wakata, who is due to take over command of the complex, a project of 15 nations, in March. Wakata's backup was current Commander Oleg Kotov, one of three Russian cosmonauts aboard the space station.
Meanwhile Mastracchio, a veteran of seven spacewalks, rummaged through storage bags outside the station for the tools needed to hook up the new pump's electrical connections and ammonia fluid lines.
The U.S. side of the $100 billion space station, which includes Japanese and European laboratories, has been without half its cooling system since Dec. 11 when a valve failed inside a pump.
The six-member crew was never in any danger, but both cooling systems are needed to radiate heat from the station's modules and laboratories.
With just one cooling loop, astronauts had to turn off unnecessary equipment and some science experiments. The Russian part of the station has a separate cooling system.
During Saturday's spacewalk, Mastracchio and Hopkins removed the failed pump and put it in a temporary storage location outside the station.
NASA is considering a possible future spacewalk to repair the pump and keep it as another spare. In addition to the new pump being installed on Tuesday, the station currently has two other spares.
Saturday's outing was NASA's first spacewalk since July when the water-cooled spacesuit worn by Italian astronaut Luca Parmitano developed a leak. His helmet began filling with water, a situation that could have caused him to drown if the spacewalk was not quickly aborted.
Engineers traced the problem to contaminated water in a device that circulates water and air in a spacesuit and takes moisture out of the air. How the water became contaminated remains under investigation.
As a precaution, the helmets worn by Mastracchio and Hopkins were outfitted with moisture-absorbent pads and homemade snorkels that could draw air from the belly of the spacesuits in case the problem reoccurred.
No leaks were reported during Saturday's spacewalk. (Editing by Kevin Gray and Maureen Bavdek)