Two American astronauts have floated outside the International Space Station for 5,5 hours, completing a first urgent spacewalk to repair a broken cooling line at the research outpost.

Astronauts Rick Mastracchio and Mike Hopkins began their spacewalk, broadcast live on NASA television, at 7:01am (12:01 GMT) on Saturday to remove a failed ammonia pump that caused the shutdown of the ISS cooling system for more than a week.

"Beautiful day. Awesome view," Mastracchio said as opened the airlock's hatch and looked out from 418km above the southern Atlantic Ocean.

On December 11 a broken valve caused the failure in one of the station's two external cooling loops, which circulate ammonia outside the station to keep both internal and external equipment cool.

The breakdown forced the six-man crew to turn off all nonessential equipment aboard, including some science experiments, but the crew was not in any danger, NASA said.

The work on Saturday went smoothly, with the astronauts finishing up an hour earlier than expected. They were able to not only disconnect the old pump, but also remove it from its pallet on the station's exterior truss, a task originally slated for a second mission next week.

Spacewalk postponed

At least one more spacewalk is needed to install a fresh spare.

Several hours after the pump was removed, NASA pushed the second spacewalk from Monday to Tuesday to size a new suit for Mastracchio.

At the end of Saturday's operation, Mastracchio's suit was compromised when he inadvertently turned on a water switch. He was not in any danger, but mission control decided to switch to a backup suit.

Japanese astronaut Koichi Wakata operated the station's 15-meter robotic arm, which hoisted Mastracchio and hefty equipment from one section of the $100bn research lab to another.

It was Mastracchio's seventh career spacewalk, while Hopkins ventured outside the ISS for the first time.

The astronauts had snorkels affixed to their spacesuits to protect them from a possible water leak. During NASA's last spacewalk in July, Italian astronaut Luca Parmitano's helmet began filling with water and could have caused him to drown.

Source: Agencies