Astronauts in Space (VIDEO)

Two Nasa astronauts, their spacesuits newly modified with snorkels in case of another water leak, floated outside the International Space Station for a second day as they continue the marathon three-day task of fixing the outpost's cooling system.

The spacewalk is only the second for Nasa since July when the spacesuit helmet worn by Italian astronaut Luca Parmitano began filling with water, a situation that could have caused him to drown.

The latest spacewalks were prompted by the December 11 shutdown of one of the station's two ammonia cooling systems, which forced the crew to turn off non-essential equipment and shut down dozens of science experiments.

Astronaut Mike Hopkins on the robotic arm during a spacewalk outside the ISS (AFP)

While the six-member crew is not in danger, the remaining cooling system cannot support the three laboratories and other modules on the US side of the station, a project of 15 nations.

The Russian side of the station has a separate cooling system.

Engineers at the Johnson Space Center in Houston tried devising ways to bypass a suspected faulty pump valve, but with time running short, managers decided to have astronauts replace the pump, located outside the station, with a spare.