once again tonight. Now a story way above the ground tonight, a developing story. Two american astronauts facing a risky challenge. We have told you about the crucial pump on board the international... See More
once again tonight. Now a story way above the ground tonight, a developing story. Two american astronauts facing a risky challenge. We have told you about the crucial pump on board the international space station that needs fixing. Now nasa has ordered three urgent space walks, one on christmas day. Abc's neal karlinsky with the breaking details. Reporter: Nasa says a series of dangerous space walks will be necessary. The two american astronauts on board, rick mastracchio and michael hopkins will suit out and head out at 5:15 a.M. Saturday for a six hour mission to repair the troubled cooling system which keeps the station from over heating. They're designed to take away the heat that's generated and to regulate the temperature of the mechanisms and of the environment for the astronauts. Reporter: Astronaut doug wheelock performed a similar mission three years ago and knows what they're up against. Any time we open that hatch i get asked is it dangerous. As soon as we open that hatch, it automatically becomes dangerous. Reporter: The last time nasa performed a space walk, the astronaut nearly drowned when water leaked into his helmet. The repair is not a simple one. A second space walk is scheduled for monday and a third if needed will take the astronauts into the vacuum of space on christmas day, leaving nasa engineers down on earth holding their breath before they can celebrate. Neal karlinsky, abc news, seattle.
This transcript has been automatically generated and may not be 100% accurate.