Astronauts Thomas Marshburn and Christopher Cassidy conducted a spacewalk Saturday to fix an ammonia leak. They replaced a suspected faulty pump on the International Space Station.

UPDATED: 4:30 p.m., Saturday.

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As a nearly five-hour spacewalk Saturday morning drew to a close, the two astronauts replaced a suspected faulty pump in an effort to fix an ammonia coolant leak on the International Space Station.

Astronauts Thomas Marshburn and Christopher Cassidy began their spacewalk at 8:44 a.m. Saturday. The two successfully replaced a 60-pound pump box which NASA suspected was the source of the leaking ammonia coolant. They found no evidence of the frozen ammonia flakes that had originally led them to the pump box. The astronauts also found no evidence of damage to the pump box.

The walk was hastily planned after ISS crew members alerted Mission Control about the leak on Thursday when they spotted "snowflakes" of frozen ammonia floating near the pump box. NASA says that it has been aware of a slow ammonia leak, but the rate had jumped to 5 pounds per day on Thursday.

The ammonia coursing through the plumbing is used to cool the space station's electronic equipment, according to the Associated Press. There are eight of these power channels, and all seven others were operating normally. As a result, life for the six space station residents was pretty much unaffected.

NASA's space station program manager Mike Suffredini said it's a mystery as to why the leak erupted on Thursday. One possibility is a micrometeorite strike.

By 1:20 p.m. Saturday, Cassidy and Marshburn were finished with the space walk, and were heading back to the airlock. They saw no sign of leaks coming from the new pump. NASA engineers continued to pressure check the system and be certain that the new pump is working properly.

Excerpt from:

NASA astronauts fix leak on International Space Station