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    Astronauts Perform Emergency Space Walk To Fix Leak

    In this still image taken from NASA TV, International Space Station astronauts Chris Cassidy (top) and Tom Marshburn work on repairs to the ISS on May 11, 2013.
    In this still image taken from NASA TV, International Space Station astronauts Chris Cassidy (top) and Tom Marshburn work on repairs to the ISS on May 11, 2013.
    Astronauts carried out an unscheduled space walk Saturday to replace a pump that they believe was the cause of an ammonia leak from the International Space Station that was first detected on Thursday. NASA flight engineers plan to monitor the site of the leak closely in the weeks ahead to make sure no other problem exists.

    NASA Astronauts Chris Cassidy and Tom Marshburn spent five and a half hours outside the orbiting space station, removing a 97-kilogram pump controller box and replacing it with a spare. After completing the task, tethered to the orbiting craft in the weightlessness of space, the two astronauts looked for any further sign of leakage and saw none.

    But in a news conference at the Johnson Space Center in Houston later, Deputy International Space Station Program Manager Joel Montalbano said it could be some time before engineers can be certain that the problem is completely resolved.

    "It is going to take the teams a few weeks to go ahead and just watch the system, understand the system, see how it is performing, look at the different day and night cycles and, again, just watch it over time, before they are ready to tell us that we were 100 percent successful," Montalbano said.

    On Thursday the space station crew and engineers monitoring the orbiting laboratory from earth noticed white ammonia crystals, resembling snow flakes, that were floating out of an area where power units are located. Ammonia is used to cool the solar power systems that supply the station with electricity. In November there had been a similar leak in that area that crew members thought they had fixed.

    International Space Station Flight Director Ed Van Cise said engineers thought the pump was the problem, but they could not be certain until it was replaced.

    "That is the reason we kept the crew out there for 40 minutes after everything was done is that we wanted to have their eyes there at the scene while we turned it on and they could look for not only a big flow of ammonia crystals, but any smaller things that may tell us that there is still a leak going on," he said.

    NASA engineers say the ammonia in that power channel would have run out within a day had it not been shut down after the leak was discovered Thursday. But they say it never posed a serious threat to the space station or the six-man crew.

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