(CAPE CANAVERAL, Fla.) Astronauts ventured out Saturday on the first of a series of urgent repair spacewalks to revive a crippled cooling line at the International Space Station.

The two Americans on the crew, Rick Mastracchio and Michael Hopkins, will need to perform two and, quite possibly, three spacewalks to replace an ammonia pump containing a bad valve.

Next will be one Monday, followed by the third on Christmas Day.

The breakdown 10 days ago left one of two identical cooling loops too cold and forced the astronauts to turn off all nonessential equipment inside the orbiting lab, bringing scientific research to a near-halt and leaving the station in a vulnerable state.

Mastracchio, making his seventh spacewalk, and Hopkins, making his first, wore extra safety gear as they floated outside. NASA wanted to prevent a recurrence of the helmet flooding that nearly drowned an astronaut last summer, so Saturdays spacewalkers had snorkels in their suits and water-absorbant pads in their helmets.

Beautiful day, Mastracchio said as the orbiting complex approached the west coast of Africa.

And then: The ammonia tank over here looks familiar.

The pump replacement is a huge undertaking attempted only once before, back in 2010. The two astronauts who tackled the job three years ago were in Mission Control, offering guidance, as Saturdays drama unfolded 260 miles up.

The 780-pound pump is about the size of a double-door refrigerator and extremely cumbersome to handle, with plumbing full of toxic ammonia. NASAs plan fine-tuned over the past several days called for the pump to be disconnected Saturday, pulled out Monday and a fresh spare put in, and then all the hookups of the new pump completed Wednesday.

It would be the first Christmas spacewalk ever for NASA.

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NASA Astronauts Begin Urgent Spacewalks