• Pump replacement has been attempted only once before

This article is more than 6 years old

This article is more than 6 years old

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 Saturday's 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 Saturday's drama unfolded 260 miles up.

The 780-pound (353kg) pump is about the size of a double-door refrigerator and extremely cumbersome to handle, with plumbing full of toxic ammonia. Nasa's plan fine-tuned over the past several days called for the pump to be disconnected Saturday, pulled out Monday and a new spare put in, and then all the hookups of the new pump completed Wednesday.

It would be the first Christmas spacewalk ever for Nasa.

In the days following the 11 December breakdown, flight controllers attempted in vain to fix the bad valve through remote commanding. Then they tried using a different valve to regulate the temperature of the overly cold loop, with some success. But last Tuesday, Nasa decided the situation was severe enough to press ahead with the spacewalks. Although the astronauts were safe and comfortable, Nasa did not want to risk another failure and a potential loss of the entire cooling system, needed to radiate the heat generated by on-board equipment.

Nasa delayed a delivery mission from Wallops Island, Virginia, to accommodate the spacewalks. That flight by Orbital Sciences Corp., which should have occurred this past week, is now targeted for 7 January.

Until Saturday, US spacewalks had been on hold since July, when an Italian astronaut's helmet was flooded with water from the cooling system of his suit. Luca Parmitano barely got back inside alive.

Engineers traced the problem to a device in the suit that turned out to be contaminated how and why, no one yet knows.

For Saturday's spacewalk, Hopkins wore Parmitano's suit, albeit with newly installed and thoroughly tested components.

Just in case, Nasa had Mastracchio and Hopkins build snorkels out of plastic tubing from their suits, before going out. The snorkels will be used in case water starts building up in their helmets. They also put absorbent pads in their helmets; the pads were launched from Earth following the July scare.