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 a third on Christmas Day if it's needed.
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 got off to a quick start on the repairs as he worked to disconnect the flawed pump.
"Looking good so far," he told Mission Control as he unhooked four ammonia fluid lines. He encountered a loose bolt and informed Mission Control he was afraid it might come off, but was assured everything was fine. A few minutes later, he reported that some flakes of frozen ammonia — a toxic substance — brushed against his suit. "More flakes coming out," he radioed. "A steady stream of them."
Mastracchio, a seven-time spacewalker, 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.
Two hours into the spacewalk, the spacewalkers were still dry.
The pump replacement is a huge undertaking attempted only once before, back in 2010 on this very unit. The two astronauts who tackled the job three years ago were in Mission Control, offering guidance, as Saturday's drama unfolded 260 miles up. Mastracchio promised to bring back a wire tie installed on the pump by the previous spacewalkers. "Oh, awesome, thanks Rick," replied the astronaut in Mission Control who put it on.
The 780-pound pump is about the size of a double-door refrigerator and extremely cumbersome to handle, with plumbing full of dangerous ammonia. Any traces of ammonia on the spacesuits must be dissipated by the time the astronauts go back inside, to avoid further contamination.
NASA's 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. If the work is finished on the first two spacewalks, the third wouldn't be needed.
It would be the first Christmas spacewalk ever for NASA.
In the days following the Dec. 11 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, Va., to accommodate the spacewalks. That flight by Orbital Sciences Corp., which should have occurred this past week, is now targeted for Jan. 7.
Until Saturday, U.S. 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.