NASA astronauts Rick Mastracchio and Michael Hopkins have successfully completed their first spacewalk to repair the failed coolant system of the International Space Station (ISS). It was a debut spacewalk for Hopkins and seventh for Mastracchio. Both wore extra safety gear to prevent a recurrence of the helmet flooding that nearly drowned an Italian astronaut last summer.

The astronauts unplugged a faulty ammonia pump from its nesting spot during the first stage of urgent repairs. The spacewalk lasted 5 hours 28 minutes instead of the planned 6.5 hours because Mastracchio complained that he was feeling cold in his spacesuit.

On his way back into the station, he compromised his suit by inadvertently turning on a water switch in the air lock. To give him time to get a spare suit ready, NASA's Mission Control postponed the second spacewalk until Tuesday.

If the astronauts keep up the pace, just two spacewalks may be enough, which means there is a chance to complete the entire repairs by Christmas. That may not be the case if the third spacewalk is required.

NASA wanted the first spacewalk to last longer, but midway through the excursion Mastracchio felt his toes were getting so very cold he had to crank up the heat in his boots. Two hours later, he told Mission Control that a number of other things were bothering him but declined to elaborate when asked what was wrong.

The 354-kilogram pump the size of a double-door refrigerator with plumbing full of toxic ammonia is difficult to handle. Its replacement was attempted only once before, in 2010. Any traces of ammonia on the spacesuits had to be removed before the crew re-entered the ISS.

Mastracchio unhooked the ammonia fluid and electrical lines on the pump with relative ease but let go an important O-ring. Luckily, he managed to retrieve it.

As the worked outside the station, a smoke alarm went off inside, but it turned out to be a false alarm.

On December 11, one of the coolant loops of the ISS shut down automatically when it reached temperature limits, prompting NASA to reroute the coolant systems. Some of the non-essential electrical equipment had to be turned off. The situation is serious but not critical.

"The equipment involved takes heat from all the electronic equipment inside the space station and transfers it outside the space station radiating heat away into space. The thing is operating perfectly all right, but if the second unit were to fail, that would mean that they were into a really critical situation, where they couldn't cool the space station properly, and they would likely have to abandon it,"Dr. Patrick Fullick, founder of Capital Science Connections, told RT.

Saturday's spacewalk was the first one by American astronauts since July, when Luca Parmitano's helmet was flooded with water from the cooling system of his suit and he barely got back alive.

Besides Mastracchio and Hopkins, the current ISS crew consists of three Russian cosmonauts and a Japanese astronaut.

Voice of Russia