CAPE CANAVERAL, Fla. – A pair of space station astronauts ventured out on an urgent spacewalk Saturday to restore a crucial cooling system — one of the most challenging repairs ever attempted at the orbiting lab.

As the sun rose over NASA's spaceport, astronauts Douglas Wheelock and Tracy Caldwell Dyson exited the International Space Station. Their mission was to replace a broken ammonia coolant pump, a job considered so difficult that two spacewalks are required. Saturday was part one.

Wheelock quickly made his way to the broken pump as the space station circled 220 miles above the planet.

"Do I get extra credit?" Caldwell Dyson asked as she gathered tools.

Mission Control said she would and joked that she even might earn an extra brownie afterward.

Wheelock said all the connections looked good on the pump, and he saw no signs it had been hit by micrometeorites or other debris.

The ammonia pump shut down last weekend and knocked out half of the space station's cooling system. The pump is supposed to push ammonia coolant through the lines on the right side of the complex and prevent equipment from overheating. To cope with the failure, the six-person crew had to turn off all unnecessary equipment and halt science experiments.

The cooling line on the left side — unaffected by the trouble — has had to manage everything.

Engineers worked nonstop over the past week to come up with the emergency repair plan. In addition, astronauts in Houston rehearsed every step of the spacewalk while submerged in NASA's huge training pool.

There was a risk the fully pressurized ammonia lines could leak; the spacewalkers would have to get the toxic substance off their suits before going back inside. In addition, the lines were stiff because of all the pressure, making the repair job especially difficult.

NASA originally targeted Thursday for the spacewalk, the same day that Wheelock and Caldwell Dyson were supposed to go out and do some advance work for a November shuttle visit. But flight controllers needed two extra days to refine all the procedures.

Although space station managers knew an ammonia pump would fail one day, they did not expect it to happen so soon in the 12-year life of the complex. The pump was launched in 2002, but did not start operating fully until 2006.

The space station is equipped with four spare pumps; the one that will replace the broken unit went up in 2006.

Each pump is a boxy 5 1/2 feet by 4 feet by 3 feet and has a mass of 780 pounds. Several cooling lines will have to be connected to the new pump, at least some of which will be done on the second spacewalk planned for Wednesday.

NASA said the breakdown is serious but has not endangered the crew, and the one functional cooling loop has kept the space station stable. Additional breakdowns could leave the station in a precarious situation, however, and that's why managers wanted to get the broken line working again as soon as possible.

Saturday's spacewalk was the first by Americans, without a shuttle present, since 2008.

The crew includes three Americans and three Russians. Caldwell Dyson has been on board since April, and Wheelock since June.

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