CAPE CANAVERAL, Fla. — A pair of space station astronauts ventured out on an urgent spacewalk today 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.

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.