Space station back in order after seven-hour spacewalk. 'Sweet!'

Astronauts aboard the International Space Station successfully completed today one of the most complicated repair jobs a station crew faces, swapping a failed cooling pump with one of four spares bolted to the station's exterior.

During a spacewalk lasting 7 hours, 20 minutes, US Army Col. Douglas Wheelock, one of the two spacewalkers involved, had to wrestle a 780-pound, refrigerator-size cooling pump into an exterior berth vacated last Wednesday when he and Tracy Caldwell Dyson removed the failed unit.

The installation also relied heavily on crew member Shannon Walker, who guided the station's robotic arm with Wheelock and the cooling unit on the end of it. The pump slid into its new home without a hitch.

As Wheelock followed Dr. Caldwell Dyson into the airlock after the trio finished the job, he called down to mission control and asked about the pump's status.

"The pump is looking good," came the reply.

"Aw, sweet! We have our station back," Wheelock said.

Beyond the immediate success for the space station, the repair job holds broader lessons for the future of human space exploration, notes Kirk Shireman, deputy head of the space station program at NASA.

It's important for the space program "to deal with these kinds of things in space without without a resupply vehicle readily available," he said during a post-spacewalk briefing today. "When we go to the moon and beyond, the only way you'll be able to repair things is with whatever you brought with you."

The repair job was all the more remarkable for the unusually short amount of preparation time mission planners had to choreograph the spacewalks.

The reason for the urgency: The pump's failure cut in half the station's capacity to cool the interior of the orbiting laboratory, where heat builds up from a range of electrical equipment as well as from six active astronauts.

The two spacewalkers failed to extract the failed pump on the first try on Aug. 7, due to an unexpected coolant leak as they disconnected coolant lines. Mission planners essentially worked around the clock to prepare the repair sequence astronauts would have to follow during their second try at removing the pump on Aug. 11.

Normally, the station operates with two cooling loops. Inside the station, water courses through cooling lines for each loop, picking up excess heat. But water would immediately freeze if exposed to the cold temperatures as the stations slips out of sunlight on the night side of Earth.

So the water flows through heat exchangers, which transfer the heat to the ammonia. The station's cooling system uses two pumps housed along the station's truss, each of which sends ammonia into one of two radiators that extend from the truss.

With one loop out of service, the station crew was in no danger, mission managers emphasize.

But if the second half of the cooling system failed before astronauts could make repairs to the first half, the situation "could get really ugly for us," Mr. Shireman said.

By the time the two spacewalkers were back in the airlock, all of the pump's electrical systems had checked out, and the pump had been pressurized. Engineers will begin flowing coolant through it Monday night.