CAPE CANAVERAL, Fla. -- Spacewalking astronauts relied on brute force to remove a balky ammonia line from a broken pump Wednesday, overcoming a hurdle that bogged down their previous effort to restore full cooling to the International Space Station.

It was the second spacewalk in five days for Douglas Wheelock and Tracy Caldwell Dyson. This time, to everyone's relief, there was no burst of ammonia from the jammed connector, just a few frozen flakes of the toxic stuff that leaked out.

The space station has been operating with only half its usual cooling capability ever since an ammonia coolant pump failed 1 1/2 weeks ago. Science research is on hold and unnecessary equipment is off until the pump can be replaced, a repair job that's considered one of the most challenging in the 12-year history of the orbiting lab. The cooling system is crucial for keeping electronics from overheating.

Wheelock had to yank the connector back and forth for several seconds before it popped off the failed pump, which could not be removed until that last hose was unhooked. He shook so hard with his gloved hands that the TV images beamed down from his helmet camera were bumpy and full of static.

"We didn't tell the guys inside to hold on when you did that," Mission Control joked.

It also took some tugging with a tool to get the connector off.

Before Wednesday's spacewalk, flight controllers lowered the pressure in the disabled cooling line -- one of two identical loops. That made all the difference. There was none of the major leakage that occurred during Saturday's spacewalk. The leak developed around the jammed connector; the spacewalkers had to hammer the connector loose, then plug it back in to stop the stream of ammonia.

As it turns out, there was no need Wednesday for the astronauts to isolate the troublesome connector by closing off other valves, or by venting out any residual ammonia. Instead, Caldwell Dyson jumped ahead and began unhooking electrical cables on the pump.

Their objective Wednesday was to remove the broken 780-pound pump, about the size of a bathtub. A spare would be installed during a third spacewalk Sunday.

NASA originally figured two spacewalks would suffice. The jammed connector and ammonia leak on the first outing, however, set everything back.

Since the July 31 malfunction, the space station has had to get by on a single cooling loop. NASA wants the second line up and running again as soon as possible, in case the first one ends up broken, too. That would leave the orbiting lab in a precarious position, with only a limited amount of time for emergency repairs before the crew would have to abandon ship.

Three Americans and three Russians are on board. Their safety has not been jeopardized by the cooling system trouble, and their comfort has not been compromised as they work and live 220 miles above Earth.
The space station is meant to continue working until 2020. NASA will have to rely on Russia and other countries for crew and cargo transport once the shuttle fleet is retired next year.

Only two shuttle visits remain, in November and February 2011. A third shuttle mission is under consideration for next summer.

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