NASA working on Plan B for space station repair

NASA engineers are once again scrambling to find a way to restore the International Space Station's ailing cooling system to full strength after weekend spacewalk repair efforts were stalled.

Leaking ammonia coolant and a stuck hose forced American astronauts Douglas Wheelock and Tracy Caldwell Dyson to cut short their attempts to replace a faulty cooling system pump during a Saturday spacewalk. The astronauts plan to stage another spacewalk to tackle the problem on Wednesday, and then – if all goes well – finish replacing the pump during an extra spacewalk on Sunday.

"We are pressing ahead with the second spacewalk on Wednesday," NASA spokesperson Rob Navias told Space.com.

Wheelock and Caldwell Dyson spent more than eight hours working outside the space station Saturday to try to disconnect the oven-sized ammonia pump from its mooring on the space station's right side. But one of four liquid ammonia hoses initially refused to budge, and then began leaking ammonia once Wheelock freed it by hammering on the connector with a tool.

The ammonia pump failed July 31 and knocked out half of the space station's main cooling system. The space station crew has turned off some systems and left others running without backups to prevent the orbiting lab from overheating. [Graphic: Inside and Out: The International Space Station]

The faulty pump is in one of two cooling loops serving the space station's U.S. segment. A second loop is working fine, mission managers have said. The space station's Russian segment also has its own independent cooling system, they added.

NASA engineers and mission managers spent all of last week planning Saturday's attempted spacewalk repair. The job was already so complicated that two spacewalks would be necessary, station managers said at the time.

Now, a third repair spacewalk will be required to replace the pump with a spare one and restore the space station's ammonia cooling system.

On Wednesday, the astronauts will have to disconnect two additional ammonia plumbing hoses elsewhere on the space station in order to staunch the leak that occurred during Wheelock's initial effort to remove the faulty pump, Navias said. Once that work is complete, the spacewalkers will vent any remaining ammonia from the final ammonia hose, then disconnect it along with five support cables and bolts in order to remove the pump.

Each pump weighs 780 pounds (353 kilograms) and is 5.5 feet long (1.6 meters) by 4 feet wide (1.2 meters). They are about 3 feet (almost 1 meter) tall.

The disabled pump will be stored on the station's railcar-like mobile transporter system, Navias said.

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"That's probably as far as we're going to get on Wednesday, and that then would set us up for a Sunday spacewalk," Navias said.

It would be on Sunday that, if all goes according to plan, Wheelock and Caldwell Dyson would be able to retrieve the spare ammonia pump and install it, Navias added.

The $100 billion International Space Station is currently home to six people. Three are American astronauts with NASA, and the rest are cosmonauts with Russia's Federal Space Agency.

Construction on the space station began in 1998 and is slated to be completed next year.

• Graphic: Inside and Out: The International Space Station
• FAQ: International Space Station Cooling System Pump Failure
• NASA: Third Spacewalk Required to Fix Space Station Cooling System