This story was updated at 11:48 a.m. EDT.
NASA is planning a pair of emergency spacewalks to repair the International Space Station after half of its cooling system shut down unexpectedly Saturday, forcing astronauts to power down several systems.
The two repair spacewalks will be performed by American astronauts Doug Wheelock and Tracy Caldwell Dyson and could begin Thursday to replace a pump module in one of the space station's two cooling system loops. Engineers are assessing the spacewalk plans before making a final decision.
"They already have procedures for this," NASA spokesperson Kelly Humphries told SPACE.com. "This is a significant item and one that there has been a lot of work to prepare for. They're just working on the final details."
A spacewalk had already been planned for Thursday for the Wheelock and Caldwell Dyson to upgrade some station systems. So their spacesuits and other general tools have been prepared for that excursion.
The details for the first repair spacewalk will likely be finalized by Wednesday morning. The second spacewalk could occur by Sunday, based on how Thursday's excursion goes, NASA officials said.
NASA has stressed that Wheelock, Caldwell Dyson and the rest of the six astronauts living aboard the International Space Station are in no danger from the cooling system problem. But it is something Mission Control would like to address quickly since a Sunday attempt to reset the circuit breaker and reactivate the pump module has failed.
"The bottom line is that the station is a good configuration right now," NASA spokesperson Kyle Herring said during daily mission commentary. The space agency will give an update on the space station's status and spacewalk planning at 4 p.m. EDT (2000 GMT).
Cooling system problems
The space station's cooling system troubles began late Saturday when a circuit breaker tripped and shut down the Loop A cooling system pump module, which is used to move super-cold liquid ammonia through the station to remove excess heat from its systems and laboratories.
The shutdown triggered alarms in the space station, awakening the outpost's six astronauts so they could begin turning off systems to reduce the heat load on the space station's only remaining working cooling system string Loop B. The space station's cooling system is critical to the outpost's operation since it prevents the station from overheating.
Station astronauts shut down two control moment gyroscopes (used to maintain the station's orientation in space without thrusters), some station power converters and command-routing equipment, and backup systems for the station's S-band communications antenna and Global Positioning System. So some station systems were running without backups in place if they fail.
Mission Control reactivated one of the two offline gyroscopes on Sunday and astronauts installed jumper cables between some station systems and the power system on the outpost's U.S. Destiny laboratory to preserve redundancy. The space station crew also got a change to rest from the long hours they worked late Saturday, Humphries said.
"The crew has basically gone back to normal activities," Humphries said.
Repair spacewalks ahead
The next step is a spacewalk repair. To prepare for the first spacewalk, Mission Control vented any remaining liquid ammonia from the Loop A system.
The space repair would likely call for two spacewalks by Wheelock and Caldwell Dyson, who would physically replace the afflicted pump module in the first excursion and hook up ammonia fluid and electrical connections in the second one.
There are two spare ammonia pump modules stored outside the space station on spare parts platforms attached to the orbiting laboratory's backbone-like main truss.
The space station is currently home to three Americans (NASA astronaut Shannon Walker is the third) and three Russian cosmonauts representing Russia's Federal Space Agency.
Wheelock and Caldwell Dyson had originally planned to perform a spacewalk Thursday to hook up a power extension cord for a future Russian laboratory and install equipment on part of the station's Russian segment. That work will be rescheduled to a later date, NASA officials said.
Astronauts have been living aboard the $100 billion International Space Station, which is being built by five international space agencies representing 15 different countries, for nearly 10 years. Construction began on the orbiting laboratory in 1998.
NASA plans to fly two space shuttle missions (in November and February, respectively) to complete construction of the space station before retiring the shuttle fleet next year. Congress is discussing the possible addition of a third and final shuttle mission, which if approved would likely carry spare parts and other supplies to the space station in next summer.
NASA will broadcast a space station status update live at 4 p.m. EDT (2000 GMT) on NASA TV. Click here for space station mission updates and SPACE.com's NASA TV feed.