This story was updated at Aug. 3 at 8:28 a.m. ET.
NASA engineers are working round-the-clock to firm up plans to send astronauts on two emergency spacewalks outside the International Space Station to repair a serious cooling system failure.
The move comes after the weekend breakdown of a pump on the right side of the space station that pushes liquid ammonia through one of two cooling system loops that keep the outpost from overheating.
The malfunction occurred late Saturday and forced astronauts on the station to shut down many systems and leave others just one glitch away from failure. The first spacewalk is set for Friday.
Currently, the space station's condition is stable, with its second cooling system loop working well, and its crew is in no danger, NASA officials have stressed. But the space agency wants to fix the problem soon so the station crew can resume science experiments and other work that have been postponed until the pump can be replaced.
"Folks are working hot and heavy to get everything ready," said space station flight director Courtenay McMillan.
McMillan and her team had hoped to complete preparations in time to send American astronauts Douglas Wheelock and Tracy Caldwell Dyson outside the space station for the first repair spacewalk Thursday morning. But they decided late tonight to wait until Friday based on the results of practice dives in NASA's huge spacewalk rehearsal pool in Houston.
That spacewalk would likely begin around 7 a.m. ET (1100 GMT) if all goes well.
'Dramatic' spacewalk changes ahead
Normally, it takes up to two weeks for mission managers to plan a major International Space Station repair spacewalk, McMillan said. But two things have allowed NASA to fast track the upcoming repairs.
First, Wheelock and Caldwell Dyson already planned to perform a spacewalk this Thursday to prepare the space station for future work, so the spacesuits and station airlock are primed for the excursion. Only the plan for the spacewalk, known in NASA parlance as an extravehicular activity (EVA), will change.
Still, space station program manager Mike Suffredini said the repair plan marked "a dramatic change for the EVA."
Second, NASA already has a baseline plan to replace cooling system pump modules on the space station because they are vital components.
"This is an anomaly we knew someday would happen," Suffredini said. "It's an anomaly we have trained for. It's an anomaly we have planned for."
The space station's Russian segment runs on a cooling loop that is independent from U.S. cooling system and could handle the load for a short time in the unlikely event that the second loop in the U.S. segment failed, he added.
Space pump preparation
Suffredini said the pump module replacement is one of the "Big 14,"14 key space station repairs that space station mission managers prepare for on the off chance the failures occur while a NASA space shuttle is not at the orbiting lab.
Wheelock and Caldwell Dyson last trained on the general skills required for the tricky repair about a year ago, but are eager to tackle the job, he added.
"I did talk to the crew this morning ? they're in great spirits," Suffredini said.
Both repair spacewalks will be challenging, McMillan said.
Each pump module weighs 780 pounds (353 kg) and is 5 1/2 feet long (69 inches) by 4 feet wide (50 inches). They are also about 3 feet tall (36 inches), making them very bulky and tough to move, McMillan added.
The space station's faulty pump module has been at the orbiting laboratory since 2002 and in use since 2006. There are four spare pumps available at the space station.
Station engineers would prefer to return the pump to Earth to determine how it failed, but there is no room on the remaining scheduled shuttle missions, Suffredini said.
The space station is currently slated to be extended through at least 2020, though NASA only plans to fly two more space shuttle missions (in November and February) to complete construction of the orbiting laboratory 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 next summer.
If that third shuttle flight is approved, there would be room to return the pump module to Earth, Suffredini said. More pump modules could also be delivered to the station on non-shuttle spacecraft, such as Japan's H-2 Transfer Vehicle and the private Dragon spacecraft being developed by Space Exploration Technologies in California, he added.
Station astronauts set for spacewalks
The space station is currently home to three Americans (NASA astronaut Shannon Walker is the third) and three cosmonauts representing Russia's Federal Space Agency.
Thursday's spacewalk will follow a Russian spacewalk last week to outfit a space station module with gear that allows incoming Soyuz and Progress spacecraft to dock on autopilot.
Two NASA astronauts ? veteran spaceflyers Sunita Williams and Catherine "Cady" Coleman ? have been practicing the tricky repair in the Neutral Buoyancy Laboratory, an enormous pool at NASA's Johnson Space Center in Houston where astronauts practice for spacewalks using spacesuits and life-size versions of the space station and shuttles.
NASA hopes to be able to use the space station's Canadarm2 robotic arm to help move the hefty ammonia pumps, but the arm system ? like many others ? is without a backup. It was powered down with the rest of the heat-reducing measures.
Station engineers are confident they can keep operating the robotic arm without proper cooling if the system fails during the spacewalk while an astronaut is perched at the tip. They are refining the procedures for such an event, NASA officials said in a statement late Monday.
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, NASA officials said.
Astronauts have been living aboard the $100 billion International Space Station in shifts for nearly 10 years. Construction began on the orbiting laboratory in 1998. It is being built by five international space agencies representing 15 different countries.
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NASA will broadcast the International Space Station spacewalk repairs live from space on NASA TV, with the first spacewalk slated to begin Thursday at 7 a.m. ET (1100 GMT). Click here for space station mission updates and SPACE.com's NASA TV feed.