This story was updated at 12:05 p.m. ET.
NASA has delayed the start of major repairs on the International Space Station until Friday to give engineers more time to plan two spacewalks required to fix the outpost's ailing cooling system.
Space station managers had hoped to be ready to begin replacing a faulty ammonia pump in half of the space station's U.S. cooling system by Thursday, but the results of an underwater practice session by astronauts on Earth forced a delay. The complicated spacewalk repairs are now targeted for Friday and Monday.
The failed pump shut down over the weekend due to a tripped a circuit breaker caused by a power spike. It is a critical malfunction since the cooling system, which pushes liquid ammonia through plumbing lines, is vital to keep the space station's systems from overheating. [Graphic: Space Station's Cooling System Problem Explained]
American astronauts Douglas Wheelock and Tracy Caldwell Dyson will perform the two emergency spacewalks to replace the faulty ammonia pump with one of four spares stored on the space station's exterior. Two other astronauts are performing several practice dives at NASA's massive spacewalk rehearsal pool in Houston to help plan the complicated repair job.
'Gigantic' pump repair ahead
Caldwell Dyson said Monday that the ammonia pumps are "gigantic" and about the size of a laundry dryer box. It weighs 780 pounds (353 kg) and is 5 1/2 feet long (69 inches) by 4 feet wide (50 inches). They are about 3 feet tall (36 inches), making them very bulky and difficult to move.
"We're going to split up and one of us is going to take the failed one out and the other is going to prep one in a different location and then we're going to join up, swap one out for the other," Caldwell Dyson told the World Class Rockers, a band that was visiting Mission Control at the time. Caldwell Dyson is a lead singer for the all-astronaut rock band Max Q.
The first spacewalk is set to begin just before 7 a.m. EDT (1100 GMT) on Friday, NASA officials said.
Caldwell Dyson said the first spacewalk is dedicated to just swapping out the failed pump with a new one. On Monday, she and Wheelock will physically hook up the electrical and liquid ammonia lines, she added.
"That's, in a nutshell, what we're going to do," Caldwell Dyson said.
NASA has said repeatedly that the six astronauts aboard the station are in no danger, but they have had to shut down many systems and leave others without backups on the outpost's U.S. segment to reduce the amount of heat generated. The station's second U.S. cooling system loop is working fine with the reduced heat load.
The space station's Russian segment runs on a cooling loop that is independent from the 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, NASA's station program manager Mike Suffredini said Monday.
A failed ammonia pump is one of the more serious space station failures engineers can imagine, and is one of 14 major malfunctions that NASA keeps repair plans for just in case it occurs, Suffredini said. This is the first time any of those major repair jobs has been required, he added.
Pump's return to Earth desired
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 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 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, Suffredini said.
NASA plans 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.
- Graphic: NASA's Space Station Cooling System Malfunction Explained
- Gallery: NASA's 1st 100 Spacewalks
- NASA Scrambling to Plan Vital Space Station Repairs
NASA will broadcast the International Space Station spacewalk repairs live from space on NASA TV, with the first spacewalk slated to begin Friday at 7 a.m. ET (1100 GMT). Click here for space station mission updates and SPACE.com's NASA TV feed.