Two astronauts successfully installed a replacement coolant pump aboard the International Space Station Tuesday, wrapping up a high-stakes two-spacewalk repair job and clearing the way for flight controllers to re-activate a critical coolant system.

Credit: NASA

"We'd just like to say thanks to all the great folks doing all this hard work to get this space station back up and running," spacewalker Rick Mastracchio radioed as he returned to the Quest airlock module.

"No, thank you guys," astronaut Doug Wheelock replied from mission control in Houston. "It's the best Christmas ever. Thanks, guys."

"Yeah, I'd just like to add to that," Mike Hopkins said from the airlock. "Fantastic work, Merry Christmas to everybody. It took a couple of licks to get her done, but we got it."

The only problem of any significance during the seven-hour 30-minute spacewalk was trouble getting one of four ammonia lines disconnected from a so-called jumper box where it was temporarily plugged in during a spacewalk Saturday.

Hopkins and Mastracchio finally freed a quick-disconnect fitting holding the half-inch line in place, and were sprayed with ammonia ice crystals trapped in the connector. The toxic chemical posed no threat to the astronauts but they spent a few extra minutes in vacuum to ensure any ice stuck to their suits had time to dissipate before they re-entered the station.

The ammonia contamination barely slowed the astronauts down. After freeing the stuck line, Hopkins and Mastracchio finished hooking up all four ammonia lines to the replacement pump module and plugged in five electrical cables. Flight controllers then carried out a so-called "bump" test, briefly spinning up the pump and verifying the operation of internal valves.

"Good news! The thermal control officer reports a good bump start test on the newly installed pump module," NASA's mission control commentator, Rob Navias, reported. "We have a pump that is alive and well."

Later today, flight controllers planned to begin re-activating coolant loop A, clearing the way to restart scientific experiments and other hardware that was shut down when the coolant loop suffered a malfunction Dec. 11.

The successful spacewalk also clears the way for Expedition 38 commander Oleg Kotov and Russian flight engineer Sergey Ryazanskiy to carry out a spacewalk of their own on Friday. They plan to venture outside to mount cameras on the hull of the Zvezda command module as part of a commercial venture to beam down high-definition Earth views to subscribers around the world.

Today's spacewalk was the 258th by U.S. astronauts, the 176th devoted to station assembly and maintenance since construction began in 1998, the 10th so far this year, the eighth for Mastracchio and the second for Hopkins.

One hundred and 14 astronauts and cosmonauts have now spent 1,107 hours and 37 minutes of spacewalk time building and maintaining the International Space Station since construction began in 1998. Mastracchio's total through eight spacewalks now stands at 51 hours and 28 minutes, moving him up to sixth on the list of most experienced spacewalkers. Hopkins total EVA time through two spacewalks stands at 12 hours and 58 minutes.

The goal of today's outing was to install a replacement ammonia pump module in coolant loop A, one of two cooling systems used to dissipate the heat generated by the station's electrical systems. A valve in the loop A pump assembly malfunctioned Dec. 11, partially disabling the system and forcing flight controllers to power down non-essential systems.

More important, the malfunction left the station one failure away from a much more drastic powerdown should the lone operational coolant loop break down.

During a five-hour 28-minute spacewalk Saturday, Mastracchio and Hopkins disconnected the suspect ammonia pump module on the right side of the station's main power truss, pulled it from its slide-in rack in the S1 segment of the truss and mounted it on a nearby storage fixture to complete phase one of the coolant system repair job.

Because of an accidental switch throw inside the airlock at the end of the spacewalk, a small amount of water got into the plumbing of Mastracchio's backpack, raising the possibility that potentially damaging ice could form during the second EVA.

As a result, Mastracchio assembled a different suit for the second spacewalk while Hopkins used the one he wore Saturday, the same suit that developed a potentially dangerous leak during a July spacewalk.

After exhaustive troubleshooting, engineers concluded the leak was caused by contamination that clogged a filter. While the root cause of the contamination has not yet been determined, the astronauts replaced suspect components and both suits performed normally during both coolant repair spacewalks.

During their second spacewalk Tuesday, Mastracchio and Hopkins focused on installing the replacement pump module.

In spectacular video downlinked from the station, Mastracchio and Hopkins, anchored to the end of the station's robot arm, could be seen unbolting the spare pump module from its storage pallet and pulling it from its insulated enclosure.

"Looks like you're almost there," Mastracchio radioed as arm-operator Koichi Wakata, working inside the Destiny lab module, slowly pulled Hopkins and the pump assembly away. "You're out of the groove there, Mike, I think you're in charge now of the pump module. ... It's stable, Mike, it looks good, you're doing a great job, it looks beautiful."

Hopkins held the tethered pump module in his gloved hands as Wakata slowly moved him inboard from the storage pallet on the S3 truss segment to the pump's install location in the S1 segment.

The astronauts then guided the big module into place and drove home bolts to lock it down. They had no trouble hooking up the first two ammonia lines, known as M1 and M2. Another two lines, M3 and M4, were attached to a jumper box Saturday to allow the ammonia in the coolant loop to expand and contract as needed when the station flew into and out of sunlight.

The astronauts initially were unable to disconnect the half-inch-wide M4 line from the jumper box. Mastracchio retrieved a tool designed to apply additional force to the quick-disconnect bale holding the mechanism together. The idea was to push the lever far enough over to allow the astronauts to depress a locking button, allowing them to separate the two sides of the fitting.

But the stubborn fitting refused to cooperate.

"One thing we never expected," Mastracchio muttered at one point.

After positioning the quick-disconnect tool with varying degrees of force, flight controllers decided to lower the pressure in the line. Right around that point, the astronauts successfully demated the stubborn fitting, but reported "we do have snow coming out."

A few moments later, astronaut Douglas Wheelock in mission control asked the spacewalkers if ammonia was still leaking out "or has it dissipated."

"Yes. It's about one every second, one little snowflake a second," Mastracchio replied.

Asked if their spacesuits had been hit by any ice, Mastracchio said "absolutely," adding a few moments later "they are just completely drowning us now."

"Copy, Rick," Wheelock said. "And we have no video, Rick, so we'll just take your continued description."

"OK. They're pretty good size particles, much bigger than anything we've ever seen. See that big one going by you, Mike?"

"I do," Hopkins said.

"It looks like they're coming inboard of the pump module, all around the pump module, looks like," Mastracchio said. "I can't see it everywhere, where the light is, but they're hitting the wrist cluster of the SSRMS (robot arm), they're enveloping Mike, probably enveloping me, also."

"Yes they are," Hopkins said. "Big chunks, big chunks."

The ammonia presumably was trapped in the line and expelled when flight controllers sent commands to vent the jumper box. The leak rate diminished a few moments later and the astronauts pressed ahead with work to attach M4 and then M3 to the replacement pump module.

The ammonia posed no threat to the spacewalkers, but they had to spend a few extra minutes in vacuum to give any ice crystals that may of stuck to their suits time to dissipate. Decontamination procedures have been required during past spacewalks involving the ammonia coolant system to make sure returning spacewalkers don't introduce any toxic material into the station's air supply.

NASA originally held open the possibility of a third spacewalk to complete the coolant system repair work, but with the successful pump installation and activation Tuesday, the station crew now can look forward to a quiet Christmas in orbit before making preparations for the Russian spacewalk Friday.