Astronauts Rick Mastracchio and Mike Hopkins prepared their spacesuits Monday for a Christmas Eve spacewalk, their second in four days to complete an urgent cooling system repair job, installing a spare 780-pound ammonia pump module on the right side of the International Space Station's main power truss.
If the spacewalk goes as smoothly as the initial repair work did during a five-hour 28-minute excursion Saturday -- and assuming the replacement pump module works as expected to restore lost cooling -- a third spacewalk will not be required.
That would clear the decks for an already-planned Russian spacewalk Friday to install high-definition cameras on the hull of the Zvezda command module as part of a commercial venture with a Canadian company that plans to beam down Earth views to subscribers around the world.
But first, Mastracchio and Hopkins, switching roles for their second spacewalk, must finish the coolant system repair work.For identification, Hopkins, call sign EV-1, will be wearing an unmarked suit with helmet camera No. 18. Mastracchio, EV-2, will be wearing a suit with red stripes around the legs and using helmet camera No. 20. This will be the 176th spacewalk 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.
If all goes well, the astronauts, floating in the Quest airlock module, will switch their suits to internal battery power around 7:10 a.m. EST (GMT-5) Tuesday to officially kick off a planned six-hour spacewalk.
Because of an accidental switch throw inside the airlock at the end of the first repair spacewalk Saturday, 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 will be using a different upper torso and backpack, No. 3005, while Hopkins will continue using the one he wore Saturday, No. 3011. That's 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 Saturday's spacewalk.
The cooling system repair work was ordered in the wake of a valve malfunction that partially disabled one of the space station's two ammonia coolant loops, which dissipate the heat generated by the lab's electronic systems.With one loop only partially functional, flight controllers had to power down non-essential equipment in the U.S. segment of the station, including much of the crew's research equipment. More important, the failure left the station with just one fully operational coolant loop. As a result, a subsequent problem with that system could require extensive power downs and a possible evacuation.
Engineers initially hoped to restore coolant loop A to normal, or near-normal, operation with a software "patch," but mission managers eventually decided to simply replace the pump module with one of three spares currently on board the station.
During their first spacewalk Saturday, Hopkins and Mastracchio successfully removed the coolant loop A pump module, disconnecting four ammonia lines and five power cables. Mastracchio, anchored to the end of the station's robot arm, then pulled the bulky pump module out of its rack in the S1 truss segment.
After a grapple fixture was attached to the pump housing, Mastracchio carried it to a nearby attachment fixture where the box was locked down and plugged into station power for long-term storage.
The original flight plan called for up to three spacewalks to replace the pump module, with its actual removal planned for the second EVA. But Mastracchio and Hopkins had no problems with the sometimes troublesome ammonia connectors and they were able to complete the removal task on Saturday.
Running well ahead of schedule, flight controllers asked the crew about extending the spacewalk to accomplish more get-ahead work, but Mastracchio said he preferred to end the excursion at that point. He did not say why he was reluctant to continue and flight controllers did not press him for an explanation.
During the second spacewalk, the astronauts plan to remove a replacement pump module from its storage pallet on the S3 truss segment, install it in the S1 slot where the faulty pump had been and re-attach the four ammonia lines and five electrical cables. This time around, Hopkins will be anchored to the end of the station's robot arm while Mastracchio, safely tethered, free floats.
NASA originally planned the second spacewalk for Monday, but the inadvertent switch throw at the end of the first outing that allowed water to enter Mastracchio's backpack prompted flight controllers to change their plans. While the suit was not damaged, its internal systems must be thoroughly dried out and that can take a week or so to accomplish.
"Unless something goes awry, we should wrap up all of this pump replacement work in just two spacewalks," said NASA mission control commentator Rob Navias.
If so, the crew will enjoy a holiday break Wednesday while two Russian cosmonauts, Expedition 38 commander Oleg Kotov and Sergey Ryazanskiy, prepare for a spacewalk of their own on Friday to install high-definition and medium-resolution cameras for a Canadian company, UrtheCast, and to replace external materials science experiments.
They are expected to open the hatch of the Pirs airlock module around 7 a.m. Friday to begin the year's 11th and final planned space station spacewalk.