Spacewalking NASA astronauts Rick Mastracchio and Mike Hopkins completed repairs to the six person International Space Station’s crippled external thermal control system on Tuesday, wrapping up the complex task in two rather than the three spacewalks originally envisioned.
During Tuesday’s 7 1/2 hour excursion, the two men installed a new refrigerator-sized pump module for the ammonia thermal control system in an onboard segment of the station’s starboard solar power system truss.
As they prepared to return to the ISS airlock to end the spacewalk at 2:23 p.m., EST, NASA activated the new module in a brief “bump test,” suggesting the new pump and its control valves were electronically functional. The successful re-pressurization of the Loop A thermal control apparatus with nitrogen and a more exhaustive post-repair check out followed on Tuesday night.
"The new pump (module) is considered fully functional," NASA said in a statement late Tuesday.
The measured reactivation of ISS dual Loop A and B cooling was under way Wednesday. The reactivaton of electronic systems deactivated after Loop A was degraded by a Dec. 11 flow control valve electrical failure was to follow through the weekend.
NASA astronaut Mike Hopkins, center, grips new ISS thermal control system pump module from the tip of the Canadian robot arm. Photo Credit: NASA.
Unlike the first repair spacewalk on Saturday, Mastracchio and Hopkins had to contend with the possible contamination of their spacesuits from a small amount of frozen ammonia, a toxic substance, that leaked during reconnection of one of four coolant lines to the new pump.
Loop A and its Loop B twin circulate ammonia coolant through radiators that extend from the ISS power truss to cool critical electrical systems, including those assigned to life support and scientific research. Scientific investigations were among those activities suspended by the loss of Loop A thermal control two weeks ago.
“Thank you guys," Mission Control told Mastracchio and Hopkins as they wrapped up their work. "This is the best Christmas ever."
The old pump module, which was removed during a spacewalk on Saturday, remains temporarily attached to a fixture on the Mobile Base System -- a rail car that runs along the truss parked close to the worksite. NASA expects to relocate the old 780-pound pump control module to a permanent external storage site during a future spacewalk. The bulky deactivated module can remain at the temporary site until June, according to a weekend thermal analysis by NASA engineers.
NASA is hopeful it can be refurbished while in orbit.
As on Saturday, neither Hopkins nor Mastracchio experienced a repeat of the worrisome July 16th leak of water that flowed into the helmet of European Space Agency astronaut Luca Parmitano, who was wearing a NASA spacesuit. However, Mastracchio was required to change NASA spacesuits between the Saturday and Tuesday spacewalks after the cooling system sublimator in his personal life support system backpack activated in the ISS airlock following the first excursion and raising concerns of a water intrusion.
During Saturday’s 5 ½ hour first spacewalk, Hopkins and Mastracchio detached four ammonia coolant lines, five electrical connectors and four hold down bolts to extract the old pump module from its perch on the power truss. The most challenging task was detaching the ammonia lines without a significant leak of the toxic coolant.
They reversed those steps on Tuesday, but were slowed by balky attach bolts as well as a spray of frozen ammonia while remating the third of four coolant lines.
Troublesome pump module ammonia line connections. Photo Credit: NASA TV
“We do have snow,” Mastracchio reported to NASA’s Mission Control, a reference to the flaky appearance of the frozen coolant as it emerged from a connection point.
Later, the spacewalkers inspected their spacesuits for evidence of frozen ammonia. They reported no obvious signs of contamination.
Mission Control, however, asked the two men to carry out a 15 minute airlock ammonia decontamination procedure before rejoining their four station colleagues.
The spacewalkers were assisted during both excursions by Japanese astronaut Koichi Wakata, who served as the internal operator of the Canadian robot arm. The 58-foot-long arm moved the spacewalkers and the new pump module around the outside of the station.
Cosmonaut Oleg Kotov, left, the current ISS commander, assists Japanese astronaut Koichi Wakata with space station robot arm operations on Tuesday.
Photo Credit: NASA TV
An electrical problem on Dec. 11 disabled the flow control valve in the older pump module, allowing low temperature ammonia to reach heat exchangers in the station’s U.S. segment Harmony and Destiny modules. Those exchangers transfer heat from water cooling lines in the U.S. segment’s habitable modules to the external ammonia loop.
The cold ammonia temps could cause the water to freeze, expand and damage plumbing that would allow unwanted ammonia into the ISS living volume.
With Loop A degraded, NASA flight control teams were forced to shutdown non-essential electronics inside the station, including those in the European Columbus and Japanese Kibo science modules. Loop B, took on additional thermal control chores as well so that both loops could continue to dissipate the heat generated by the station’s solar arrays, power storage batteries and switching units.
On Friday, Russian ISS cosmonauts Oleg Kotov and Sergey Ryazanskiy are scheduled for a six to seven hour spacewalk to install a pair of external Earth observing cameras and service several experiments. The excursion will represent the 11th this year by ISS crews, the sixth by cosmonauts.