The six-person International Space Station is headed toward a resumption of normal operations following a pair of unscheduled late-December spacewalks by NASA astronauts to replace a degraded external thermal control system (TCS) pump module.

The astronauts followed the excursions with other measures to restore cooling to all U.S. segment electrical systems deactivated in response to a Dec. 11 flow control valve failure.

The last of the electrical loads – all in the European Space Agency’s Columbus laboratory module – were expected to be restored to the repaired cooling system on Dec. 31, according to Kelly Humphries, a spokesman at NASA’s Johnson Space Center.

Research activities that ramped up as the assembly of the station’s U.S. segment was completed in July 2011 were among the operations interrupted by the balky valve housed within the pump module. So were the anticipated Dec. 18 launching of Orbital Sciences Corp.’s “ORB-1” resupply mission to the ISS and a Russian spacewalk. Launching of the Antares/Cygnus rocket/commercial freighter combination on what is to be the first mission flown under a $1.9 billion, eight-flight Cargo Resupply Services contract between NASA and Orbital Sciences was rescheduled for no earlier than Jan. 7. ORB-1 was postponed to clear the way for the contingency spacewalks on Dec. 21 and 24 by ISS astronauts Rick Mastracchio and Mike Hopkins.

Full ISS thermal control was restored Dec. 24-25. NASA’s Mission Control orchestrated the gradual return of nonessential electrical systems that were shut down in response to the thermal control restrictions in the aftermath. Those included research activities in the U.S. Destiny, Japanese Kibo and Columbus labs as well as connecting U.S. nodes.

A third ISS spacewalk — an unrelated Russian excursion on Dec. 27 to install a pair of Earth observing cameras for UrtheCast, a Vancouver, Canada-based commercial venture in collaboration with the Russian federal space agency Roscosmos — was not so successful. Cosmonauts Oleg Kotov, the ISS commander, and Sergey Ryazanskiy quickly installed but were then forced to retrieve the medium- and high-resolution still and video imagers when ground controllers were unable to establish the desired telemetry links for the planned Internet-based, subscriber service.

The setback pushed the spacewalk to 8 hr., 7 min., a Russian record.

Initially, the Mission Operations Directorate at NASA’s Johnson Space Center responded to an electrical failure of the flow control valve in Loop A of the station’s dual-loop ammonia TCS system with plans for three spacewalks. The successful strategy, accomplished with one fewer spacewalk than envisioned, included measures to avoid a repeat of the worrisome leak that allowed water to flow into the helmet of European Space Agency astronaut Luca Parmitano, who was wearing a U.S. spacesuit, during a July 16 spacewalk.

But Mastracchio had to don a different NASA spacesuit after an inadvertent airlock activation of the sublimator on his protective garment following the Dec. 21 excursion. Both spacewalks included brief but ultimately inconsequential exposures to leaking frozen ammonia, a toxic substance, as Mastracchio and Hopkins detached and reattached coolant lines.

During the 5 1/2 hr. Dec. 21 spacewalk, Mastracchio and Hopkins removed the 780-lb. Loop A pump module containing the balky flow control valve from its niche on the inboard starboard region of the station’s vaulting solar power truss. The two men worked ahead of their timeline, but Mastracchio noted discomfort in his toes from the cold temperatures.

The Dec. 24 spacewalk stretched to 7 1/2 hr. as the two astronauts replaced the older pump module with a spare stowed on the starboard truss in mid-2009.

The older pump module remains temporarily stowed near the worksite, where it can remain until mid-2014, according to the results of a new NASA thermal analysis. Engineers hope to refurbish the three-year-old component, eventually installing a new external flow control valve to replace the failed internal valve. A permanent stowage site on a solar power truss spare parts platform awaits. The station is also equipped with two additional spare pump modules, components that were designed to be launched and deorbited aboard the space shuttle.

Hopkins was assigned the same NASA spacesuit worn by Parmitano. But the suit’s personal life support system backpack was equipped with a new fan pump separator. Engineers believe water flow ducts in the fan pump separator in Parmitano’s suit became clogged with particulates that formed over time in the circulation of cooling water within the shuttle-era protective garment. Rather than shut off as it was immersed in water, the fan continued to turn on July 16, pushing water into an air circulation vent in Parmitano’s helmet.

NASA directed Mastracchio and Hopkins to place absorbent pads stretching from the top to the back of their heads for the two spacewalks, a protocol that is likely to remain in force. In Parmitano’s case, cooling water flowed through a helmet air vent at the back of his neck and massed around his eyes, ears and nose as he made a blind retreat to the safety of the airlock on July 16.

Mastracchio and Hopkins also installed makeshift snorkels in their suits. Fashioned from 20-in. pieces of plastic drain line, the snorkels were fastened inside the suits close to the lips of the spacewalkers. The tubing extended to their waists.

During the Dec. 21-24 spacewalks, the two astronauts were asked by Mission Control to check for signs of water in the absorbent pads every 30 min. Each check produced no evidence of leaking water.

Japanese astronaut Koiche Wakata served as the ISS robot arm operator during the excursions, moving Mastracchio and Hopkins around the outside of the orbiting research lab.

The ISS is equipped with Loop A and B cooling loops to dissipate internal as well as external heat from electrical systems, including life support and research hardware. The heat is released to space with the circulation of ammonia through radiators that extend from the solar power truss.

The Dec. 11 flow control valve failure allowed low-temperature ammonia to reach heat exchangers in the Harmony and Destiny modules in the station’s U.S. segment. The heat exchangers are designed to transfer heat from internal water cooling loops to the ammonia. Exposure to low-temperature ammonia could cause the water to freeze, expand and damage the water loop pumping and potentially introduce ammonia to habitable volumes of the space station.