Two U.S. astronauts are preparing for a possible May 11 spacewalk outside the International Space Station (ISS) in response to a significant ammonia coolant leak in the thermal control system radiator of the orbiting lab’s oldest solar power module.

The NASA-led ISS Mission Management Team has scheduled a meeting for late May 10 to consider final approval for a 6-7-hr. extravehicular activity (EVA) to replace the source of the suspected leak, a pump and flow control system (PFCS) electronics box, or to carry out further troubleshooting.

The urgency of the spacewalk is timing, Mike Suffredini, NASA’s ISS program manger told a May 10 news briefing. By the time the spacewalk occurs, station managers are hopeful there will be enough residual pressure in the thermal control system for the astronauts spot the liquid ammonia as it freezes and escapes in distinctive white flakes. “We are talking about a very very small hole,” Suffredini said. “The hard part will be finding the location.”

NASA astronauts Chris Cassidy and Tom Marshburn would begin the outing just after 8 a.m. EDT. Two spare PFCS assemblies that regulate the flow of toxic ammonia through the station’s 2B power channel to provide thermal control for the electronics are stowed next to the work site.

The spacewalk is not expected to interfere with plans for ISS commander Chris Hadfield, cosmonaut Roman Romanenko and Marshburn to undock from the six-person station late May 13 in their Soyuz TMA-07M spacecraft and descend to a landing in southern Kazakhstan, ending a 146-day mission. Undocking is set for 7:08 p.m. EDT.

If more than one spacewalk is necessary, the troubleshooting will likely fall to three new ISS crew members scheduled to reach the station May 28.

While Cassidy, Marshburn and Hadfield readied the U.S. segment Quest airlock, U.S. spacesuits and tools for the excursion, NASA and European Space Agency astronauts Terry Virts and Samantha Cristoforetti rehearsed the procedures and exercised the timeline in the Neutral Buoyancy Laboratory, a large training pool at NASA’s Johnson Space Center.

“Everyone is comfortable with what is going on,” Hadfield, the first Canadian to command the ISS, informed NASA’s Mission Control as the preparations unfolded.

The leak, discovered May 9, was expected to drain the 2B cooling loop by late May 10. Cooling for ISS power systems was being shifted to the similar 2A loop to keep all power systems operating as long as possible. As of mid-day, the station was operating at normal power levels, said NASA spokesman Rob Navias. NASA stressed the astronauts were in no danger.

Cassidy and Marshburn carried out three previous EVAs during a July 2009 space shuttle ISS assembly mission. Two of the excursions, carried out as a team, took the two astronauts to the P-6 solar power module for a scheduled replacement of the power storage batteries.

In addition, the leak troubleshooting is ranked as one of the “Big 12” ISS maintenance tasks that U.S. crewmembers train extensively for before they lift off.

Hadfield, an experienced spacewalker who has also trained for the Big 12, would choreograph the EVA from inside the station if ISS managers give final approval.

Early May 10, the station’s three Russian cosmonauts joined in the efforts to pinpoint the source of the leak, with one noting a seepage of “frozen drops” 1-3 cm in diameter coming from the area of the P-6, 2B radiator panel. The cosmonauts also transmitted a photo of a suspicious “mark” on the solar module to ground controllers for evaluation.

On Nov. 1, previous ISS crewmembers Sunita Williams and Akihiko Hoshide carried out a near 7-hr. spacewalk to install jumper cabling in an effort to circumvent a persistent, low-rate coolant leak in the same region of the station’s far port-side solar power truss. The source was never pinpointed by the repair work, which included re-activation of a radiator panel that had been active during the station’s early assembly phase.

The P-6 is the oldest of the station’s four big solar power modules. Launched in late 2000, it was mounted in a temporary position on the station to serve as a solar power source during initial assembly. The 30,000-lb. truss piece with its twin solar panels was re-located to its current position during a late 2007 shuttle ISS assembly mission.

The departing crewmembers are scheduled to be replaced on May 28 by cosmonaut Fyodor Yurchikhin, NASA astronaut Karen Nyberg and European Space Agency astronaut Luca Parmitano. They are scheduled to launch aboard the Soyuz TMA-09M at 4:31 p.m. EDT, on an expedited transit that would have them dock at 10:17 p.m. EDT.