Mission control on Friday told astronauts to prepare for a spacewalk as soon as Saturday to diagnose the source of an ammonia coolant leak in one of the International Space Station's main power loops.

NASA astronaut Chris Cassidy works with spacesuits inside the space station airlock Friday. Credit: NASA/Spaceflight Now
 
NASA astronauts Chris Cassidy and Tom Marshburn, both experienced spacewalkers, would climb into spacesuits and exit the space station's airlock Saturday to help engineer isolate the leak, which appeared Thursday morning and was first reported by space station commander Chris Hadfield.

The ammonia leak is in the system used to cool avionics in one of eight electrical channels powered by the space station's main solar panels. The complex is primarily powered by the huge U.S. solar panels, which are mounted in four sections on each end of the space station's truss backbone.

Each power truss segment includes two panels extending 115 feet long, which rotate to follow the sun as the outpost races around the Earth at a velocity of five miles per second.

The leaky cooling system is on the space station's P6 truss section, mounted at the port end of the 357-foot-long truss. It was launched in 2000 and contains the oldest set of the space station's large solar array wings.

Mission control reconfigured the space station's power system overnight Friday to transfer the electrical load of power channel 2B to other channels before losing cooling capability.

A minor leak in the same cooling system was first noticed in 2007, and spacewalkers in 2011 added ammonia to the coolant loop replace the lost fluid. But the leak rate increased, forcing another spacewalk last year to try to isolate the leak by reconfiguring ammonia lines and activating a spare radiator.

The leak rate spiked Thursday, forcing engineers to address the problem immediately.

Read our earlier story for details on the background of the leak.

Mission control in Houston radioed the space station crew early Friday to start preparing for a spacewalk to locate the source of the leak that appeared Thursday.

Cassidy and Marshburn would begin the spacewalk Saturday morning. The astronauts conducted two spacewalks together on the STS-127 shuttle mission in July 2009.

Canadian astronaut Chris Hadfield, now in command of the space station, commented on the situation on his Twitter account.

"Good Morning, Earth! Big change in plans, spacewalk tomorrow, Chris Cassidy and Tom Marshburn are getting suits and airlock ready. Cool!"

Hadfield later tweeted:

"The whole team is ticking like clockwork, readying for tomorrow. I am so proud to be Commander of this crew. Such great, capable, fun people."

There was no word Friday morning whether the departure of Hadfield, Marshburn and Russian cosmonaut Roman Romanenko would go ahead as scheduled Monday. The three-man crew launched to the space station in December and is due to end their mission Monday with a parachute-assisted landing in Kazakhstan aboard a Russian Soyuz spacecraft.