Flight controllers are monitoring an ammonia coolant leak in the International Space Station's left-side power truss, NASA officials said late Thursday.

File photo of the International Space Station. Credit: NASA

The leak, apparently located in the outboard port-six (P6) solar array truss segment, was reported by the station crew around 11:30 a.m. EDT (GMT-4). Downlinked video, sources said, showed a stream of white flakes dissipating in the vacuum of space.

NASA officials said the crew was not in any danger.

The leak is in the system used to cool electronics associated with solar array power channel 2B, one of eight fed by the station's huge solar panels. Ammonia flowing through a large radiator is used to carry away heat generated by the array's batteries and electrical systems.

The coolant system requires at least 40 pounds of ammonia to operate normally. Based on the observed leak rate, NASA said in a web update, the channel 2B coolant loop could drop below that level and shut down within 48 hours if nothing is done to resolve it.

In that case, the station's six-man crew would be forced to reconfigure the station's coolant loops, using a different loop to cool at least some of the channel 2B systems.

While the crew would lose redundancy in the cooling system, flight controllers do not believe any major systems would have to be shut down to reduce cooling requirements.

The station is equipped with spare parts for the coolant system and the U.S. astronauts are trained for possible spacewalk repair jobs. But as of this writing, it is not known whether a spacewalk might be required at some point or whether some other repair option might be implemented.

But a spacewalk would require two U.S. astronauts. At present, two U.S. astronauts, a Canadian flier and three Russians are aboard the outpost. But NASA astronaut Thomas Marshburn, Canadian space station commander Chris Hadfield and cosmonaut Roman Romanenko are scheduled to return to Earth early Tuesday aboard a Russian Soyuz spacecraft.

"We don't see anything technically that we can't overcome," astronaut Doug Wheelock radioed the crew from Houston. "But we are still getting our arms fully around that issue."

This is not the first time a leak has been observed in the channel 2B system.

This annotated photo shows the P6 truss segment where astronauts worked on the coolant system on a Nov. 1 spacewalk. Credit: NASA

A slight 1.5-pound-per-year leak in the channel 2B cooling system was first noticed in 2007. During a 2011 shuttle visit, two spacewalking astronauts added eight pounds of ammonia to the reservoir to boost it back up to a full 55 pounds. The plan at that time was to top off the system every four years or so to "feed the leak," replacing the lost ammonia as required.

But over the next few months, engineers saw the leak rate suddenly quadruple, either because something changed at the original leak site or, more likely, because another leak developed elsewhere in the system.

On the assumption that the leak was in the solar array 2B coolant radiator, astronauts Sunita Williams and Akihiko Hoshide staged a spacewalk Nov. 1, 2012, to reconfigure coolant lines and to deploy a spare radiator, isolating the section of the loop where the leak was suspected.

The system operated normally until Thursday when the crew reported the visible leak. Whether the latest problem is related to the earlier issue is not yet known.

The lion's share of the International Space Station's electrical power comes from four sets of dual-panel solar arrays, two on the right side of a 357-foot-long truss and two on the left side.

Each set of solar arrays features two 115-foot-long panels that extended in opposite directions. The Russian segment of the station taps into the U.S. power grid to supplement electricity generated by two relatively small solar panels on the Zvezda command module.

The two U.S. arrays at the far left end of the station's integrated power truss -- the port six, or P6 arrays -- feed power to electrical channels 2B and 4B. The P6 set of arrays, like its three counterparts, routes power from the solar panels directly into the station's electrical grid during daylight passes, at the same time charging dual sets of batteries that take over during orbital darkness.

Each power channel generates between 150 and 160 volts of direct current, but downstream equipment near the center of the power truss -- equipment that uses a separate cooling system -- steps that down to 125 volts DC for use by the station's internal systems.

To keep the power generation components cool, each of the four sets of arrays uses two independent coolant loops that circulate ammonia through cold plates to carry heat out to a single shared radiator that extends from each module. The photo-voltaic radiator weighs 1,650 pounds and is made up of seven panels measuring 6 feet by 11 feet.

The space station can operate without the full complement of cooling channels, but the total loss of a coolant loop would require a significant reconfiguration to prevent electrical systems on the affected loop from overheating.