Two NASA astronauts will venture outside the International Space Station on an emergency spacewalk today (May 11) in an attempt to o fix a leak of ammonia coolant.

NASA's Chris Cassidy and Tom Marshburn will perform the unprecedented spacewalk. The spacewalk will start at 8:15 a.m. EDT (1115 GMT) with NASA TV coverage beginning at 7 a.m. EDT (1100 GMT). You canwatch the spacewalk here on SPACE.comvia NASA TV.

The six residents of the orbiting laboratory noticedfrozen flakes of ammonialeaking from a station coolant loop on the leftmost side of the space station on Thursday (May 9). Liquid ammonia is a vital coolant for chilling the electronics that produce the outpost's power. [Emergency Spacewalk to Fix Space Station Leak in Photos]

"The plan for this EVA [spacewalk] really is to see if we can identify the leak," Mike Suffredini, NASA's International Space Station program manager said during a briefing yesterday. "The plan is to change out the pump on this particular EVA. The most likely sources of the leak is this particular pump."

The space station crew is in no danger, NASA officials said, but the pump responsible for the movement of ammonia through that part of the system was shut off in order to conserve coolant.

Spacewalk repair on tap

For today's spacewalk, Cassidy and Marshburn plan to inspect, and potentially replace, an ammonia coolant pump controller box on the space station's Port 6 truss, a segment of the station's scaffolding-like backbone that is on the far left side. NASA space station engineers suspect the box may be the source of the leak since the ammonia was spotted coming from an area near it on Thursday.

This will not be the first time space station crewmembers have spacewalked to repair an ammonia leak on the space station's cooling sytem.

NASA's Sunita Williams and Japanese spaceflyer Akihiko Hoshide took a spacewalk to correct a leak in a coolant loop on theInternational Space Station's Port 6 truss (its scaffolding-like backbone). The 2012 coolant leak was in the same loop as the current leak, but engineers don't yet know if the two leaks are related.

The station's Port 6 truss is the oldest piece of the space station's backbone and carries two of the outpost's eight wing-like solar arrays. It was launched in November 2000 and originally installed on the station's roof, extending up. In 2007, visiting shuttle astronauts relocated the P6 truss to its final location on the station's far left side.

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Astronauts Taking Spacewalk to Fix Space Station Ammonia Leak Today: Watch Live