Two American astronauts will make an unplanned spacewalk tomorrow to diagnose and try to repair an ammonia leak.  The liquid is critical for cooling the components of the International Space Station and is circulated in tubes running throughout the orbiting complex.  American astronauts Chris Cassidy and Tom Marshburn are experienced spacewalkers and have been trained for this kind of repair work.  

The small leak was first diagnosed back in 2007 but spiked on Thursday, prompting the need for unplanned spacewalk.  The two American astronauts and a Russian colleague were scheduled to leave the space station on Monday but it's unclear if the repair work will delay their return.