At around 10:30 a.m. CDT on Thursday, small white flakes were seen by the International Space Station (ISS) crewmembers as they were floating away from an position on the space station's P6 truss structure. The flakes turned out to be caused by a leakage of ammonia used to cool the ISS's power channels.
This ammonia loop is the same one that spacewalkers attempted to troubleshoot a leak on during a spacewalk on Nov 1, 2012. It is not yet known whether this increased ammonia flow is from the same leak, which at the time, was not visible. However, the rate of leakage is depleting the store of ammonia that cools one of the station's eight electrical power channels. The location of the leak is shown below.
Late Friday afternoon NASA officials made the decision to undertake an unscheduled spacewalk to repair the leak. NASA astronauts Tom Marshburn and Chris Cassidy are currently scheduled to begin their repair mission about 8:15 a.m. EDT on Saturday May 11. The spacewalk should last about six hours.
The station continues to operate normally otherwise and the crew is in no danger.
Source: NASA