NASA astronauts Tom Marshburn and Chris Cassidy are attempting to locate the source of an ammonia coolant leak on the International Space Station.

The operation began Saturday at 7 a.m. EDT. NASA has reported the crew of the ISS is not in any danger. NASA estimated the spacewalk would last roughly 6-1/2 hours.

Watch a live stream of the NASA repair operation below, courtesy of Ustream:

Live Video Streaming by Ustream