The ammonia supply from the impacted 2B power channel was expected to be depleted late Friday morning.
"Station's power relies on ammonia coolant. A few hours ago, we determined that the ammonia was leaking out of the Station and into space," Expedition 35 Commander Chris Hadfield reported via Twitter on Thursday.
This morning, as plans were underway for a potential emergency spacewalk, Hadfield was upbeat about the situation.
"Good Morning, Earth! Big change in plans, spacewalk tomorrow, Chris Cassidy and Tom Marshburn are getting suits and airlock ready. Cool!" he said. "The whole team is ticking like clockwork, readying for tomorrow. I am so proud to be Commander of this crew. Such great, capable, fun people."
PHOTOS: Inside Atlantis' Final Space Station Mission
NASA astronauts Cassidy and Marshburn are currently working in the station's Quest airlock, checking out the U.S. spacesuits they would wear if a spacewalk is approved, according to a NASA news release. Hadfield is also preparing to assist as the "intravehicular" crew member, or spacewalk choreographer.