One of two cooling loops on the International Space Station failed late Wednesday, creating a problem that was “urgent” but not “life-threatening” for the ISS crew, NASA told NBC News. A spacewalk may be necessary to repair the loop known as Loop-A. And for the time being, Loop-B is bearing all of the cooling systems' load.
A NASA spokesperson said that cooling system Loop-A was shut down due to an anomalous temperature imbalance, which triggered a shutdown after it reached a pre-set temperature. As NBC News reports, “engineers think the problem was caused by a malfunctioning flow control valve for the station's ammonia coolant. Mission managers are trying to determine whether a software fix can get the valve working again or whether a spacewalk will be required. Sorting through the issues might take a couple days or as much as a couple of weeks.”
The problem is considered one of 14 issues (together called “The Big 14”) that NASA expects to face occasionally over the course of the ISS' life. According to NBC News, it generally takes two weeks to prepare for a “Big 14” repair, but depending on resources, it can take as little as two days.
Because of the failure, some non-critical systems were shut down, including the Japanese Kibo lab and the European Columbus lab, “but external electrical equipment is still working fine,” the spokesperson said. Life support systems and electrical systems have been prioritized, as well as the systems running science experiments, including those dependent on freezers preserving samples.
Problems with the cooling system aren't new. Back in May, an ammonia leak was discovered on the ISS, necessitating an emergency spacewalk in which astronauts Chris Cassidy and Tom Marshburn replaced a PFCS module successfully.