Saturday night just before 8 p.m. Eastern time, some 200 miles above the Earth’s surface, a circuit breaker tripped: No one on board the International Space Station is in danger, but the outpost is now one cooling loop down. NASA said in a statement that they are planning an emergency spacewalk to fix this part of the station’s cooling system later this week.

The cooling loop moderates the station’s temperature and regulates other station avionics controls, by keeping cooling ammonia circulating through it. Without any cooling system, the three Americans and three Russians currently on board might find conducting research difficult:

According to NASA figures, without thermal controls the ISS’s sun-facing side would roast at 250 degrees Fahrenheit (121 Celsius), while the outpost’s dark side would plunge to some minus 250 degrees Fahrenheit (-157 Celsius). A statement posted some years ago on NASA’s website suggested: “There might be a comfortable spot somewhere in the middle of the Station, but searching for it wouldn’t be much fun!” [AFP]

Besides the cooling loop, the malfunction also took down two of the station’s four gyroscopes, which are used to position the station, but one is already again up and running, and NASA says that three out of four is enough to operate the station for now.

Flight control has already given preliminary approval for a spacewalk later this week, the first of two walks to fetch repair supplies stored elsewhere on the station and install them. The walks will replace a scheduled spacewalk to install a power cable and platform for robotics work.

Although a final decision on a new spacewalk plan is still pending engineering and timeline analysis, the most likely scenario would call for an initial spacewalk no earlier than Thursday by [Doug] Wheelock and [Tracy] Caldwell Dyson to replace the Pump Module and structurally bolt it into place on the S1 truss, with an additional spacewalk by the duo two or three days later to mate fluid and electrical connections. [NASA]

NASA plans to vent any residual ammonia in the cooling loop’s lines (for the repair-astronauts’ protection) and says that the astronauts will perform the repairs no sooner than Thursday. A NASA TV briefing planned for 4 p.m. (Eastern time) today may give more details.

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Image: NASA