The International Space Station's cooling system has shut down, forcing crew to plan repairs during a spacewalk later this week.
Cooling loop A failed on Saturday night just before 7pm Central time, tripping a circuit breaker. This shut off power to the pump module for loop A which feeds ammonia to the cooling system.
An attempt on Sunday night to close the circuit breaker and restart the pump module failed.
But most systems are still receiving cooling, and many are operating with redundancy following the installation of jumper cables from the Destiny Lab’s power system overnight.
"The crew is not in any danger and is monitoring systems and relaxing on an otherwise off duty day," says NASA.
Two Control Moment Gyroscopes were taken off line on Saturday night, but one was spun up once again this morning, giving crew control over the station's orientation once again.
Temperatures on the Main Bus Switching Units, which route power to various systems, are a little higher than normal, but stable and well within normal parameters, says NASA.
The flight control and management teams today decided that a spacewalk planned for Thursday should be used to repair the cooling system.
Crew members Doug Wheelock and Tracy Caldwell Dyson will need at least two spacewalks to swap out the failed pump module, which is located on the station’s S1 truss.
There are two spare pump modules, and the team plans to use the one on External Stowage Platform 2, next to the Quest airlock.
Thursday's spacewalk was originally intended to install a power extension cable to the Unity module in preparation for the delivery of the Permanent Multipurpose Module on the STS-133 mission in November.
The team had also been planning to install a Power and Data Grapple Fixture on the Zarya module to support future robotics work. This work will be carried out at a later date, says NASA.
Although a final decision on a new spacewalk plan hasn't yet been made, Wheelock and Caldwell Dyson are likely to replace the pump module and structurally bolt it into place on the S1 truss on Thursday, with another spacewalk two or three days later to mate fluid and electrical connections.
The flight control team plans to vent residual ammonia in the lines between the Ammonia Tank Assembly on the S1 truss and the failed pump module on Tuesday to prepare for the module’s replacement.