The third spacewalk to replace a failed ammonia pump unit on the International Space Station will take place on Monday, NASA has announced.

Expedition 24 flight engineers Tracy Caldwell Dyson and Doug Wheelock will venture outside the orbiting outpost to finally install a replacement unit and restore the station's full cooling capability.

The job has proved somewhat bothersome, due mainly to a leaky ammonia line which delayed removal of the failed unit last Saturday. At the second attempt on Wednesday, Wheelock managed to disconnect the line from the module using brute force.

With the old pump now removed, Caldwell Dyson and Wheelock can bolt in the new kit, which is currently sitting on a stowage platform outside the ISS's Quest airlock.

They're due to start Monday's EVA just before 11:00 GMT. Wheelock will be making his sixth spacewalk, while Caldwell Dyson will be clocking up her third. ®