Astronauts work to fix broken ISS cooling system

Two US astronauts began a second spacewalk Wednesday to fix a cooling system that broke last month prompting the shut down of some systems aboard the International Space Station, dpa reported.

Doug Wheelock and Tracy Caldwell Dyson began the complicated work to replace the broken 350-kilogramme ammonia cooling loop pump with a spare part stored about 10 metres away during a first spacewalk on Saturday. But they were unable to make much progress after a hose proved difficult to disconnect and leaked dangerous ammonia.

The two spacewalkers must catch up on tasks they were unable to complete Saturday and will focus on disconnecting the tricky hose before removing the pump and installing the new one.

A third spacewalk was added to hook up hoses and the wire the part into the station to allow the cooling system to operate. The final spacewalk to complete the repairs is scheduled for no earlier than Sunday.

   The cooling system is one of two used to keep electronics on the ISS from overheating. Alarms were triggered on July 31 signalling that the ammonia-based refrigeration system on the ISS had failed due to a power surge.

   NASA officials stressed that the six crew members were in no danger, and all critical and many non-critical systems on the ISS were since operating as normal despite the malfunction.

   The ISS could continue to operate without the cooling system indefinitely, but would be in trouble if the second ammonia cooling loop were to also fail before the first is replaced.

   The US space agency had known that the cooling unit would eventually need to be replaced after about 100,000 hours of use and had installed four extra units outside the ISS as a precaution