An astronaut attempts to fix the cooling unit on the ISS
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NASA astronauts make second spacewalk to fix ISS cooler

NASA astronauts Doug Wheelock and Tracy Caldwell Dyson embarked on a second spacewalk to repair a vital cooling unit on the International Space Station.

A first attempt on Saturday failed despite the two astronauts conducting one of the longest spacewalks in history.

During Saturday's eight hour mission, Wheelock and Caldwell Dyson managed to detach the unit's hoses but a toxic ammonia leak disrupted efforts to replace the pump.

This time, controllers lowered the pressure in the disabled cooling line in an effort to prevent another leak.

NASA has said the three Russian cosmonauts and three NASA astronauts aboard the station are not at any risk.