Two astronauts are working outside the orbiting International Space Station to try to bypass a leak in a cooling system.

Engineers suspect the radiator was hit by a micrometeoroid or a piece of space debris, which created a tiny hole that has allowed ammonia to escape. The radiators dissipate heat from the station's equipment.

Station commander Sunita (Sunny) Williams and flight engineer Akihiko (Aki) Hoshide left the station (at 1230 GMT) to reconfigure the lines and hook up a spare radiator. If the problem is not resolved, another space walk will be needed, and it could take several weeks to determine the source of the leak.

Williams and Hoshide are in constant contact with engineers at NASA's ground control as they maneuver around the space station to attach the coolant lines.

While the two astronauts worked outside, their four crewmates were busy unpacking cargo from the Russian Progress supply ship that docked with the ISS on Wednesday.