Astronauts conduct spacewalk to fix ammonia leak on space station MIAMI (Reuters) - Two astronauts aboard the International Space Station took a hurriedly arranged spacewalk on Saturday to try to fix an ammonia leak in a cooling system on one of the station's solar arrays that provide electricity to the orbital outpost. The astronauts used a pistol-shaped screwdriver to remove a panel, install a spare pump and add new ammonia coolant, and were to spend several more hours outside the station troubleshooting the plumbing problem. The crew spotted a steady stream of small, white frozen ammonia flakes floating away from a coolant line outside the station on Thursday, according to NASA. Mission managers reviewed images and data before deciding to send American astronauts Chris Cassidy, 43, and Tom Marshburn, 52, out on Saturday morning to try to stop the leak by replacing a pump on the cooling system. "The crew is not in danger, and the station continues to operate normally otherwise," NASA said in a statement. Ammonia is used to cool the power systems that independently operate each of the station's eight solar arrays. The leak is on the far left side of the station's truss structure, in an ammonia loop that astronauts previously tried to troubleshoot during a spacewalk in November 2012. While Cassidy and Marshburn, both experienced spacewalkers, worked outside the space station, crew commander Chris Hadfield, a Canadian astronaut, choreographed their movements from inside the station. Russian cosmonauts Pavel Vinogradov, Alexander Misurkin and Roman Romanenko make up the rest of the crew. Crew also rerouted the remaining power channels to maintain full operation of the systems normally controlled by the solar array that is cooled by the leaking loop. The space station, a $100 billion research laboratory that orbits 250 miles above Earth, is owned by the United States and Russia in partnership with Europe, Japan and Canada. The spacewalk was being live-streamed on NASA TV (www.nasa.gov/multimedia/nasatv). Reporting by David Adams and Jane Sutton; editing by Jackie Frank