WASHINGTON (BBC):Two US astronauts have replaced a pump on a spacewalk aimed at fixing a leak of ammonia from the International Space Station’s cooling system. Chris Cassidy and Tom Marshburn completed the work an hour ahead of schedule, reporting no further escape. The crew had spotted particles of ammonia drifting away from the laboratory on Thursday. Nasa said the crew were not at risk but managers wanted to solve the problem before Mr Marshburn left the station. He is due to return to Earth early next week along with the space station’s Canadian commander Chris Hadfield and Russian cosmonaut Roman Romanenko, after six months in space.