Cape Canaveral - Astronauts have stepped out on the first of a series of urgent repair spacewalks at the International Space Station. The two Americans on the crew floated outside on Saturday morning. They will disconnect an ammonia pump that contains a bad valve. The breakdown has crippled the space station's critical cooling system for the past week and a half. The pump replacement is a huge undertaking attempted only once before, in 2010. Nasa has ordered up three spacewalks to complete the job. The next one will be on Monday. Astronauts Rick Mastracchio and Michael Hopkins will go back out again Christmas Day, if any work remains. The six-man crew had to turn off all nonessential equipment inside the orbiting lab following the malfunction on 11 December. Scientific research came to a near-halt.