CAPE CANAVERAL, Fla. (AP) — Astronauts have stepped out on the first of a series of urgent repair spacewalks at the International Space Station. The six-man crew had to turn off all nonessential equipment inside the orbiting lab following the malfunction on Dec. 11. Scientific research came to a near-halt following the problem. Scientific experiments aboard ISS are controlled by technicians working at the Marshall Space Flight Center in Huntsville, Alabama. The two Americans on the crew floated outside Saturday morning. They disconnected an ammonia pump that contains a bad valve. The breakdown has crippled the space station's critical cooling system for the past 1½ weeks. 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 Monday. Astronauts Rick Mastracchio and Michael Hopkins will go back out again Christmas Day, if any work remains.