Two NASA astronauts, working more quickly than expected, completed a spacewalk on Saturday in which they took the first steps to repair a malfunctioning pump module that is part of the cooling system for the International Space Station.

The astronauts, Col. Michael S. Hopkins of the Air Force and Richard A. Mastracchio, were far ahead of schedule throughout the nearly five-and-a-half-hour spacewalk as they detached tubing and electrical connectors from the pump. They were able to remove the 780-pound module and move it to a temporary storage location, a task that had been scheduled for completion during a second spacewalk.

Koichi Wakata, a Japanese astronaut, was operating a robotic arm from inside the station to help with the repairs.

Colonel Hopkins and Mr. Mastracchio stepped out of an airlock at 7:01 a.m. Eastern time, and even though they accomplished more than they had set out to do, they were able to return at 12:29 p.m., an hour earlier than scheduled. The two encountered few complications.