Two space shuttle Discovery astronauts moved a failed ammonia cooling pump during a more than six-hour spacewalk on Monday outside the International Space Station, dpa reported.
NASA officials called Monday's work a "textbook" spacewalk.
It was the first of two spacewalks in the 11-day shuttle mission, which is Discovery's last before the spacecraft is retired.
Astronauts Steve Bowen and Alvin Drew also installed a back-up power extension cable and camera equipment and did other work to make future spacewalks easier.
Before heading back inside, they filled a metal container with the vacuum of space as part of a Japanese experiment called "Message in a Bottle" that will later be displayed on Earth.
The cooling pump broke last year and had to be relocated from a temporary storage location where it had been stashed on an earlier spacewalk. It had to be moved before it can be taken back to Earth for analysis.
The six-hour, 34-minute spacewalk wrapped up at 2220 GMT.
A second spacewalk is set for Wednesday.
Two more shuttle flights are planned for later this year in the final missions for the rest of the fleet, Endeavour and Atlantis.
Discovery's mission is delivering the last major US contribution to the ISS - an extra room - along with supplies and equipment, including a human-like robot, known as Robonaut 2 (R2), the first such robot ever sent to space.