NASA TV image shows Atlantis astronaut Bobby Satcher (C) on a robotic arm, working during a planned spacewalk. Astronauts from the shuttle Atlantis completed their first space walk of the mission, replacing a vital communications antenna on the exterior of the International Space Station. [Photo: Xinhua/AFP]

Two U.S. astronauts conducted the first spacewalk for space shuttle Atlantis and installed an antenna on the International Space Station (ISS) Thursday, NASA said.

According to NASA, mission specialists Michael Foreman and Dr. Robert Satcher Jr. began the spacewalk at 9:24 a.m. EDT (1424 GMT), which lasted six hours and 37 minutes.

The duo installed a spare antenna to the outside of the ISS in just two hours after venturing out. They also hooked up cables and a handrail, and greased some mechanisms.

After finishing all their scheduled chores with two hours to spare, Foreman and Satcher were assigned a task that had been planned for the second spacewalk. They deployed the outboard Payload Attach System on the Earth-facing side of the Starboard 3 truss, after overcoming some initial difficulties.

Atlantis lifted off from Kennedy Space Center in Florida on Monday. Its 11-day flight will include three spacewalks and the installation of two platforms to the station's truss, or backbone. The platforms will hold spare parts to sustain station operations after the shuttles' retirement.

Atlantis' STS-129 mission is NASA's fifth and last shuttle mission for 2009. There are just five more shuttle launches scheduled before the planned September 2010 retirement of the space shuttle fleet.