U.S. astronauts Ron Garan and Mike Fossum have completed the final spacewalk of NASA's 30-year shuttle program.

Garan and Fossum spent about six-and-a-half hours outside the International Space Station Tuesday. The two astronauts performed a variety of tasks, including retrieving a broken ammonia pump from outside the space station and installing a robot used for refueling and repairing satellites.

This has been the only spacewalk scheduled while the space shuttle Atlantis is docked at the orbiting outpost. The four astronauts aboard Atlantis assisted with the spacewalk.

The Atlantis crew arrived at the ISS on Sunday to deliver more than four tons of spare parts, equipment, food, and other supplies that will sustain space station operations through 2012.

Upon its return to Earth next week, Atlantis will be retired and the three-decade U.S. space shuttle program will end.

In other developments, NASA says a piece of Soviet space debris it has been tracking poses no risk to the space station or shuttle. There had been concern the debris could collide Tuesday with one of the spacecraft. The debris was expected to make its closest approach during Tuesday's spacewalk.

NASA is stopping the shuttle program to concentrate resources on deep-space exploration. The agency is working with several commercial U.S. aerospace companies to develop vehicles to replace the shuttles. Until then, Russia's Soyuz spacecraft will ferry U.S. astronauts to and from the space station, while Russian, European and Japanese cargo spacecraft will continue their resupply and waste disposal missions to the station.