Two American residents of the International Space Station are set to step outside the facility on Tuesday to begin the final spacewalk of NASA's shuttle era.

Astronauts Ron Garan and Mike Fossum will complete the mission, which involves retrieving a broken ammonia pump from outside the space station. They will also install a robot used for refueling and repairing satellites.

The four astronauts aboard the recently-arrived space shuttle Atlantis will assist with the mission, which should take six and a half hours to complete. The spacewalk is slated to begin at 1244 UTC.

Meanwhile, 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 is expected to make its closest approach during Tuesday's planned 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 30-year U.S. space shuttle program will be over.

NASA is ending 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.