NASA officials say a piece of orbiting "space junk," passing near the International Space Station Friday, is a safe distance away from the station, and the docked shuttle Discovery.

NASA says the orbiting "junk" is part of the body of an European rocket and is 19 square meters in size.   

The space agency expected the old rocket to pass within three kilometers of the station.  Engineers considered the distance to be safe and proceeded with a planned spacewalk on Thursday.  

During the walk, two shuttle astronauts successfully installed a massive new ammonia tank outside the station.

Mission control managers said Friday the new tank is working perfectly.  

Shuttle astronauts have a third and final spacewalk scheduled for Saturday.  

The ammonia tank is needed for the station's cooling system and replaces an older, smaller tank.

At 800 kilograms, the new tank is the largest item spacewalking astronauts have handled during assembly of the space station.  Objects are weightless in zero gravity conditions in space, but they still have mass and can be difficult to move.

The space shuttle arrived at the station Sunday, carrying equipment that included an exercise treadmill named for U.S. television comedian Stephen Colbert.

When Discovery undocks from the outpost, astronaut Nicole Stott will remain aboard the space station as flight engineer for the next three months.  She is replacing astronaut Timothy Kopra, who will join the rest of the shuttle's crew for the flight back to Earth.

The shuttle is expected to return to Earth September 10 with the space station's old ammonia tank in its cargo hold.

Some information for this report was provided by AFP and AP.