NASA officials say astronauts are still on schedule to conduct a spacewalk Thursday at the International Space Station despite the threat of an approaching piece of "space junk."

The "space junk" is part of the body of a three-year-old European rocket and is 19 square meters in size.  

NASA engineers said the rocket debris is far enough away to not pose a threat to Thursday's planned walk.   They said if their projections change the astronauts would still have time to complete the spacewalk before the station would steer out of the way.

Engineers expect the debris to pass within three kilometers of the International Space Station on Friday.  NASA officials say they do not expect the "space junk" to change course.

During the walk Thursday, astronauts Danny Olivas and Christer Fuglesang will install a new coolant tank on the station's exterior. The tank replaces an older, smaller one Olivas and Nicole Stott removed in a spacewalk that ended early Wednesday.

At 800 kilograms, the new tank is the most massive 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 Discovery arrived at the station Sunday, carrying equipment that included an exercise treadmill named for U.S. television comedian Stephen Colbert. 

When Discovery returns to Earth, Stott will remain aboard the space station as flight engineer for the next three months.  She is replacing U.S. astronaut Timothy Kopra who will join the rest of the shuttle's crew for the return flight. 

The shuttle, scheduled to land on September 10, will have the space station's old ammonia tank in its cargo hold. 

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