A spacewalk by two astronauts is still on for Thursday evening even though a large piece of space junk is headed their way.

Part of an old European rocket is expected to pass within two miles of space shuttle Discovery and the international space station late Friday morning. NASA officials say that should be a safe distance and they do not expect the linked spacecraft to change course. But if the projections change, the astronauts would complete the spacewalk before steering out of the way.

The debris is believed to have an area surface of 200 square feet.

Astronauts Danny Olivas and Christer Fuglesang will install a new ammonia tank outside the space station. It will be the second spacewalk of the mission.

Mission Control was keeping close tabs on the piece of European rocket because there is a chance, however unlikely, it could come too close or even hit the linked space shuttle Discovery and international space station if their path is not altered.

Right now, the debris is expected to pass within seven miles of the outpost on Friday, "a fairly healthy" miss distance, said Mission Control commentator Rob Navias.

"I would emphasize that it is only a remote possibility that we would have to ... steer clear of this piece," Navias said.

Experts will continue to track the debris — part of a 3-year-old Ariane 5 rocket — to make sure it stays at a safe distance. Pieces of uncontrolled space junk sometimes stray from their orbit, however, and that is the concern. The object's oval-shaped orbit — stretching as far out as 20,000 miles — made it especially difficult to monitor.

The late-breaking news did not affect the work of the two crews aboard the complex. They moved more cargo into the space station and even installed some of the new big-ticket items, including a sleeping compartment.

If Mission Control determines the shuttle-station complex needs to dodge the junk, that move into a higher orbit would not happen until after Thursday night's spacewalk. The joined spacecraft currently are flying about 220 miles above the planet.

A final decision was not expected until Thursday.

The astronauts performed the first of three planned spacewalks Tuesday, removing an old ammonia tank from the space station. On Thursday, two spacewalkers will install a new, fully loaded tank to replenish the cooling system of the outpost.

Discovery will remain at the space station until Tuesday.