NASA is tracking a piece of leftover space junk from a 2007 Chinese anti-satellite test that is expected to fly near the International Space Station twice on Wednesday, a day after the shuttle Discovery leaves the orbiting lab.

The satellite debris is expected to come within 31 miles (50 km) of the space station at about 4:30 a.m. EDT (0830 GMT) Wednesday morning, then zip around again two hours later to pass within 15 miles (25 km), NASA officials said.

Shuttle flight director Tony Ceccacci said NASA engineers are tracking the orbital debris to determine whether it could pose a threat to the space station. If so, the space station would have to fire its thrusters to in order to dodge the satellite remnant after Discovery undocks Tuesday afternoon.

"The latest and greatest confidence is that it probably won't be a concern," Ceccacci told reporters late Monday. "We won't really know until we get the latest tracking this evening and determine what the probability of collision is."

The potential space junk threat comes on the heels of another piece of orbital trash that buzzed the linked station and space shuttle last week.

That debris, a massive chunk of a 3-year-old European rocket body, came within a mile (1.3 km) of the space station, but it never posed an impact risk to the spacecraft, mission managers said. Trajectory analysts found that despite the near miss, the rocket trash had zero chance of hitting either Discovery or the space station.

Unlike that large debris, which was part of an Ariane 5 rocket, the current piece of space trash under NASA scrutiny is small. It is from the Chinese weather satellite Fengyun 1C, which the Chinese military intentionally destroyed with a missile during a 2007 anti-satellite test.

"It is just big enough to be tracked," Ceccacci said.

The resulting debris from the satellite's destruction has come near spacecraft before, most recently in May when a piece flew near the shuttle Atlantis while it was linked to the Hubble Space Telescope.

NASA typically moves the space station if the potential for a space debris impact is within a 1-in-10,000 chance. The agency likes to maintain a safety perimeter that extends out 15 miles (25 km) around the space station, as well as about a half-mile (0.75 km) above and below it.

While Mission Control tracked the space debris late Monday, the 13 astronauts aboard the linked shuttle and space station held a brief farewell ceremony to mark the end of more than a week of joint work to upgrade the orbiting laboratory.

"All good things come to an end," Discovery commander Rick Sturckow told the station crew.

Discovery astronauts launched to the station Aug. 28 and delivered nearly eight tons of new supplies, science gear and a treadmill named after TV comedian Stephen Colbert. On Monday, the astronauts plucked their portable cargo pod from its parking spot on the station and packed it away in Discovery's payload bay for the return trip to Earth

The shuttle is due to undock from the space station Tuesday at 3:26 p.m. EDT (1936 GMT) and land Thursday at NASA's Kennedy Space Center in Florida.

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