The shuttle Discovery crew fired steering rockets today to dodge a piece of space debris during what was expected to be the final day of a successful resupply run to the International Space Station.

NASA doesn't know what the space junk is, except that it likely came from the shuttle or space station on Saturday during the last of the Discovery crew's three spacewalks.

The shuttle departed the station on Tuesday. It is scheduled to return to the Kennedy Space Center in Florida at 7:05 p.m. EDT (2305 GMT) today.

"Exactly what (the debris) is is not known, but it's been moving toward the orbiter so it is a concern," said mission commentator Pat Ryan.

During Discovery's nine-day stay at the station, two other pieces of orbital debris sent engineers scrambling to prepare avoidance maneuvers, which were later determined to be unnecessary. Those pieces of space junk were identified as part of a spent upper-stage European rocket motor and a fragment from an obsolete weather satellite China destroyed in January 2007 during a widely condemned weapons test.

Discovery blasted off a minute before midnight on Aug. 28 with more than 7.5 tons of food, laboratory equipment, science experiments, spare parts and a new treadmill and crew quarters for the space station, which is nearing completion after more than a decade of construction.

NASA plans six more shuttle missions to the orbital outpost, a $100 billion project of 16 nations, before the fleet is retired.