Astronauts aboard the International Space Station (ISS) may have to maneuver the station to evade flying junk as the space shuttle Discovery closes in for docking. The warning comes just four days after the crew was forced to take refuge in an escape capsule as a last-minute risk of debris strike was discovered.

Like last week's chunk of debris, which passed without incident as the three ISS members huddled in the station's Soyuz capsule, tomorrow's threatening object is not related to last month's collision between a Russian satellite and a commercial communications satellite.

The space garbage, currently projected to pass within a half-mile (0.79 kilometer) of the station at 3:14 A.M. (Eastern Daylight Time) tomorrow, appears to have originated from the 1981 breakup of a Soviet satellite called Kosmos 1275, says NASA spokesperson Bill Jeffs at Johnson Space Center in Houston. He did not know the size of the object.

Jeffs says a decision on whether to fire thrusters to move the ISS out of harm's way would be made "sometime later this afternoon." In an update to the NASA Web site posted at 1:13 P.M., the space agency said such a maneuver appeared unlikely but that better tracking data would be available at 4:00 P.M. If evasive action is taken, Discovery may have to adjust its approach as it closes in for its scheduled rendezvous and docking at 5:13 P.M. tomorrow. The shuttle is delivering the final pieces of the ISS solar array and a replacement unit for a faulty urine-to-water recycling system in anticipation of the station's crew size expanding from three to six later in the year.

UPDATE (3/17/09): NASA announced last night that it had informed ISS commander Michael Fincke that an avoidance maneuver would not be necessary.

Photo of Discovery docked to the ISS in 2005 courtesy of NASA