Space station commander Mike Fincke, flight engineer Yury Lonchkov and Sandra Magnus were told to board the lab's Russian Soyuz spacecraft today because of a possible close approach by a piece of space debris.

At 12:15 p.m., Fincke was told to be inside the Soyuz ferry craft in the next 20 minutes. Russian flight controllers recommended leaving the Soyuz hatch open and Fincke concurred, saying the crew would make sure it could be quickly closed if necessary.

A NASA source said the debris in question was listed as "PAM-D" debris, an acronym implying it was a spent payload assist module solid-fuel motor used to boost payloads to higher orbits. It was not immediately clear if that was, in fact, what the debris might be.

There are more than 18,000 pieces of space junk in low-Earth orbit the size of a baseball and larger. U.S. Strategic Command prioritizes radar tracking to protect manned spacecraft first, followed by high-priority military and civilian payloads.

Normally, the station's rocket thrusters are used to change its orbit slightly when close encounters are predicted. Putting the crew in the station's Soyuz lifeboat is unusual. It was not immediately clear why a maneuver was not carried out today or whether the alert came in too late.

Additional details will be posted as they become available.