Space station commander Mike Fincke, flight engineer Yury Lonchkov and Sandra Magnus evacuated to the lab's Russian Soyuz spacecraft today because of a predicted close encounter with debris from a spent upper stage booster rocket. But the space junk streaked harmlessly past the station around 12:39 p.m. and the crew was cleared to re-enter the lab complex.

"OK guys, based on all the confirmations, most likely the object passed us by," a Russian flight controller said, according to a translator. "So you are go to egress Soyuz. Yury, also please tell us are you ready to start the leak check?"

"And Houston, space-to-ground 1, can you hear us?" Fincke called over the NASA communications circuit.

"We have you loud and clear Mike, on space-to-ground 1," a controller in Houston replied.

"OK, we understand we're clear (to) ingress back to the USOS (U.S. segment of the station)," Fincke said.

"And we're all very happy the (debris) has passed with no impact," Houston replied. "That's great news."

According to a NASA statement, the station crew members evacuated to the Soyuz as a precautionary measure "due to space debris that has been determined to be within the range where a collision is possible. News of the close approach came too late for flight controllers to coordinate an avoidance maneuver. A portion of a spent satellite motor is within the distance of the station's debris avoidance maneuver requirement 'box.'

"Crew members are entering their Soyuz TMA-13 capsule and soft-locking the hatches, in case the debris should affect the space station and they are required to undock. The closure of the hatches ensures the safety of the crew and the ability to quickly depart the station in the unlikely event the debris collided with the station causing a depressurization."

Time of closest approach was 12:39 p.m. EDT. It was not immediately known just how close the debris from the spent payload assist module, or PAM-D, upper stage rocket motor came to the international space station.

But all objects in low-Earth orbit are traveling at 5 miles per second and impacts are potentially catastrophic. A 0.4 inch-wide sphere of aluminum moving at orbital speeds packs the same punch as a 400-pound safe moving at 60 mph.

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 with little warning is unusual and it was not immediately clear why an alert about today's close encounter was not delivered or received in time to carry out a course change.

Aerospace Corp. Space Debris Background here

NASA Orbital Debris FAQs here