NEW YORK — Space debris that came alarmingly close to the international space station March 12, forcing crew members to take refuge inside a Russian-built Soyuz lifeboat, was in an unpredictable orbit and nearly too small to track, a space debris expert said.

Notice of the incoming debris – a small piece of spent rocket motor – came too late for flight controllers to boost the space station out of the way, NASA officials said. As a precaution, the station’s two U.S. astronauts and Russian crewmate moved into the station’s attached Soyuz spacecraft for nine minutes as the threat passed. The astronauts were ready to evacuate the space station if the debris hit the station and depressurized its living space.

NASA said the debris was a roughly 12 centimeter fragment of a spent Payload Assist Module commonly used to boost satellites out of low Earth orbit. The U.S. Air Force tracks about 19,000 pieces of debris that are about 10 centimeters and larger.

Bill Ailor, director of the Center for Orbital and Re-entry Debris Studies at The Aerospace Corp. in El Segundo, Calif., said the size of the debris was a major factor in NASA not being notified sooner about the potential threat to the space station.

“That is right at the lower limit of the ability to track things,” Ailor said. “And the orbit itself dips into the atmosphere, and when it does that it makes it unpredictable.”

The wayward satellite motor part was flying at about 31,900 kilometers per hour, NASA officials said. The space station orbits the Earth at about 28,200 kilometers per hour.

NASA spokeswoman Brandi Dean said the astronauts did not fully close the hatches between the Soyuz and the rest of the space station during the drill, but they were prepared to do so if required.

It was not immediately known exactly how close the debris came to the space station. Initial calculations indicated that the motor fragment passed within 5 kilometers, but Dean said NASA flight controllers were refining their estimates.

Space station commander Michael Fincke and flight engineer Sandra Magnus, both of NASA, and Russian cosmonaut Yury Lonchakov told Mission Control that they could not see the space debris.

“Sandy kept her eyes out the Soyuz windows and we didn’t see anything, of course,” Fincke radioed down to Mission Control. “But we were wondering how close things were. We’d be interested in that.”

The space station has fired its engines to avoid space junk before, and astronauts have sought shelter in their attached Soyuz spacecraft – which doubles as a lifeboat in emergencies – at least one other time, Dean said.

The last time astronauts sought refuge from space debris in their Soyuz spacecraft was Nov. 17, 2008, when the remains of an old Proton rocket flew within 19 kilometers of the space station, according to the European Space Agency.

Ailor said he is aware of eight times that NASA has had to maneuver the space shuttle to avoid space debris and at least two times the space station has had to take avoidance measures.

While space debris is usually less of a threat to spacecraft in low Earth orbit, the amount of debris in general is on the rise, Ailor said.

“Everyone is predicting we will be seeing more, and recent events are creating more debris,” he said.

NASA officials said the Feb. 10 collision between a defunct Russian military satellite and an operational Iridium telecommunications satellite has increased the risk of debris impacts during a space shuttle mission by about 6 percent.

The unprecedented Feb. 10 collision occurred 790 kilometers above Siberia and created two large clouds of debris in low Earth orbit.

Staff writer Becky Iannotta contributed to this report from Washington.