CAPE CANAVERAL, Florida (Reuters) - A passing piece of potentially dangerous space debris forced astronauts at the International Space Station to temporarily seek refuge in escape ships early on Saturday, U.S. officials said.

The debris, a fragment from an old Russian satellite named Cosmos 2251 that smashed into an Iridium Communications spacecraft in 2009, passed harmlessly by the $100 billion orbital outpost at 2:38 a.m. EDT (0638 GMT), NASA said.

With enough advance notice, NASA will maneuver the space station, which orbits about 240 miles above the planet, to put more space between it and passing debris. The other option is for the station's six crew members to shelter inside the two Soyuz capsules berthed at the station in case the outpost is struck and depressurizes.

"This was a very erratic piece of Cosmos 2251 debris and tracking it was very difficult," NASA spokesman Michael Curie wrote in an email to Reuters.

"Its size and exact distance are unknown, and the crew sheltered in place as a highly-conservative, cautionary measure. The predicted miss distance prior to its passing was 11 to 14 kilometers (6.8 to 8.7 miles) in overall miss distance. But again, we do not know its exact distance at 2:38 am EDT, the time of closest approach," he said.

It was the third time a crew has had to shelter in Soyuz spacecraft when debris was predicted to pass close to the space station, NASA said.

More than 20,000 pieces of man-made debris larger than a softball currently orbit Earth. Space junk travels at speeds of up to 17,500 mph, so even small pieces have enough energy to cause significant damage upon impact.

NASA says the greatest risk from debris comes from untrackable objects. The February 10, 2009, collision of the Russian and Iridium satellites added more than 2,000 pieces of trackable debris to the growing list of space junk. Two years earlier, China intentionally destroyed one of its defunct weather satellites to test a missile, generating more than 3,000 pieces of debris.

The U.S. military's Space Surveillance Network tracks objects as small as two inches in diameter in orbits close to Earth, such as where the space station flies, and about one yard (.9 meter) in orbit in higher orbits.

(Editing by Paul Simao)