That was a close call.

The two American astronauts and one Russian cosmonaut aboard the International Space Station had to duck for cover Thursday as space debris passed perilously close to the orbiting platform.

Crew members Sandra Magnus, Michael Fincke and Yury Lonchakov were ordered into one of the Soyuz TMA-13 escape capsules at 12:35 p.m. EDT, and given the all-clear 10 minutes later.

In case the space station was hit, the astronauts could have undocked and headed back to Earth. Even a tiny hole could have caused a catastrophic loss of air pressure and rendered the station uninhabitable.

The debris, part of a mechanism to put a satellite in proper orbit, measured about 5 inches, a size that "will wreck your whole day," said Mark Matney, an orbit debris scientist for NASA.

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"We were watching it with bated breath," Matney told The Associated Press. "We didn't know what was going to happen."

Normally, the space station would have fired its positioning thrusters to get out of the way of any oncoming objects, but NASA said the news of the possible collision came too late for that.

Matney, who's been with NASA since 1992, said it was the closest call he can remember.

The debris was expected to come within the 2.8-mile-wide box of space around the station that makes up NASA's danger zone, said NASA spokesman Kyle Herring.

"We were looking out the Soyuz window," Fincke radioed to Houston. "We didn't see anything of course. We were wondering how close we were."

Because the U.S. Strategic Command, which monitors space debris, could not get a good enough look at the debris, NASA may never know exactly how close it came, said NASA spokesman Josh Byerly. It was traveling 5.5 miles per second -- about 20,000 mph, he said.

The debris is likely a small weight followed by a 39-inch string or strand that was used to stabilize a global positioning satellite placed in orbit in May 1993, said Harvard astrophysicist Jonathan McDowell, who tracks all objects in orbit.

One of the reasons NASA got such late warning on the debris is that it is an unusual orbit that keeps dipping into the atmosphere and changing, McDowell said. It was in the worst kind of orbit to track, Matney said.

The GPS satellite went out of daily use in January, McDowell said.

Thousands of pieces of "space junk" orbit the Earth, so much that it's getting to be a problem.

The 2007 deliberate destruction of a satellite by China added hundreds of pieces, and the collision last month of an American commercial communications satellite and a dead Russian satellite only made it worse.

NASA is even considering scrapping an upcoming shuttle mission to repair the Hubble Space Telescope because the telescope orbits at an altitude close to the trails of debris left by the satellite collision.

The space station orbits at a lower altitude and is generally not at risk from space debris.

Space shuttle Discovery was to have launched from Cape Canaveral Wednesday evening for arrival at the space station Friday, but the launch was postponed to at least Sunday due to an unexpected hydrogen leak.

Meanwhile, one retired rocket scientist has proposed sending up giant squirt guns to blast water at space debris, sending each piece to such a low orbit that it would fall into the atmosphere and burn up.

The Associated Press contributed to this report.