One of the hundreds of thousands of pieces of space-age litter orbiting Earth zipped uncomfortably close to the International Space Station on Tuesday.

The six crew members of the space station took refuge in their “lifeboats” — two Soyuz space capsules they would use to escape a crippled station — as the unidentified object hurtled past them at a speed of 29,000 miles per hour, missing the space station by only 1,100 feet. The episode took place at 8:08 a.m. Eastern time.

“We believe the probability that it would the hit the station was about 1 in 360,” said Lark Howorth, who leads the team at National Aeronautics and Space Administration (NASA) that tracks the space station's trajectory. NASA rules call for precautions when the risk of impact is greater than 1 in 10,000.

In the section of the station run by the United States, astronauts closed the hatches in case the debris — commonly known as space junk — crashed through, to limit the danger of explosive decompression. To prepare for a rapid departure, the clamps holding the Soyuz capsules to the station were released.

“They would be one command away from releasing the hooks and undocking,” said Edward Van Cise, NASA's lead flight director.

Mission controllers gave the all-clear signal four minutes later, and the crew members returned to work. There was no sign of damage or impact to the station.

Second time

It was only the second time in the 10-year history of people living on the space station that the crew needed to take such precautions; on March 12, 2009, a piece of an old satellite motor went zipping by.

If the station had been hit, the crew could have quickly undocked and returned to Earth. The risk of space junk hitting a Soyuz capsule is much slimmer.

Usually, when NASA gets a warning, several days in advance, that something that might come too close to the station, it moves the station by firing thrusters. Or, if a space shuttle happened to be visiting at the time, the shuttle would nudge the station out of danger. That has happened 12 times.

This time, however, the warning came Monday evening, less than 15 hours in advance, too little time to plan a manoeuvre.

Since the first artificial satellite, Sputnik 1, was launched was in 1957, the space neighbourhood has become cluttered with human-made detritus — more than half a million pieces, by recent estimates, from the size of a marble on up. If the orbits of two intersect, the result can be a destructive collision.

“It's getting kind of dangerous,” said Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who has become an expert on space debris. “Most active satellites now have a regular process of manoeuvring to avoid debris.”

NASA estimates that for each six-month period, there is a 1-in-100 chance that some or all of the space station crew might need to evacuate, and most of that risk comes from the possibility of impact from debris or natural micrometeroids. Over 10 years, the current planned lifetime of the station, the cumulative risk is nearly one in five.

“It's at the level where it probably won't happen in the lifetime of the station, but it could easily,” Dr. McDowell said.

The debris includes spent rocket stages, and sometimes over time residual fuel combines and explodes. “You now no longer have a rocket stage,” Dr. McDowell said. “You have 500 pieces of shrapnel.”

Also still in orbit are broken satellites or almost incidental litter. In the past, lens covers on satellite cameras and sensors were simply popped off and left to float away. Now satellite makers put the lens cap on a hinge.

Military antisatellite tests also make a big mess, notably when the Chinese blew up one of their satellites in 2007. — © New York Times News Service

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