A small piece of space junk will fly uncomfortably close to the International Space Station late Friday and may force astronauts aboard the outpost to take shelter in their Russian lifeboats.
NASA?s Mission Control radioed the six astronauts on the station earlier today to alert them of the approaching space junk, which will fly within 1,640 feet (500 meters) of the orbiting laboratory Friday night at 10:48 EST (0348 Saturday GMT).
Sending the astronauts into their Soyuz lifeboats would be a precaution only, NASA officials said. Currently, the space junk poses no threat to the station or its crew, they added.
?It?s pretty unusual,? said Kirk Shireman, NASA?s deputy station program manager, said of the shelter plan in an interview. ?I wouldn?t be surprised if the need to do it for this [debris event] goes away.?
The object is likely very small because it is difficult to track, NASA spokesperson Rob Navias told SPACE.com. News of its close approach to the station came too late to steer the massive orbiting laboratory clear using its Russian thrusters.
?We don?t know what it is or the size,? Navias said.
Station astronauts will find out before the end of the day today whether they?ll have to sleep inside the two Soyuz spacecraft that serve as ferries to and from the station, as well as lifeboats in an emergency escape. Another option is to wake up before the space junk zooms by and then take shelter for a short while until the threat has passed, Navias said.
The station crew is due to go to sleep at about 4:30 p.m. EST (2130 GMT).
This is not the first time a piece of wayward space junk has come close enough to send astronauts into their Russian lifeboats.
A close pass by an old rocket engine remnant sent three station astronauts into their Soyuz spacecraft earlier this year in March. Since then, however, the space station?s crew size has doubled to six astronauts, so two Soyuz vehicles are currently docked to the station.
NASA also delayed the departure of a Japanese cargo ship from the space station last week because of a space debris threat.
The space station is currently home to two Americans, two Russians, a Canadian and Belgian astronaut Frank De Winne of the European Space Agency, who commands the team?s Expedition 21 mission.
NASA typically prefers to move the space station when the odds of a space debris impact are within a 1-in-10,000 chance. Astronauts take shelter when debris is expected to fly within a so-called ?red zone? and the space station doesn?t have time to dodge, Shireman told SPACE.com
There is also a box-like buffer around the station that mission managers prefer to keep free of any debris. That safety zone extends about 15 miles (25 km) around the space station, as well as about a half-mile (0.75 km) above and below it. The station flies in an orbit about 220 miles (354 km) above Earth at a speed of about 17,500 mph (28,163 kph).
But a debris avoidance maneuver ? as dodging space junk is known at NASA ? can take days to plan. The space station?s Mission Control team did not have that time because of the short lead time, Navias said.
Space debris has been a growing threat for manned spacecraft and other satellites in orbit today.
The collision between two communications satellites earlier this year brought the issue to the forefront. It, as well as China?s intentional destruction of a satellite during a 2007 anti-satellite test, have sparked a renewed push to better track, and possibly reduce, the more than 20,000 pieces of space junk currently watched by various agencies.
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- SPACE.com Video Show - Inside the International Space Station
- Video - The Expanding Danger of Space Junk