CAPE CANAVERAL, Fla. — The three astronauts aboard the International Space Station no longer have to worry about a small piece of space junk heading their way.
Mission Control informed the crew Tuesday afternoon that the debris no longer poses a threat. Eight hours earlier, Mission Control told the astronauts they might have to seek shelter in their attached Soyuz capsule. That precaution is no longer needed.
The 6-inch (15-centimeter) piece of debris is from a Chinese satellite that was deliberately destroyed in 2007 as part of a weapons test. Initial estimates put it passing within 3 miles (5 kilometers) of the space station late Thursday afternoon. But as the afternoon wore on, the threat level went from red to green, due to further refinements in the orbital calculations.
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Just last Friday, the space station had to move out of the way of an orbiting remnant from a two-satellite collision in 2009. Tuesday's alert came too late for an avoidance maneuver, which is why the crew was warned to be ready to take shelter in the Soyuz.
Debris is an increasingly serious problem in orbit, because of colliding and destroyed spacecraft. At 5 miles a second, damage can be severe, even from something several inches big. Decompression, in fact, is at the top of any spacefarers' danger list. More than 12,500 pieces of debris are orbiting Earth — and those are the ones big enough to track.
NASA issues an alert if a piece of orbital debris is projected to pass through an imaginary "pizza box" in space, centered on the space station and measuring 30 by 30 miles wide (1.5 by 50 by 50 kilometers).
Mission Control notified the crew of Tuesday's potential threat a few hours after the risk was identified. The three crew members are Dmitry Kondratyev, the station's Russian commander, American Catherine Coleman and Italian Paolo Nespoli.
The three arrived at the station in a Soyuz craft last December. That spacecraft serves as a lifeboat in case of an emergency, and will deliver the trio back to Earth at the end of their six-month mission in May.
If the risk level had remained red, the astronauts would have had to remove ventilation lines running from the space station's major modules, seal the hatches to the rooms, and switch the radio channels so they could remain in contact with flight control teams in Houston and Moscow while they took shelter.
The last time a station crew took refuge in a Soyuz was in 2009. That time, the crew had less than an hour's advance warning.
A fresh three-person crew is en route to the 220-mile-high (350-kilometer-high) outpost after rocketing away from Kazakhstan. That Soyuz is due to arrive Wednesday evening.
This report was supplemented by msnbc.com.
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