The six-strong crew of the International Space Station took shelter in their Soyuz escape craft late on Friday as a piece of orbital debris came close enough to warrant the precautionary measure.
The debris was tracked from early Friday morning. Analysis showed there was a remote possibility of the debris colliding with the space station so the astronauts moved to their escape capsules. NASA explained that there was not enough time to perform the usual avoidance maneouvre.
Whilst sheltering in the Soyuz craft the debris passed below the ISS at a distance of approximately 15 kilometers (9 miles) at 07:38 CET on Saturday March 24 and the six crew were given the all clear to move back inside the station.
This is the third time in the history of the ISS that a crew has sheltered from the threat of space junk hitting the station. In June 2011 the crew moved to the escape capsules as a piece of junk came to within just 1,100 feet of the orbiting station.
The six astronauts currently on the ISS are Commander Dan Burbank, Anatoly Ivanishin, André Kuipers, Oleg Kononenko, Don Pettit and Anton Shkaplerov. After the debris had passed André Kuipers described the experience in his blog:
"Its orbit was hard to predict but it would pass by us at a distance of approximately 10 kilometres. That means code red. We had to go to our safe house to wait and see if the debris woudl hit us. The two Soyuz spaceships function as safehouses and as lifeboats. If something were to hit us with heavy impact, then we would be already safe in our spaceship and could return to Earth".
Even though the ISS is the most heavily-shielded spacecraft ever, even a small piece of space debris the size of a grain of sand travelling at around 28,000 kilometres per hour (17,500 mph) could cause damage.
The ISS regularly performs collision avoidance manoeuvres to reduce the risk of this happening.
NASA's Twitter feed stated the piece of debris was part of a Russian Cosmos 2251 satellite, destroyed in collision with a US communications satellite, Iridium 33, in February 2009. According to estimates, the accident created a huge cloud of debris containing over 2000 pieces, with many more too small to track.
Space debris ranges in size from flecks of paint to the third stage of Apollo 12’s Saturn V launch vehicle - at over 11,000 kg. The majority of debris is wreckage from satellite collisions and explosions.
There are probably over half a million pieces of space debris that are smaller than a melon 10 cm and larger than a cherry. These are generally too small to track and cause the greatest risk to space missions.
Approximately 22,000 items of space debris are tracked by Space Situational Awareness (SSA) systems. SSA systems are usually carried out by the military for security purposes.
Analysis: space debris
An attempt to draw up an International Code of Conduct for Outer Space Activities is in progress.