Statistics show the International Space Station came under growing danger from space junk after 2007, with half of the orbiting lab's close calls since then due to near-collisions with debris from a Chinese anti-satellite missile test, the mysterious explosion of a Russian military spacecraft, and the cataclysmic high-speed crash of two satellites.

Artist's concept of space debris in low Earth orbit. Credit: ESA
 
The space station, assembled in orbit beginning in 1998, has fired its thrusters 14 times to avoid space debris, with half of the maneuvers coming since August 2008.

If ground controllers recognize a debris threat, or conjunction, too late, they ask the station crew to take refuge inside their Soyuz escape capsules during the predicted closest approach. The lab's crew has moved into their Soyuz lifeboats three times, first in March 2009.

NASA says the number of hazardous debris conjunctions per month more than tripled between 2006 and 2008.

Officials blame the change on three debris-creating events:

• China intercepted an orbiting satellite with a ground-launched missile in January 2007. The anti-satellite test destroyed China's polar-orbiting Fengyun 1C weather satellite 530 miles above Earth, creating the largest cloud of space debris in history. More than 3,200 objects from the destroyed satellite were catalogued by the U.S. military, and only about 6 percent of the debris had re-entered the atmosphere by the end of 2011.
  
  
• A Russian military satellite broke apart in early 2008, spreading more than 500 fragments in low Earth orbit. The Cosmos 2421 satellite, launched in June 2006, liberated debris three times in March, April and June 2008. Analysts say 22 out of 50 similar satellites launched since 1974 have exploded in orbit. Cosmos 2421, which was designed to eavesdrop on U.S. naval vessels, was orbiting 255 miles high when it spread debris, and almost all of the satellite's fragments have re-entered the atmosphere.
  
  
• The Iridium 33 communications satellite and a retired Russian relay spacecraft struck each other 490 miles over Siberia. The satellites collided at a relative velocity of more than 24,000 mph, throwing more than 1,700 objects through a region of space trafficked by the International Space Station, numerous operational satellites, and more than 3,000 other catalogued objects. It was the first collision of two intact satellites. The bulk of the fragments from Cosmos 2251 and Iridium 33 satellites remain in orbit.

Artist's concept of an Iridium communications satellite. Credit: Iridium
 
The higher altitude break-ups of Fengyun 1C, Cosmos 2251 and Iridium 33 left long-lasting debris in orbit. It could be decades for all of the fragments to fall back to Earth and burn up in the atmosphere.

NASA says increasing solar activity, which balloons the atmosphere and creates more drag, is helping rid low-altitude orbital zones of some debris.

At least 48 percent of the space station's near-misses since 2007 were due to debris generated by China's anti-satellite test, Russia's Cosmos 2421 satellite, and the in-orbit collision in 2009, according to NASA data.

Officials commanded four of the space station's last seven debris avoidance maneuvers to move the international complex out of the path of debris created by the three incidents.

An emergency burn in August 2008 moved the space station out of the way of debris from Cosmos 2421. Another thruster firing in April 2011 altered the station's orbit to avoid a close call with a fragment from Cosmos 2251, the Russian satellite annihilated in the collision with Iridium 33.

Two maneuvers in January dodged debris from Iridium 33 and Fengyun 1C, the craft destroyed in China's satellite weapons test in 2007.

The last debris avoidance burn before 2008 was in May 2003. Experts credit more accurate tracking technology for the reduction in maneuvers, but the rate of near-misses picked up again in 2008 as fragments from Russian, Chinese and Iridium satellites spread around the globe.

File photo of the International Space Station. Credit: NASA
 
The space station's six residents most recently scurried to their Soyuz lifeboats March 24 due to the late notice of a threat from Cosmos 2251 satellite debris produced in the 2009 orbital crash.

The space station is armored to protect against impacts of the tiniest debris, and officials have a good handle on the trajectories of well-known, large objects. But there is some dangerous debris too small or erratic to accurately track.

U.S. Space Command, the military division which tracks objects in orbit, notifies mission control in Houston of potential threats from space junk.

The Air Force keeps tabs on more than 22,000 objects in orbit, and experts believe there are hundreds of thousands more too small to be spotted from existing radars. About 1,100 of those objects are active satellites.

If an object is estimated to have a greater than 1-in-10,000 chance of hitting the space station, managers will order a rocket burn to change the orbit of the 450-ton complex. But it takes time to program an avoidance maneuver, and late warnings force astronauts into their Soyuz capsules to wait out the danger.