The ISS (International Space Station) and the Space Shuttle Discovery currently attached to it were forced March 22 to change orbit to avoid being smacked by 10-year-old debris from a Chinese satellite launch. The maneuver was successful but marked the third time in three weeks the ISS has been threatened by orbiting space junk.

Last week, a breakaway piece of a Russian satellite came close enough to the ISS for NASA engineers March 17 to consider moving the space station and recalibrating Discovery's track to the ISS. The previous week, a piece of a Russian spacecraft motor came close enough to the ISS that the three-man crew was forced to evacuate to the Soyuz TMA-13 capsule, which is attached to the space station to transport astronauts back in an emergency.

"Space debris is becoming an ever-increasing challenge. When it comes to dodging junk, it's a big deal. It's very tiring. Sometimes it's exhausting," said flight director Kwatsi Alibaruho.

NASA's ongoing problems with increasing space debris raises the obvious question: just how much junk is out there?

The U.S.-operated SSN (Space Surveillance Network) tracks 17,300 artificial objects in space larger than 10 centimeters. About 800 of those objects are operational satellites. But the SSN tracks only a fraction of the junk orbiting Earth. According to the Secure World Foundation, which is meeting in Washington this week to promote space situational awareness, estimates 300,000 total objects are out there, the flotsam of a half-century of space exploration.

The junk includes discarded fuel tanks, screws, blots, paint chips, foil scraps and other objects. The Secure World Foundation estimates there are also billions of bits and pieces smaller than one centimeter circling the planet, each following its own orbit. During an eight-year period ending in 2002, the Hubble Space Telescope's solar panels were struck 725,000 times with approximately 5,000 of those impacts large enough to be seen by the naked eye.

"All sizes of orbital debris, even down to less than a millimeter, can have a devastating effect on anything they hit because of their high relative impact velocities," the Secure World Foundation explains on its site. "While overall levels of space debris are somewhat manageable at this time, we face a looming problem with escalating amounts of debris in the most heavily used orbits. This scenario will sharply increase the probability of damaging collisions between space debris and operational spacecraft."

The ISS was designed with shielding to protect against impacts from debris ranging from one millimeter to one centimeter in size.

"A hit in a critical area by larger objects would cause significant and serious damage. Most space systems do not even have this level of shielding, because of the added weight and launch costs, and thus are highly vulnerable to loss of service from debris impacts," the Secure World foundation states.

As the quantity of space debris increases within popular obits, the organization predicts a ramp up in space collisions, "creating a cascade effect leading to increasing amounts of threatening debris." A NASA study forecasts a 10-fold increase in the probability of collision with debris over the next 200 years.

"While this forecast is a cause for concern in itself, it is considered an underestimate because for modeling purposes the study's forecast was based on the assumption that no further launches would be undertaken during this period," the Secure World Foundation states. "Since orbital launches are expected to continue, the probability of future collisions with orbiting debris will almost certainly be higher."

NASA was also consulted before the Chinese government in 2007 successfully launched a missile to destroy a defunct satellite. Striking the dead satellite at an altitude of 869 kilometers, the explosion created a cloud debris generating more than 150,000 pieces of space junk larger than one centimeter.

There are no existing international agreements banning the deliberate creation of debris but consensus is building in the international scientific community to address the problem of space debris.

"In order to manage international problems that will naturally develop as the space environment surrounding Earth becomes more crowded, we must begin on the creation of space resource management mechanisms to reduce the creation of new debris," says the Secure World Foundation. "Our continued ability to maintain a secure and safe space environment depends on it."