February 15, 2009 When we are born, we soil ourselves and other people clean it up for us. As we mature, we take responsibility for our own excrement. Strangely, as a society, we're not at all good at toilet training ourselves regarding the excrement produced by industry, transport or agriculture. Human beings capacity to eschew short term gain when faced with long term harm is notoriously woeful so it’s not surprising we've done exactly the same thing in space, leaving so much debris from space missions that it's now dangerous to be in the orbital band around earth due to the likelihood of being hit by junk traveling at 18,000 mph. The latest evidence: last week saw the first ever accidental collision between two intact spacecraft, a deactivated Russian satellite and an Iridium 33 satellite, which left a fresh cloud of debris 497 miles above the Earth.

Space debris, orbital debris, space junk, call it what you will, consists of anything in orbit around the Earth created by humans that no longer serves any useful purpose. This includes derelict spacecraft, upper stages of launch vehicles, carriers for multiple payloads, debris intentionally released during spacecraft separation from its launch vehicle or during mission operations, debris created as a result of spacecraft or upper stage explosions or collisions, solid rocket motor effluents, and tiny flecks of paint released by thermal stress or small particle impacts. Even clouds of very small particles can cause damage when traveling at such high velocities causing a sandblasting effect.

Some other pieces of orbital debris reportedly include a glove lost by astronaut Ed White on the first American space-walk, a camera Michael Collins lost near the spacecraft Gemini 10 and garbage bags jettisoned by the Soviet Mir cosmonauts throughout the space station's 15-year life. Since most objects such as these were released in orbits that led them to re-enter the Earth's atmosphere and burn up within weeks, they are not major contributors to the space debris environment. The main source of space debris is satellite explosions, especially old upper stages left in orbit with stored energy sources resulting in residual propellants and high pressure fluids. These constitute the largest component of cataloged orbital debris.

There are approximately 17,000 objects larger than 10cm (3.9-inches) known to exist and the estimated population of particles between 1 and 10cm in diameter is greater than 200,000, while the number of particles smaller than 1cm (0.39-inches) probably exceeds tens of millions. This debris is not uniformly spread out about the Earth. Most orbital debris can be found in Low Earth Orbit (LEO) within distances below 2,000km (1,243 miles) where debris circles the Earth at speeds of 7 to 8km/s. Objects larger than 10cm in diameter are routinely tracked by the US Space Surveillance Network and objects as small as 3mm can be detected by ground-based radars, but assessments of the population of debris smaller than 1mm is made by examining the impact features on the surface of returned spacecraft.

In 2006 a piece of debris no bigger than a grain of rice came close to piercing a coolant tube on the space shuttle Atlantis that helps regulate shuttle temperature. Instead it pierced a radiator panel and the Atlantis was spared an early trip home by a matter of inches. As the larger pieces of debris are relatively rare and easy to track the space shuttle can be warned in advance and maneuvered away from the object if it is projected to come within a few kilometers – this occurs once every year or two. Smaller pieces of debris however are of bigger concern – particularly worrying are the marble-sized pieces that are too small to track, but big enough to do some real damage.

The International Space Station (ISS) is the most heavily shielded spacecraft ever flown with critical components such as habitable compartments and high pressure tanks normally able to withstand the impact of debris as large as 1cm in diameter, It also has the capability of maneuvering to avoid tracked objects and even though the risk of a critical ISS component being struck by debris 1-10cm in diameter is slight, the ISS has moved out of the way of orbital debris 8 times since construction began in 1998. And it looks like it will be on the move again with news this past week that two satellites, one a deactivated Russian satellite and the other a still operational Iridium 33 satellite, collided 800km (497 miles) above the Earth. NASA said it was tracking two large clouds of debris and that the ISS, which is orbiting at 350km (217 miles) altitude, may be forced to evade the collision debris.

While the odds of such impacts are still rare, NASA consultant Donald J. Kessler has proposed a scenario dubbed the Kessler Syndrome that suggests that even if humans were to stop contributing to the existing amount of orbital debris tomorrow, the amount of debris would still continue to increase due to the fact that objects struck by debris would cause even more debris and result in an even greater risk of further impacts. Taken to its worst possible conclusion the escalating amount of debris in orbit could eventually render space exploration, and even the use of satellites, unfeasible for generations.

Fortunately, at the most commonly used Low Earth Orbits residual air drag brings debris down to lower altitudes where it finally re-enters the atmosphere meaning altitudes under around 300 miles will be swept clear in a matter of months. Although this also occurs at higher altitudes, the process can take millennia, and for any orbital debris in the geostationary orbit at 36,000km altitude it would never happen. However, since the geostationary orbit is a special natural resource, many spacecraft operators boost their old spacecraft into higher, disposal orbits at the end of their mission. Notice I said many spacecraft operators – not all.

The problem of orbital debris has been recognized by the international community for decades, and with no viable proposals to eradicate the threat you might think that minimizing the amount of garbage produced would be a high priority. Apparently not for the Chinese military who in 2007 blew up one of their own satellites with a missile, a month before the UN agreed to guidelines calling on nations to “avoid intentional destruction and other harmful activities” in a bid to cut down on the amount of orbital debris created. NASA called the resultant debris field the “most severe artificial debris cloud in Earth orbit since the beginning of space exploration.”

Today’s global village is hugely dependent on satellites and the militarization of space and the prospect of nations destroying each others satellites could see space become virtually inaccessible and cripple the world economy by blinding civilian satellites. This was one of the reasons one of President Barack Obama's first acts as President was to issue a policy statement calling for a "worldwide ban on weapons that interfere with military and commercial satellites." This was in contrast to the Bush administration’s efforts to resist the de-militarization of space. While it would be nice to see other world leaders follow President Obama’s lead, unfortunately human history does not provide a lot of precedents to suggest a de-militarized space in our immediate future – but hope springs eternal. Meanwhile, with the prospect of cleaning up the existing orbiting debris already circling the planet seemingly out of the question, it is up to space faring nations to ensure that the amount of debris doesn't rise and doesn't stunt mankind reaching for the stars.

Darren Quick

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