SatNews found the following article to be of potential interest to our subscribers, courtesy of Trevor Williams University of Victoria Mechanical Engineering PhD candidate:

After launching 6000 satellites since 1957, on 4600 rockets, 70% of the debris in-orbit today remains in low earth orbit, up to 2000kms above the planet surface. An ESA/ESOC (European Space Agency/European Space Operations Centre) artist impression of this debris is shown above (not to scale of course). Orbital debris is a growing problem and one that needs to be dealt with before large parts of the orbital planes become un-usable for centuries.

Many satellites have their orbits at roughly 800kms above the planet Earth, almost exactly at the same altitude that the Iridium (900kgs/1980lbs) and Cosmos 2251 (685kg/1507lbs) spacecraft recently collided (485 miles, 780 km, above the Russian Arctic on Tuesday 10th February 2009 at a combined speed of 15,000mph/24,000kph). The collision most likely produced 1000-3000 thousand extra orbital debris fragments. These fragments are dispersed around the original orbit paths of these satellites but will continue to drift in this orbit path, most likely affecting other Iridium satellites (currently there are 66 in multiple orbit planes) and others. It will take some time to determine exactly where all the fragments will end up.

In the 800km orbit plane there are numerous communication and earth observation satellites, which are at this altitude because it offers an orbit where there is a balance between repeat visit times over the same spot on the planet, sun illumination with only short eclipse periods where there is no electrical power available so they rely on battery storage. Low Earth Orbit Debris: Image Courtesy of ESA/ESOC.

However, it seems it has been getting a little crowded in this orbit – not just with spacecraft but also with various other space junk and orbital debris. Iridium Corporation had been receiving hundreds of weekly notifications from the US Strategic Command's Joint Space Operations Centre, the one that tracks debris in space, about potential close approaches within five kilometres (3 miles) of their satellites.

There is not much you can do about it anyway, the margin of error in prediction and the difficulty in altering trajectory means it is just as much a ‘crap-shoot’ to attempt to move a satellite’s orbital path, as it is to not do anything about it. It is kind of like closing your eyes when you think you are about to hit something in your car and hope that you miss it.

With at least 18,000 pieces of known orbiting junk there is always a risk of collisions. The odds of hitting something remain quite remote at around 0.000002% though the risk is growing every time a satellite is launched, not only from the satellite itself but more so from the spent rocket engines that have delivered it to orbit. These spent rocket engines are programmed to de-orbit but this can take months. Before de-orbiting, there is always the risk of collision with other debris before they de-orbit themselves, explode due to fuel residues igniting or simply when bits fall off them. Photo on right: Image courtesy of Analytical Graphics, Inc.

It seems wherever humans go, we leave a tell-tale trail of trash behind us. Right now there are no binding international rules forcing satellite owners and launch vehicle to clean up their mess in an orderly and timely fashion. There is a ‘gentlemen’s agreement’ to either de-orbit the satellite when in low earth orbit, or raise it to a ‘graveyard’ orbit  some 300kms above the geo-synchronous orbit of most large communications and television broadcast satellites.

The problem stems from when the satellite is designed and it requires a large quantity of fuel to de-orbit, fuel that most satellite operators prefer to use for station-keeping and orbit control (momentum dumping), thereby prolonging its useful, paying life. When geo-communication satellites have yearly revenues of 60-80 million dollars, it is difficult for an operator to use-up a year’s supply of fuel to scrap the satellite, so they wait until the very last predicted time and then run-out of fuel before they get to the graveyard orbit. Sometimes the satellites fail before they can be scrapped and they drift slowly backwards around the planet, with other operators moving their spacecraft out of the way.

Orbital ‘real-estate’ is another finite resource on this planet, one that is rapidly being used and abused, so it would be a wise decision to ensure all users to be legally bound to dispose of their future-trash wisely, safely and assuredly. For those that fail to do so, penalize them, fine them sufficient monies to attempt a clean-up operation – which would be a remarkable feat of engineering, though not completely impossible to achieve. The US Defense Advanced Research Projects Agency (DARPA) flew the Orbital Express Satellite rendezvous and re-fuelling mission that successfully docked two spacecraft and refueled one of them. The same procedure could be used to attach a module that de-orbits or moves out of the way, defunct satellites.

Resources

Analytical Graphics, Inc.: http://www.agi.com

The Orbital Express Project: http://www.darpa.mil/tto/programs/oe.htm

ESA/ESOC article on debris and spacecraft: http://www.esa.int/SPECIALS/ESOC/SEMN2VM5NDF_mg_1.html

Trevor Williams is a University of Victoria Mechanical Engineering PhD candidate specialising in renewable energy, power grid modelling and plug-in hybrid electric vehicles. He has a bachelors in Aeronautical Engineering, a Masters in Management Science and over 23 years international experience in the space industry, having worked on Earth observation and telecommunications satellites.