Monday, January 30, 2006

The sky is not falling - orbital debris threatens satellites

Last January, a 30 year old discarded booster from a US satellite collided with a piece of a Chinese launch vehicle from 2000. The accident released even more debris into orbit. According to a recent article in Science last week (Risks in Space from Orbiting Debris by Liou and Johnson) this type of incident will be more and more likely in the coming decades as the space near earth fills up with more an more orbiting junk and the debris that sheds off of it.

Data from a 1992 survey shows a large peak around 900km and 1500km (plot created from data in this report, caution .pdf) The LEGEND model of orbiting debris shows the amount of stuff in low earth orbit increasing three times over the next two hundred years, which the article states will result in a 10-fold increase in collisions. Ironically, these catastrophic collisions produce even more debris. The biggest increase is at the peak at 900km altitude.

What does this mean for current and future satellites? Many commercially important satellites orbit in the ranges discussed here. They are both the source of debris and the source of concern. The Iridium satellites orbit at about 800km, NOAA polar orbiting satellites orbit around 850km, the International Space Station orbits at 380km, Hubble around 600km, while this discussion is not relevant for geostationary satellites which orbit far out of the range discussed here at 35,800km.

Besides the postmission disposal of orbital vehicles advocated by the major space faring nations the authors outline several approaches to actually remediating low earth orbit. The schemes involve attachment of electrodynamic tethers or drag enhancement structures to increase orbital decay to dispose of large pieces of orbital debris. Ion rockets are mentioned to actively deorbit the debris and even ground based lasers to affect the orbits are proposed. The laser idea seems to verge on science fiction, and I wouldn't want to be in its path when it fires.

Finally Liou and Johnson caution that unless there is some plan to address the debris buildup the risks to space system operations will climb.

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