19 January 2007

The Tragedy of the Commons in Low Earth Orbit

The media have jumped on the report published in Aviation Week a couple days ago concerning a successful Chinese anti-satellite test. Most of the articles discuss the possibility of a newly emerging "space race" and the possible militarization beyond the atmosphere. I'm not going to speculate on the true intention of the Chinese in the asat test -- hey, for all I know this is just a ploy to introduce a new sport, satellite shooting, into the Beijing Olympics. Instead I want to look at the "environmental" impact such an action causes.

In this case the environment is the increasingly crowded near earth space we call Low Earth Orbit (LEO). In terms of outer space real estate this is the prime beachfront property. The only other tony spot is the geosynchronous zone -- but that's a more specialized neighborhood. LEO doesn't really have a well-defined boundary. Basically it starts where the atmosphere is thin enough that orbits are possible (without frictional heating slowing the satellite so much that it plunges and burns up). Wikipedia suggests this is roughly 200km up from the surface. It extends a couple thousand kilometers further. The idea is that fuel/payload costs just keep rising the higher up an orbit you want. Additionally the power required to communicate with a satellite increases as well, of particular concern for satellite phones. Thousands of satellites currently reside in this space, and with the exception of the Apollo moon voyages, *all* of manned space experience is in LEO. We depend on it for communications and imaging.

The article states that the destroyed satellite was the victim of a kinetic energy kill (as opposed to, say, an explosion or laser pulse). Basically, we're talking about a smart bullet, delivered by a suborbital ballistic missile, which scored a direct hit and blasted the satellite to bits (see http://www.armscontrolwonk.com/1359/chinese-test-asat).

The trouble with blasting an orbital object is that the bits don't "settle down". Instead they create a debris field. One satellite in a nice, circular polar orbit at 800km becomes 40,000 tiny fragments all spreading outward. But because this is LEO, and because the fragments don't have escape velocity, they just go into individual eccentric orbits around Earth. As their orbits are now all wonky they don't move "in-step" with satellites moving through LEO, and thus present a hazard. They become fast moving shrapnel that weave back and forth through the surrounding circular orbits. Only after years will their orbits degrade (from minute atmospheric friction effects at perigee) and they burn up on re-entry. A single hit from one of these fragments would likely knock out another satellite, or puncture the hull of a manned spacecraft.

So for me the issue with the Chinese asat test is about polluting this finite resource that is Low Earth Orbit. I think the spacefaring nations need to get together and produce some comprehensive regulation. Without international agreement on the usage of LEO, we risk a devastating Tragedy of the Commons situation where the region becomes nearly unusable. Satellites launched will only have a short lifetime before some stray fragment destroys them. Manned missions will no longer be safe.

Will we as a species, I wonder, be any better at managing the outer space environment than we are with our terrestrial one?


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